Delocalization and geometries in the P-function of thiophosphinic derivatives; Tautomerism and supramolecular interactions

Delocalization and geometries in the P-function of thiophosphinic derivatives; Tautomerism and supramolecular interactions | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Delocalization and geometries in the P-function of thiophosphinic derivatives; Tautomerism and supramolecular interactions Mátyás Czugler, Zoltán Mucsi, Józef Drabowicz, György Keglevich This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7212964/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 24 Mar, 2026 Read the published version in Structural Chemistry → Version 1 posted 7 You are reading this latest preprint version Abstract Model series of thiophosphinic acid derivatives were scrutinized at the B3LYP/6-311 + + G(2d,2p) level of theory, and the resulting geometries were compared with experimental crystal structure data obtained from the Cambridge Structure Database (CSD). Typical bond length and bond angle values were selected for a series of similar fragments from CSD, and mean dimensions were also calculated from the experimental data accumulated. Linking the results of theoretical calculations and the evidences of data mining allowed the reconsideration of a few past experimental models with (thio)phosphinate - anion - (thio)phosphinic acid type associations. It seems also from this survey that thiophosphinic acid derivatives may be suitable vehicles for transgressing boundaries from a single molecule to supramolecular relations and vice versa . The study shows that a few intramolecular distances, such as the P-O bond lengths both in the anionic and in the neutral forms may significantly depend on intermolecular contacts as well. The tautomerism involving the thionic acid and thiolic acid forms was studied by theoretical methods at the B3LYP/6-311 + + G(2d,2p) level of theory assuming monomolecular and different bimolecular mechanisms. The shift toward the thionic actor was supported by the kinetic and thermodynamic energetics. thiophosphinic acids bond lengths delocalization supramolecular interactions tautomerism Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Thiophosphinic acids and their esters are important starting materials and intermediates in organic syntheses [1]. As regards the acids, these days it became obvious that their tautomeric equilibrium is shifted to the “thionic acid” >P(S)OH ( A ) form, while the “thiolic acid” >P(O)SH ( B ) is a minor component (Scheme 1) [2]. At the same time, the esters of both tautomeric forms ( C and D ) are available as distinct species. Thiophosphinic acids A , in general, may be prepared by the hydrolysis of thiophosphinyl chlorides ( E ), or by sulfur insertion into the P–H bond of secondary phosphine oxides ( F ) (Scheme 2) [3,4]. An odorless “liquid” method has recently been elaborated for the preparation of thiophosphinic acids [5]. Very recently, our laboratories have started experiments on an odorless procedure for the synthesis of thiophosphinic acids based on the addition of sulfur to secondary phosphine oxides in the solid phase using a ball mill or mortar grinding [6,7] As regards the synthesis of thiophosphinic esters C , thiophosphinil chlorides E should be reacted with alcohols (Scheme 3/(1)) [8]. At the same time, the thiophosphinate counterparts D are available on the alkylation of thiophosphinic acids A (Scheme 3/(2)) [9]. 1. Bond Distances and Delocalization in thiophosphinic derivatives The initial research plan aimed at surveying the phosphinate acid landscape from the pure phosphinate (F1) to the pure dithiophosphinate (F4) scaffold through the corresponding monothio species (F2 and F3) in the CSD (Fig. 1) [10]. The exploratory searches showed that while fragment F1 had several hundred hit numbers ( H N = 343), F2, F3 and F4 were less populated (F2 H N = 17, F3 H N = 58 and F4 H N = 26). However, due to the multiple presence of the respective fragments within a few molecules the number of occurrences are somewhat higher for F1-F4 ( O N = 417, 19, 89 and 27). F1 and F4 are the two extremes (as these contain only O or only S pendants), they are also models of the respective (single or double) covalent bonds. However, our attention was focused on the mixed forms containing both chalcogen (O and S) atoms. As these are apparently less abundant, further distinction and extension of the search models was necessary. Thus, fragments F5 and F6 representing a thiophosphinic acid and its deprotonated form, respectively (Fig. 2) became new actors of the search. From these exploratory searches it also became obvious that although chemical variation influences the mean values, these may overlap due to the larger standard deviations. 1.1 Within the thiophosphinic-related P-function First of all, attention is paid primarily to some dimensions of the P-function including single- double- and delocalized bonds in the title molecules. Thus, covalent bond distances are in the fore in this section. There are, of course, various ways to analyze bonding relations from sophisticated ab initio approaches to simpler ones, like the Bond–Valence method [11–15]. A rough estimate of delocalization may be guessed on the basis of the stretching of a given double bond vs. the respective double bond estimate. It is also possible to compare the shortening of a single bond with the mean of single bond lengths. Table 1 contains results of a preliminary series of searches of the thiophosphinic acid-related fragments shown in the first column. Details for the searches may be found in the Supplementary Information (SI 1–6). In the first approach, let us compare the trends in the lengths of the P=S and P-O bonds if the series of >P(S)OC, >P(S)OH and >P(S)O - is compared. Table 1 contains mean bond lengths as derived from the CSD X-ray database [10]. One can see that in the above order, the P=S bond is elongated, while the P-O bond is shortened. Hence, there must be a significant delocalization for the >P((S)O – anion. To quantify the extent of delocalization, let us consider that the mean P=S length is 1.98 Å that is between the 1.94 (P=S) and 2.08 (P-S) extremes (Table 2). The delocalization may be estimated on the basis of the 0.04 Å P=S lengthening and the 0.14 Å gap as (1.98-1.94)/(2.08-1.94) = 0.04/0.14 = 29%. Looking from the P-S side, the numerator becomes 0.10 as (2.08-1.98) giving rise to a value of 0.10/0.14 = 71%. Approaching from the P-O side, considering a 0.09 Å change by the decrease of P–O from 1.61 to 1.52 and a gap of 0.14 Å between the P–O and P=O bonds, the delocalization may be taken as (1.61-1.52)/(1.61-1.47) = 0.09/0.14 = 64%. Regarding the P=O, a 0.05 Å (1.52-1.47) stretch suggests a delocalization of 0.05/0.14 = 36%. These percentages may perhaps indicate that delocalization cannot be neglected if ionic forms occur. However, as it turns out ( vide infra ) the delocalization within the C,CP(X)YC function may intimately be coupled with the presence of intermolecular interactions as well. Two such outstanding cases are CIRQAY [16] and SORKOB [17] (named as their CSD reference codes). CIRQAY is an example of a thiosphosphinic acid and a thiophosphinate anion asymmetric dimer type association, while SORKOB comprises a phosphinic acid – phosphinate analogue with an O – ... H -- O intermolecular contact type. Looking at the pairwise P:::O and P:::S distances in CIRQAY (Fig. 3), one may see only marginal P:::O difference (1.543(2) vs. 1.531(2) Å), while the P:::S distances are practically equal (1.96 Å, Fig. 3/(B)). This is in sharp contrast with the theoretical model of the dimer associate, where P-O bonds do show differences, and even the P-S bonds act in a similar way (Fig. 3/(A)). The phosphinic acid – phosphinate analogue in SORKOB appears to have the same symptoms. The P:::O distances (1.532 Å vs. 1.525 Å) of the anionic O ‑‑ H ... O – connection are only marginally differing, and the situation is similar for the other two “P=O” bonds (1.499 Å and 1.505 Å). One may suspect that these indications stem from the associative nature of these crystal structures. Thus, an interlude of a detour in this region is needed. In other words, we stumble into a situation, where "normal" (i.e. covalent) organic chemistry and supramolecular chemistry may no longer be fully separated. 1.2 Regarding supramolecular relations The CCP(S)O – anion-fragment search gave only a rather limited result yielding 9 hits (see Suppl. Info Fig. SI-6). Examples for delocalization in thiophosphinate - thiophosphinic acid systems in the solid state are indeed rare. For this, we decided to perform a combined search, where both O=P-O – and S=P-O – anions are involved, even metal-organic crystals were also allowed as search targets. Eventually, there were 201 such molecules (see Suppl. Info Fig. SI-7). Then, a search was made for contacts of CCP(X)O – anions (X=O,S) to HO-NM fragments, where "NM" may mean any Non-Metal element. There were 95 such crystal structures (see Suppl. Info Fig. SI-8), which contained 158 replicas satisfying the shorter than vdW radii sum criteria. Fig. 4 shows the scattergram of these contacts while Table 3 shows the essential statistical description characteristics of this data set. It is noted that the shortest O – ... O bridgehead distances being less than 2.5 Å (shown in red dots) were found in the crystal structures of CIRQAY, SORKOB,and CUMBOB, FASBUY, IZOXOM, NUWMUN, QIFBAL, VEZHEN, VEZKEQ, WAWVID, WIGGID and WIJHIK [16,17,20-28] (named as their CSD reference codes). These 12 cases also provide the 11 shortest O – ... H contacts. Those 12 crystals distances stem from strongly interacting species. Table 3. Mean contact distances (Å) with standard deviations for 158 O N counts. (For search results details and the fragment used look up Suppl. Info SI-8). DIST5 and DIST6 refers to O – ... O bridgehead and to O – ... H distances, respectively. Name Mean ( Å) Std. Dev Min(Å) Max( Å) Outliers Low High Total DIST5 2.73 0.13 2.40 2.97 0 0 0 DIST6 1.89 0.18 1.25* 2.35 2* 0 2 *These outliers are from an anion ... oxonium cation H-bond (1.25 Å) in NUWMUN and from a perfluoroacetato-guest in WIJHIK (1.39 Å). There are a few interesting features in this scattergram. Firstly that these 12 extremely short distance values belong to 4 perfluorinated compounds with direct anion ... cation contacts to oxonium (VEZKEQ, NUWMUN, VEZHEN) and to perfluoro-acetic acid cations (WIJHIK), while WAWVID has a pyridinium counter-ion. The other 7 structures (CIRQAY, CUMBOB, FASBUY, IZOXOM, SORKOB, QIFBAL, WIGGID) exhibit the 4 thiophosphinate ... thiophosphinic, or the 3 phosphinate ... phosphinic acid with D finite set contact type [18,19]. Regardless, whether these associative phenomena stem from pluri-molecular (hetero-molecular) or from multi-molecular (homo-molecular, self-associative) nature, all these short contacts exert influence on the covalent bonding in the vicinity of the P-atom. The linear regression model shown in Fig. 4 is characterized by the DIST6 = 1.19* DIST5 - 1.37 equation with an R 2 = 0.723 giving an R = 0.850 correlation coefficient. Further inspection revealed that the acceptor O – .... H (DIST6) vs. the acceptor ... donor (O – ... O, DIST5) distances are approaching fair correlation with an R = 0.886, when one excludes metal-organic structures (see Suppl. Info Fig. SI-9). (In that case, the number of hits, as well as the fragment count numbers were reduced to somewhat less than the 2/3 of the counts presented above.) Considering that this search did not employ strong filtering, this correlation seems acceptable. In plain words, this correlation means nothing more than that these H-bonds drift towards linearity quite well. It is also clear that where the O ... O bridgehead distances are very short (< 2.5Å), the proton position may be expected closer to the midway. A notable feature of this diagram points to the unexpected long O –. .. H distance originating from the CIRQAY X-ray model, too. This may stem from an elongated O -- H covalent bond, or possibly from fast switching between the two anchor atoms or maybe from the disorder – like averaging of two opposing O -- H positions. As far as the thiophosphinate - thiophosphinic acid ensemble is concerned, the message from these data seems to be clear. It says that thiophosphinic acid salts are prone to make very strong electrostatic contacts. A few of these compounds may represent strong association ability, thus providing a bridge between molecular (covalent) and supramolecular chemistry. This also means that understanding covalent features, such as delocalization, one may have to consider effects transgressing molecular boundaries as well. 1.3 Comparison of the geometry of the thiophosphinic and phosphinic scaffolds The obvious misplace of the H atom in the thiophosphinate - thiophosphinic acid H-bridge in the CIRQAY crystal structure and the unexpected bonding around the P-atom [16] requires further inspections. The parent diphenylthiosphospinic acid crystal structure was solved by Mattes & Rühl in 1984 [29]. The related paper was not only an excellent demonstration of an experimental technique with competent results and with alike analysis, but it was also attesting relevant observations linking traditional organic chemistry ("covalent chemistry") with the supramolecular chemistry. For host-guest chemistry audience, the statement "Molecular-weight measurements indicate that Ph 2 P(S)OH is also highly associated in solutions" may have escaped attention. It meant that a molecular association mostly attributed to crystalline state may be prevalent in such compounds even in solutions. This is in congruence with the picture we see emerging from Fig. 4, namely that the strongest interactions stem from phosphinic acid derivatives. Remarkably, thiophosphinic derivatives account more that the half of these shortest contacts. A recent publication mentions Ph 2 P(S)OH as an intermittent product (CEJSIT01, [30]). Distances around the P-function were compared with those from theory (Table 4). This table compares basic dimensions of the neutral Ph 2 P(S)OH acid with those from the two examples selected for scrutiny (CIRQAY [16] and SORKOB [17] plus an additional one (PAHSOG [31]). This latter was chosen to exemplify another experimental issue of the failure of the charged entity assignment. The two independent experimental Ph 2 P(S)OH structures show essentially the same geometry. Additionally, three dipehenylthiophosphinate aninon structures were associated with the neutral forms. This table confirms that the experimental and theoretical data align in shorter-longer tendencies well. It may also be seen that the direct anion - acid contacts alter bonding dimensions as far as these lengths go. It is notable that these alterations may be observed primarily in the bond length of the P-O bond. It is definitely shorter both in the thiophosphonic and in the phosphonic acid examples, as those predicted by calculations. It may be also a sort of accumulated effects from the grand-average nature of the crystal structure, as well as its environment. Nevertheless, this shortening seems to be significant, showing effects of the strong association power of these kinds of molecules, as also attested in their recurrence amongst the shortest interactions in Fig. 4. CIRQAY and SORKOB are also interesting as the misposition of the H atom in CIRQAY is attenuated by the SORKOB structure. Fig. 5 shows an excerpt from the latter. In SORKOB the acid P–O length calculated by theory is 1.626 Å. It falls in the 1.61(2) Å statistical mean value (c.f. Table 1 and Suppl. Info Fig. SI-10). Thus, the theory P‑O distance is a typical P-O single bond length and does not show the shortening to 1.53 Å characteristic for the crystalline state model (c.f. Fig. 5). Moreover, the acid -OH bond distance is 0.964 Å in the calculated model while this size is 1.11 Å (1.107 Å) in the SORKOB X-ray model. This latter is close to the theory- calculated 1.05 Å of the CIRQAY. All in all this means that the single molecule approach in the calculations cannot account for the anionic acceptor role. Crystal structure of the multi-functional SORKOB molecule supports that the thiophosphonate anion ... thiophosphonic acid contact is a decisive crystal engineering factor [32,33]. Indeed, a glance at the interaction energies as calculated by the CrystalExplorer program [34] using a HF-approach [35] shows that the cohesive forces along the {0 0 1} direction are an order of magnitude greater (at 161 kJ/mol) than all other interactions (c.f. Suppl. Info Table SI-1, 2nd row). The almost extended SORKOB molecule (with two knicks at the P-atoms of its backbone) defines the crystallographic c -axis via the z-1 and z+1 translations (c.f. Suppl. Info Fig. SI-11). Multi-functional armory of the SORKOB molecule is also attested by the H-bridges graphs. The dominance and the importance of the anionic-neutral H-bridge connection is emphasized even in this environment. Table 4. P-function bond lengths in experiment-theory comparison for neutral Ph 2 -, S- or O-phosphonic acid molecules involved in H-bridges (CEJSIT and CEJSIT01) and with those involved in anionic - neutral acid crystalline associations. DIST1 is the P-OH bond, DIST2 is P=S (or P=O), DIST3 and DIST4 are P-C lengths, while DIST5 values are bridgehead atom distances (those from the -OH donor to the S (or O – ) acceptor or from HO to O – donator ... acceptor atoms). NAME DIST1 DIST2 DIST3 DIST4 DIST5 CEJSIT/CEJSIT01* 1.583 1.955 1.803 1.801 3.139 CIRQAY-OH 1.543 1.957 1.817 1.815 2.434 Theory-OH 1.572 1.975 1.837 1.837 2.485 CIRQAY-O - 1.531 1.960 1.821 1.821 2.434 Theory-O - 1.528 1.998 1.848 1.848 2.485 SORKOB-OH 1.532 1.499 1.794 1.828 2.419 Theory-OH 1.626 1.485 1.813 1.823 ** SORKOB-O - 1.525 1.505 1.795 1.832 2.419 Theory-O - 1.515 1.509 1.815 1.837 ** PAHSOG*** 1.508 1.933 1.804 1.825 2.338 PAHSOG*** 1.505 1.934 1.789 1.801 * Mean distances averaged from Mattes & Rühl [29] and Shao et al., [30]. ** N/A. *** In the case of PAHSOG it was not possible to make clear assignment of neither the anionic Ph 2 P(S)O - , nor the acid form, yet another proof of association-related supramolecular effects. 2. A study of the geometry around the P- atom in Ph 2 P(O)OMe, Ph 2 P(O)SMe, Ph 2 P(S)OMe and Ph 2 P(S)SMe – theoretical calculations vs. X-ray data base analysis The computed gas-phase geometries of the four phosphorous compounds – Ph₂P(O)OMe, Ph₂P(O)SMe, Ph₂P(S)OMe, and Ph₂P(S)SMe – obtained at the B3LYP/6-311++G(2d,2p) level, show good agreement with the X-ray data obtained for similar scaffolds of crystalline compounds after a search in the data base (Table 5). The P=X (X = O, S) and P–Y (Y = OMe, SMe) bond lengths differ from the experimental mean values by only 0.01–0.05 Å, which is within the expected standard error range. Obviously, the absence of solid-state packing and/or solvent effects may also have affected the calculations. As such effects regularly appear in the X-ray diffraction experimental results, their effects may be estimated by comparing with the geometry of the molecule calculated in vacuo . Instances, where such effects may become visible are just the subject of this paper. Similarly, the P–C bond distances show deviations of less than 0.02 Å. The key bond angles, including C–P=X and C–P–Y, exhibit differences of only 1–3°, which may be attributed to crystal packing or intermolecular interactions absent in the vacuum model. As expected, the compounds containing a P=S bond show consistently longer bond lengths than their P=O analogues, both computationally and experimentally. Overall, the theoretical model accurately reproduces the molecular geometries, confirming the reliability of the chosen method for further mechanistic or electronic structure investigations. Table 5 Selected bond lengths (in Å) and bond angles (°) for Ph2P(X)YMe esters computed at B3LYP/6-311++G(2d,2p) level of theory in vacuo. Mean bond distance (Å) and angle values (°) of the respective four different search fragments with their standard deviations (in parentheses) as derived from the search hits. Ph 2 P(O)OMe C 2 P(O)O-C* Ph 2 P(O)SMe C 2 P(O)S-C Ph 2 P(S)OMe C 2 P(S)O-C Ph 2 P(S)SMe C 2 -P(S)S-C P=X 1.4814 1.47(1) 1.4880 1.48(1) 1.9557 1.94(2) 1.9664 1.94(1) P–Y 1.6168 1.59(2) 2.1331 2.08(2) 1.6263 1.61(2) 2.1466 2.10(2) P–C1 1.8097 1.80(3) 1.8205 1.83(3) 1.8203 1.80(2) 1.8320 1.81(2) P–C2 1.8202 1.80(3) 1.8262 1.80(1) 1.8308 1.80(2) 1.8377 1.81(3) C1–P=X 114.34 114(2) 113.23 112(3) 115.47 115(4) 115.04 115(3) C1–P–Y 100.07 103(3) 101.07 102(4) 99.09 102(4) 99.09 101(7) C2–P=X 111.18 113(2) 111.47 112(1) 113.63 115(3) 113.08 115(4) C2–P–Y 104.77 103(3) 107.11 105(3) 103.62 104(3) 106.84 102(6) C1–P–C2 108.78 108(5) 108.43 109(2) 106.64 104(7) 106.48 105(5) X=P–Y 116.72 115(2) 114.82 115(2) 116.69 115(2) 115.02 116(3) The relatively high standard deviations of the X-ray database statistics stem from the wide range of chemistry these searches embrace. It is also obvious that many of these crystals may also contain solvent or other molecules. Apart from molecular diversity the crystalline environment variation also pose various attractive and repulsive interactions. It is also apparent that these values conform to the theoretical values within even single s.d. values. 3. The tautomerization of thiophosphinic acids The tautomerization of thiophosphinic or “thionic acid” >P(S)OH ( A ) to “thiolic acid” >P(O)SH ( B ) – typically involving equilibrium – is a fundamental proton-transfer process that plays a crucial role in the reactivity and stability of these compounds. This isomerization influences the acidity, coordination behavior and biological activity of thiophosphinic derivatives, making them relevant in the synthetic and the related medicinal chemistry areas. Comprehension of the tautomeric preferences may promote the rational design of ligands for metal complexes, as well as the development of thiophosphinic-related biologically active agents. The position of the equilibrium and height of the activation barrier for the proton shift are dependent on the acid-base catalyst, solvent effects and nature of the substituents. Quantum chemical studies were planned to evaluate the tautomeric equilibrium involving the mechanism of the isomerisation. The tautomerism under discussion is a suitable model to study intra or intermolecular proton transfers. 3.1. The thermodynamic stability of thiophosphinic acid isomers (tautomers): >P(S)OH vs. >P(O)SH Theoretical calculations performed at the B3LYP/6-311++G(2d,2p) level of theory confirmed earlier literature findings by demonstrating that the >P(S)OH (thione-type) tautomer is significantly more stable than the >P(O)SH (thiol-type) form. This preference is valid for the series of both dialkyl- and diaryl-substituted thiophosphinic acids. For the different substituent combinations shown in Scheme 4, the calculated enthalpy change (ΔH) fell in the range of +13.7 to +17.5 kJ mol –1 , indicating that the formation of the thiol form is endothermic, and hence thermodynamically unfavorable. It is clear that the >P(S)OH tautomer (the thione form) is the dominating component of the equilibrium. The value of ΔH slightly depends on the electronic and steric properties of the substituents (Scheme 4). 3.2. Kinetics for the proton transfer isomerization (tautomerization) In this section, the tautomerization of dimethylthiophosphinic acid (Me₂P(S)OH) was studied via an intramolecular proton transfer from the oxygen to the sulfur atom (Scheme 5) in a single elementary step. The reaction may proceed through a strained four-membered transition state. This monomolecular isomerization pathway is hindered kinetically, as suggested by a relatively high enthalpy of activation (ΔH # ) of 114.6 kJ mol⁻¹. Such a high barrier means that, under ambient conditions, in the absence of suitable catalysts or solvent assistance the tautomerization is negligible. The high ΔH # value may be attributed to the strong hydrogen bond present and the geometric reorganization needed during the proton shift between the heteroatoms. The absence of an external proton shuttle or a solvent-network mediating the taking place in the gas phase limits the feasibility of a lower-energy mechanism. The kinetic barrier of a tautomerism determines the stability of thiophosphinic acid and its reactivity. In practical applications, such as ligand design, the kinetic stability of one tautomer over the other may significantly influence the binding modes, and hence the metal selectivity. 3.3. Isomerization in the presence of water as the base Theoretical modeling of the isomerisation by proton transfer in the presence of water involves a bimolecular mechanism, in which the water molecule abstracts the proton from Me₂P(S)OH to generate a thiophosphinate anion and a H 3 O + cation. Subsequently, the sulfur site of the ambident anion is protonated yielding the less stable >P(O)SH tautomer, as illustrated in Scheme 6. Despite the role of water as a proton shuttle, the overall transformation remains thermodynamically unfavorable due to the endothermic nature of this process characterized by an enthalpy change of 49.1 kJ mol⁻¹. In all, although water may assist the proton transfer, the process in whole is unfavorable. 3.4. Isomerization in the presence of water as the acid An alternative mechanistic scenario may involve protonation of Me₂P(S)OH by a water molecule, leading to the formation of the protonated thiophosphinic acid, as outlined in Scheme 7. However, this transformation is highly unfavorable thermodynamically, as the calculated enthalpy change is 134.4 kJ mol⁻¹. Such a high degree of endothermicity excludes this pathway under standard conditions. The result is the consequence of the low acidity of water relative to Me₂P(S)OH, and confirms that water cannot act as a proton donor in this process. Therefore, this mechanism should be ruled. 3.5. Isomerization via autoprotolysis Another theoretically possible pathway for the tautomerization of Me₂P(S)OH involves autoprotolysis, in which a thiophosphinic acid molecule protonates another one forming a conjugate acid–base pair as shown in Scheme 8. This bimolecular mechanism could, in principle, allow for proton transfer without the involvement of an external acid or base. However, the calculated enthalpy change for this process is 71.5 kJ mol –1 indicating that the transformation is rather endothermic, and thus thermodynamically unfavorable under ambient conditions. Hence, the autoprotolysis-related isomerization should be regarded negligible. This finding support the assumption that intramolecular or solvent-assisted pathways may be more plausible for the tautomerization of thiophosphinic acids than autoprotolysis. 3.6. Isomerization with the help of MeOH As an alternative to water-mediated proton transfer, methanol may also act as a proton donor to promote the tautomerization of Me₂P(S)OH (Scheme 9). In this case, the proton transfer from complex 1 proceeds with a low activation barrier of 40.2 kJ mol –1 indicating that this pathway is kinetically accessible under mild conditions. However, the overall transformation is endothermic, with an associated enthalpy change of 25.5 kJ mol –1 . One may conclude that methanol, due to its greater acidity, is more suitable to facilitate the proton transfer than water, and methanol has the ability to stabilize the transition-state structure via hydrogen bonding. Nevertheless, the unfavorable thermodynamics prevents the equilibrium to be shifted to complex 2 . 3.7. Isomerization via a symmetric dimer A further mechanistic possibility involves isomerization starting from a symmetric hydrogen-bonded dimer of Me₂P(S)OH ( dimer A ), as depicted in Scheme 10. In this scenario, one monomer acts as a proton donor, while the other accepts the proton enabling the proton transfer within the dimer without the involvement of an external species to afford dimer B . Quantum chemical calculations indicate that the process is endothermic, with an enthalpy change of 46.7 kJ mol –1 , and proceeds via a transition state characterized by an activation barrier of 63.2 kJ mol –1 . Although this pathway is energetically more favorable than protonation by water or autoprotolysis, if the corresponding enthalpy changes (134.4 kJ mol –1 and 71.5 kJ mol –1 , respectively) are regarded, it represents a moderately hindered process. The results suggest that the dimer-assisted tautomerization may occur under appropriate conditions, but it is unlikely to be the dominant route at room temperature. 3.8. Isomerization via an asymmetric dimer An alternative and more favorable pathway involves isomerization through an asymmetric hydrogen-bonded dimer of Me₂P(S)OH ( dimer C ), as shown in Scheme 11. In this mechanism, one Me₂P(S)OH unit functions as a proton donor, while the other acts as a proton acceptor, leading eventually to the formation of an Me₂P(O)SH tautomer (as seen in dimer E ) from only one of the two starting molecules. This concerted, intermolecular proton shift proceeds through a well-defined transition state ( TS4 ), and is characterized by a low activation barrier of 37.0 kJ mol –1 . Moreover, the overall transformation is slightly exothermic indicating that it is both kinetically and thermodynamically feasible under mild conditions. These results suggest that the asymmetric dimer pathway may represent a realistic route for tautomerization in condensed-phase environments, where such hydrogen-bonding interactions may occur and stabilize the transition structure. Experimental Theoretical calculations All computations were carried out with the Gaussian16 program package (G16C1) [36], using standard convergence criteria for the gradients of the root mean square (RMS) Force, Maximum Force, RMS displacement and maximum displacement vectors (3.0 × 10 –4 , 4.5 × 10 –4 , 1.2 × 10 –3 and 1.8 × 10 –3 ). Computations were carried out at B3LYP level of theory [37], with the basis set of 6-311++G(2d,2p). The vibrational frequencies were computed at the same levels of theory. In some cases the IEFPCM method was also applied to model the solvent effect, by using the default settings of G16 [PCM(water)], ε = 78 (for water) [38]. Thermodynamic functions U, H, G and S were computed at 298.15 K. See the Supplementary Information for details. CSD data analysis It was performed initially on the database CSD version 5.44 (updates Sep. 2023).consequentially repeated on the CSD version 5.46 Updates (Feb 2025) and 6.00 (April 2025) using ConQuest [39] and Mercury [40] for producing some molecular drawings and scatterplots as well as statistics. Search criteria were only adjusted where large enough sample number permitted this, for details consult Supplementary. Otherwise only general restriction was the omission of powder structures. Declarations Supplementary Information The online version contains supplementary material belonging to the searches in the Cambridge Structure Database and to the theoretical calculations available at https://doi.org/………………………. Acknowledgements MC thanks Dr. P. Bombicz (HUN-REN Res. Cent. Nat. Sci.) for her support in the CSD use. ZM is grateful for the possibility of using HUN-REN Cloud[REFX] within the “Szerves-Biochem” project, which helped to achieve the results described in this paper. Author contributions All authors contributed to the study conception and design. GK initiated and supervised the project. CSD analyses and critical appraisal were performed by MC. ZM performed the theoretical calculations. JD placed the project in literature context. The manuscript was written by MC and GK incorporating the remarks of the other authors. All authors read and approved the final manuscript. Funding This project was supported by the National Research, Development and Innovation Office (NKKP-ADVANCED 149447). Data availability: All data can be found in the article and the Supplementary information. Ethical approval: Not applicable. Competing interests: The authors declare no competing interests. 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IUCrJ 4:575–587. https://doi.org/10.1107/S205225251700848X Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone V, Petersson GA, Nakatsuji H, Li X, Caricato M, Marenich AV, Bloino J, Janesko BG, Gomperts R, Mennucci B, Hratchian HP, Ortiz JV, Izmaylov AF, Sonnenberg JL, Williams-Young D, Ding F, Lipparini F, Egidi F, Goings J, Peng B, Petrone A, Henderson T, Ranasinghe D, Zakrzewski VG, Gao J, Rega N, Zheng G, Liang W, Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao O, Nakai H, Vreven T, Throssell K, Montgomery JA Jr, Peralta JE, Ogliaro F, Bearpark MJ, Heyd JJ, Brothers EN, Kudin KN, Staroverov VN, Keith TA, Kobayashi R, Normand J, Raghavachari K, Rendell AP, Burant JC, Iyengar SS, Tomasi J, Cossi M, Millam JM, Klene M, Adamo C, Cammi R, Ochterski JW, Martin RL, Morokuma K, Farkas O, Foresman JB, Fox DJ (2016) Gaussian 16, Revision C.01, Gaussian, Inc., Wallingford CT. Becke AD (1993) Density-functional thermochemistry. III. The role of exact exchange. J Chem Phys 98:5648–5652. https://doi.org/10.1063/1.464913 Tomasi J, Mennucci B, Cammi R (2005) Quantum mechanical continuum solvation models. Chem Rev 105:2999–3093. https://doi.org/10.1021/cr9904009 Bruno IJ, Cole JC, Edgington PR, Kessler M, Macrae CF, McCabe P, Pearson J, Taylor R (2002) New software for searching the Cambridge Structural Database and visualising crystal structures. Acta Cryst B58:389–397. https://doi.org/10.1107/S0108768102003324 Macrae CF, Sovago I, Cottrell SJ, Galek PTA, McCabe P, Pidcock E, Platings M, Shields GP, Stevens JS, Towler M, Wood PA (2020) Mercury 4.0: from visualization to analysis, design and prediction. J Appl Cryst 53 :226–235. https://doi.org/10.1107/S1600576719014092 Tables Table 1 and 2 are available in the Supplementary Files section. Scheme Scheme 1 to 11 are available in the Supplementary Files section. Additional Declarations No competing interests reported. Supplementary Files StructChemSupplInfo0725.docx Table1and2.docx Scheme1to11.docx GraphicalAbstract.docx Cite Share Download PDF Status: Published Journal Publication published 24 Mar, 2026 Read the published version in Structural Chemistry → Version 1 posted Editorial decision: Revision requested 09 Aug, 2025 Reviews received at journal 29 Jul, 2025 Reviewers agreed at journal 29 Jul, 2025 Reviewers invited by journal 29 Jul, 2025 Editor assigned by journal 29 Jul, 2025 Submission checks completed at journal 29 Jul, 2025 First submitted to journal 25 Jul, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7212964","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":492770557,"identity":"ab3c36fc-3f4f-4ae7-9280-097327bde211","order_by":0,"name":"Mátyás Czugler","email":"","orcid":"","institution":"Budapest University of Technology and Economics","correspondingAuthor":false,"prefix":"","firstName":"Mátyás","middleName":"","lastName":"Czugler","suffix":""},{"id":492770558,"identity":"6284c6cd-1eb8-488f-b0d1-aaba26e7a6e8","order_by":1,"name":"Zoltán Mucsi","email":"","orcid":"","institution":"University of Miskolc","correspondingAuthor":false,"prefix":"","firstName":"Zoltán","middleName":"","lastName":"Mucsi","suffix":""},{"id":492770559,"identity":"8422f6fc-0975-4b8c-8ca9-df52f0d569bd","order_by":2,"name":"Józef Drabowicz","email":"","orcid":"","institution":"Jan Dlugosz University in Czestochowa","correspondingAuthor":false,"prefix":"","firstName":"Józef","middleName":"","lastName":"Drabowicz","suffix":""},{"id":492770560,"identity":"12375a5f-cf2d-487d-b619-333c99c29105","order_by":3,"name":"György Keglevich","email":"data:image/png;base64,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","orcid":"","institution":"Budapest University of Technology and Economics","correspondingAuthor":true,"prefix":"","firstName":"György","middleName":"","lastName":"Keglevich","suffix":""}],"badges":[],"createdAt":"2025-07-25 09:53:29","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7212964/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7212964/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s11224-025-02596-2","type":"published","date":"2026-03-24T16:09:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":88007870,"identity":"d814db62-c962-4294-9fc2-96dca0871dae","added_by":"auto","created_at":"2025-07-31 11:21:03","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":14213,"visible":true,"origin":"","legend":"\u003cp\u003eStations on the avenue\u003cstrong\u003e \u003c/strong\u003efrom phosphinate to dithiophosphinate\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7212964/v1/c8bd2be86de106cdbeba9f1d.png"},{"id":88007876,"identity":"f858f204-f909-44cf-857e-b1b701b9d01c","added_by":"auto","created_at":"2025-07-31 11:21:03","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":8135,"visible":true,"origin":"","legend":"\u003cp\u003eFurther refinement of the searched fragments\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7212964/v1/f24601af1170cfe903fe74eb.png"},{"id":88008820,"identity":"9aea18d7-77b8-4b72-bf04-9c437dd2c784","added_by":"auto","created_at":"2025-07-31 11:29:03","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":70290,"visible":true,"origin":"","legend":"\u003cp\u003eComprehensive view of the distances computed at B3LYP/6-311++G(2d,2p) level of theory (\u003cstrong\u003eA\u003c/strong\u003e) (the details are shown later)and the published bond lengths (Å) values of the CIRQAY X-ray structure (\u003cstrong\u003eB\u003c/strong\u003e) [16].\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7212964/v1/768705f3121cfb4d6b222c1b.png"},{"id":88008845,"identity":"0e7b5c02-bbf3-41d1-a3bb-9de792e0e58d","added_by":"auto","created_at":"2025-07-31 11:29:06","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":109383,"visible":true,"origin":"","legend":"\u003cp\u003eScattergram of the O\u003csup\u003e\u003cstrong\u003e–\u003c/strong\u003e\u003c/sup\u003e ... O vs. O\u003csup\u003e\u003cstrong\u003e–\u003c/strong\u003e\u003c/sup\u003e ... H-O (DIST5-DIST6) distances (in Å). Those marked in red dots have DIST5 \u0026lt; 2.5 Å limit marked by a broken-line vertical arrow. A bent red arrow points to an outlier value of 1.82 Å originating from the CIRQAY X-ray model out of the 12 shortest of the group of both distances. The regression line equation DIST6 = 1.19* DIST5 - 1.37 has \u003cem\u003eR\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e = 0.723.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7212964/v1/3023288f258b8b2619ced03a.png"},{"id":88007881,"identity":"251bbec5-cb4c-4529-b33b-5fd9ca682b98","added_by":"auto","created_at":"2025-07-31 11:21:03","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":79397,"visible":true,"origin":"","legend":"\u003cp\u003eA blown-up view of the SORKOB H-bridge in question showing the bridgehead and O-H and H...O distances\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7212964/v1/0510dad9e7150f830c6e4533.png"},{"id":105754841,"identity":"a23302b4-97be-4b3c-9133-fd68b69f8a2c","added_by":"auto","created_at":"2026-03-30 16:22:20","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1378423,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7212964/v1/cb165157-42fd-4fac-a549-73fdd327b587.pdf"},{"id":88007871,"identity":"4d55079f-cdba-43b5-b57d-aab672f5abe3","added_by":"auto","created_at":"2025-07-31 11:21:03","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":504649,"visible":true,"origin":"","legend":"","description":"","filename":"StructChemSupplInfo0725.docx","url":"https://assets-eu.researchsquare.com/files/rs-7212964/v1/a520b3f08150d78378b20265.docx"},{"id":88008818,"identity":"b7789884-eea4-4c2c-82be-be0fe8f08afa","added_by":"auto","created_at":"2025-07-31 11:29:03","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":63318,"visible":true,"origin":"","legend":"","description":"","filename":"Table1and2.docx","url":"https://assets-eu.researchsquare.com/files/rs-7212964/v1/329c17aa142a8ada72a7abb1.docx"},{"id":88008821,"identity":"9c221cc3-6494-420c-b92b-0234f0c14907","added_by":"auto","created_at":"2025-07-31 11:29:03","extension":"docx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":186762,"visible":true,"origin":"","legend":"","description":"","filename":"Scheme1to11.docx","url":"https://assets-eu.researchsquare.com/files/rs-7212964/v1/224990c76a1085e32a349298.docx"},{"id":88007879,"identity":"96f1ed05-fa89-42d3-aae1-1144904b53bb","added_by":"auto","created_at":"2025-07-31 11:21:03","extension":"docx","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":36788,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbstract.docx","url":"https://assets-eu.researchsquare.com/files/rs-7212964/v1/a3e685cc7ad0f4e4ce88f099.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Delocalization and geometries in the P-function of thiophosphinic derivatives; Tautomerism and supramolecular interactions","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThiophosphinic acids and their esters are important starting materials and intermediates in organic syntheses [1]. As regards the acids, these days it became obvious that their tautomeric equilibrium is shifted to the \u0026ldquo;thionic acid\u0026rdquo; \u0026gt;P(S)OH (\u003cstrong\u003eA\u003c/strong\u003e) form, while the \u0026ldquo;thiolic acid\u0026rdquo; \u0026gt;P(O)SH (\u003cstrong\u003eB\u003c/strong\u003e) is a minor component (Scheme 1) [2].\u003c/p\u003e\n\u003cp\u003eAt the same time, the esters of both tautomeric forms (\u003cstrong\u003eC\u003c/strong\u003e and \u003cstrong\u003eD\u003c/strong\u003e) are available as distinct species.\u003c/p\u003e\n\u003cp\u003e\u003cimg 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\" width=\"505\" height=\"159\"\u003e\u003c/p\u003e\n\u003cp\u003eThiophosphinic acids \u003cstrong\u003eA\u003c/strong\u003e, in general,\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003emay be prepared by the hydrolysis of thiophosphinyl chlorides (\u003cstrong\u003eE\u003c/strong\u003e), or by sulfur insertion into the P\u0026ndash;H bond of secondary phosphine oxides (\u003cstrong\u003eF\u003c/strong\u003e) (Scheme 2) [3,4].\u003c/p\u003e\n\u003cp\u003eAn odorless \u0026ldquo;liquid\u0026rdquo; method has recently been elaborated for the preparation of thiophosphinic acids [5]. \u0026nbsp;Very recently, our laboratories have started experiments on an odorless procedure for the synthesis of thiophosphinic acids based on the addition of sulfur to secondary phosphine oxides in the solid phase using a ball mill or mortar grinding [6,7]\u003c/p\u003e\n\u003cp\u003eAs regards the synthesis of thiophosphinic esters \u003cstrong\u003eC\u003c/strong\u003e, thiophosphinil chlorides \u003cstrong\u003eE\u003c/strong\u003e should be reacted with alcohols (Scheme 3/(1)) [8]. At the same time, the thiophosphinate counterparts \u003cstrong\u003eD\u003c/strong\u003e are available on the alkylation of thiophosphinic acids \u003cstrong\u003eA\u003c/strong\u003e (Scheme 3/(2)) [9].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1. Bond Distances and Delocalization in thiophosphinic derivatives\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe initial research plan aimed at surveying the phosphinate acid landscape from the pure phosphinate (F1) to the pure dithiophosphinate (F4) scaffold through the corresponding monothio species (F2 and F3) in the CSD (Fig. 1) [10].\u003c/p\u003e\n\u003cp\u003eThe exploratory searches showed that while fragment F1 had several hundred hit numbers (\u003cstrong\u003e\u003cem\u003eH\u003csub\u003eN\u003c/sub\u003e\u003c/em\u003e\u003c/strong\u003e = 343), F2, F3 and F4 were less populated (F2 \u003cstrong\u003e\u003cem\u003eH\u003csub\u003eN\u003c/sub\u003e\u003c/em\u003e\u003c/strong\u003e = 17, F3 \u003cstrong\u003e\u003cem\u003eH\u003csub\u003eN\u003c/sub\u003e\u003c/em\u003e\u003c/strong\u003e = 58 and F4 \u003cstrong\u003e\u003cem\u003eH\u003csub\u003eN\u003c/sub\u003e\u003c/em\u003e\u003c/strong\u003e = 26).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHowever, due to the multiple presence of the respective fragments within a few molecules the number of occurrences are somewhat higher for F1-F4 (\u003cstrong\u003e\u003cem\u003eO\u003csub\u003eN\u003c/sub\u003e\u003c/em\u003e\u003c/strong\u003e= 417, 19, 89 and 27). F1 and F4 are the two extremes (as these contain only O or only S pendants), they are also models of the respective (single or double) covalent bonds. However, our attention was focused on the mixed forms containing both chalcogen (O and S) atoms. As these are apparently less abundant, further distinction and extension of the search models was necessary. Thus, fragments F5 and F6 representing a thiophosphinic acid and its deprotonated form, respectively (Fig. 2) became new actors of the search.\u003c/p\u003e\n\u003cp\u003eFrom these exploratory searches it also became obvious that although chemical variation influences the mean values, these may overlap due to the larger standard deviations.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.1 Within the thiophosphinic-related P-function\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFirst of all, attention is paid primarily to some dimensions of the P-function including single- double- and delocalized bonds in the title molecules. Thus, covalent bond distances are in the fore in this section. There are, of course, various ways to analyze bonding relations from sophisticated \u003cem\u003eab initio\u003c/em\u003e approaches to simpler ones, like the Bond\u0026ndash;Valence method [11\u0026ndash;15].\u003c/p\u003e\n\u003cp\u003eA rough estimate of delocalization may be guessed on the basis of the stretching of a given double bond \u003cem\u003evs.\u003c/em\u003e the respective double bond estimate. It is also possible to compare the shortening of a single bond with the mean of single bond lengths. Table 1 contains results of a preliminary series of searches of the thiophosphinic acid-related fragments shown in the first column. Details for the searches may be found in the Supplementary Information (SI 1\u0026ndash;6).\u003c/p\u003e\n\u003cp\u003eIn the first approach, let us compare the trends in the lengths of the P=S and P-O bonds if the series of \u0026gt;P(S)OC, \u0026gt;P(S)OH and \u0026gt;P(S)O\u003cstrong\u003e\u003csup\u003e-\u003c/sup\u003e\u003c/strong\u003e is compared. Table 1 contains mean bond lengths as derived from the CSD X-ray database [10]. One can see that in the above order, the P=S bond is elongated, while the P-O bond is shortened. Hence, there must be a significant delocalization for the \u0026gt;P((S)O\u003csup\u003e\u0026ndash;\u003c/sup\u003e anion.\u003c/p\u003e\n\u003cp\u003eTo quantify the extent of delocalization, let us consider that the mean P=S length is 1.98 \u0026Aring; that is between the 1.94 (P=S) and 2.08 (P-S) extremes (Table 2). The delocalization may be estimated on the basis of the 0.04 \u0026Aring; P=S lengthening and the 0.14 \u0026Aring; gap as (1.98-1.94)/(2.08-1.94) = 0.04/0.14 = 29%. Looking from the P-S side, the numerator becomes 0.10 as (2.08-1.98) giving rise to a value of 0.10/0.14 = 71%. Approaching from the P-O side, considering a 0.09 \u0026Aring; change by the decrease of P\u0026ndash;O from 1.61 to 1.52 and a gap of 0.14 \u0026Aring; between the P\u0026ndash;O and P=O bonds, the delocalization may be taken as (1.61-1.52)/(1.61-1.47) = 0.09/0.14 = 64%. Regarding the P=O, a 0.05 \u0026Aring; (1.52-1.47) stretch suggests a delocalization of 0.05/0.14 = 36%. These percentages may perhaps indicate that delocalization cannot be neglected if ionic forms occur.\u003c/p\u003e\n\u003cp\u003eHowever, as it turns out (\u003cem\u003evide infra\u003c/em\u003e) the delocalization within the C,CP(X)YC function may intimately be coupled with the presence of intermolecular interactions as well.\u0026nbsp;Two such outstanding cases are CIRQAY [16] and SORKOB [17] (named as their CSD reference codes). CIRQAY is an example of a thiosphosphinic acid and a thiophosphinate anion asymmetric dimer type association, while SORKOB comprises a phosphinic acid \u0026ndash; phosphinate analogue with an O\u003cstrong\u003e\u003csup\u003e\u0026ndash;\u003c/sup\u003e\u003c/strong\u003e... H -- O intermolecular contact type.\u0026nbsp;Looking at the pairwise P:::O and P:::S distances in CIRQAY (Fig. 3), one may see only marginal P:::O difference (1.543(2) vs. 1.531(2) \u0026Aring;), while the P:::S distances are practically equal (1.96 \u0026Aring;, Fig. 3/(B)). This is in sharp contrast with the theoretical model of the dimer associate, where P-O bonds do show differences, and even the P-S bonds act in a similar way (Fig. 3/(A)). The phosphinic acid \u0026ndash; phosphinate analogue in SORKOB appears to have the same symptoms. The P:::O distances (1.532 \u0026Aring; vs. 1.525 \u0026Aring;) of the anionic O\u0026nbsp;‑‑\u0026nbsp;H\u0026nbsp;...\u0026nbsp;O\u003cstrong\u003e\u003csup\u003e\u0026ndash;\u003c/sup\u003e\u003c/strong\u003e connection are only marginally differing, and the situation is similar for the other two \u0026ldquo;P=O\u0026rdquo; bonds (1.499 \u0026Aring; and 1.505 \u0026Aring;). One may suspect that these indications stem from the associative nature of these crystal structures. Thus, an interlude of a detour in this region is needed. In other words, we stumble into a situation, where \u0026quot;normal\u0026quot; (i.e. covalent) organic chemistry and supramolecular chemistry may no longer be fully separated.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.2 Regarding supramolecular relations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe CCP(S)O\u003cstrong\u003e\u003csup\u003e\u0026ndash;\u003c/sup\u003e\u003c/strong\u003e anion-fragment search gave only a rather limited result yielding 9 hits (see Suppl. Info Fig. SI-6). Examples for delocalization in thiophosphinate - thiophosphinic acid systems in the solid state are indeed rare. For this, we decided to perform a combined search, where both O=P-O\u003cstrong\u003e\u003csup\u003e\u0026ndash;\u003c/sup\u003e\u003c/strong\u003e and S=P-O\u003cstrong\u003e\u003csup\u003e\u0026ndash;\u003c/sup\u003e\u003c/strong\u003e anions are involved, even metal-organic crystals were also allowed as search targets. Eventually, there were 201 such molecules (see Suppl. Info Fig. SI-7). Then, a search was made for contacts of CCP(X)O\u003csup\u003e\u0026ndash;\u003c/sup\u003e anions (X=O,S) to HO-NM fragments, where \u0026quot;NM\u0026quot; may mean any Non-Metal element. There were 95 such crystal structures (see Suppl. Info Fig. SI-8), which contained 158 replicas satisfying the shorter than vdW radii sum criteria. Fig. 4 shows the scattergram of these contacts while Table 3 shows the essential statistical description characteristics of this data set. It is noted that the shortest O\u003cstrong\u003e\u003csup\u003e\u0026ndash;\u003c/sup\u003e\u003c/strong\u003e ... O bridgehead distances being less than 2.5 \u0026Aring; (shown in red dots) were found in the crystal structures of CIRQAY, SORKOB,and CUMBOB, FASBUY, IZOXOM, NUWMUN, QIFBAL, VEZHEN, VEZKEQ, WAWVID, WIGGID and WIJHIK [16,17,20-28] (named as their CSD reference codes). These 12 cases also provide the 11 shortest O\u003cstrong\u003e\u003csup\u003e\u0026ndash;\u003c/sup\u003e\u003c/strong\u003e ... H contacts. Those 12 crystals distances stem from strongly interacting species.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 3.\u003c/strong\u003e Mean contact distances (\u0026Aring;) with standard deviations for 158 \u003cstrong\u003e\u003cem\u003eO\u003csub\u003eN\u003c/sub\u003e\u003c/em\u003e\u003c/strong\u003e counts. (For search results details and the fragment used look up Suppl. Info SI-8). DIST5 and DIST6 refers to O\u003cstrong\u003e\u003csup\u003e\u0026ndash;\u003c/sup\u003e\u003c/strong\u003e ... O bridgehead and to O\u003cstrong\u003e\u003csup\u003e\u0026ndash;\u003c/sup\u003e\u003c/strong\u003e ... H distances, respectively.\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"605\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd rowspan=\"2\" valign=\"top\" style=\"width: 77px;\"\u003e\n \u003cp\u003eName\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"2\" valign=\"top\" style=\"width: 94px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eMean (\u003c/strong\u003e\u003cstrong\u003e\u0026Aring;)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"2\" valign=\"top\" style=\"width: 90px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eStd. Dev\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"2\" valign=\"top\" style=\"width: 76px;\"\u003e\n \u003cp\u003eMin(\u0026Aring;)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"2\" valign=\"top\" style=\"width: 76px;\"\u003e\n \u003cp\u003eMax(\u0026nbsp;\u0026Aring;)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" valign=\"top\" style=\"width: 192px;\"\u003e\n \u003cp\u003e\u0026nbsp;Outliers\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003eLow\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003eHigh\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003eTotal\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 77px;\"\u003e\n \u003cp\u003eDIST5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 94px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e2.73\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 90px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e0.13\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 76px;\"\u003e\n \u003cp\u003e2.40\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 76px;\"\u003e\n \u003cp\u003e2.97\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003e0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003e0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003e0\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 77px;\"\u003e\n \u003cp\u003eDIST6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 94px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.89\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 90px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e0.18\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 76px;\"\u003e\n \u003cp\u003e1.25*\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 76px;\"\u003e\n \u003cp\u003e2.35\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003e2*\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003e0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e*These outliers are from an anion ... oxonium cation H-bond (1.25 \u0026Aring;) in NUWMUN and from a perfluoroacetato-guest in WIJHIK (1.39 \u0026Aring;).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThere are a few interesting features in this scattergram. Firstly that these 12 extremely short distance values belong to 4 perfluorinated compounds with direct anion ... cation contacts to oxonium (VEZKEQ, NUWMUN, VEZHEN) and to perfluoro-acetic acid cations (WIJHIK), while WAWVID has a pyridinium counter-ion. The other 7 structures (CIRQAY, CUMBOB, FASBUY, IZOXOM, SORKOB, QIFBAL, WIGGID) exhibit the 4 thiophosphinate ... thiophosphinic, or the 3 phosphinate ... phosphinic acid with \u003cstrong\u003e\u003cem\u003eD\u003c/em\u003e\u003c/strong\u003e finite set contact type [18,19]. Regardless, whether these associative phenomena stem from pluri-molecular (hetero-molecular) or from multi-molecular (homo-molecular, self-associative) nature, all these short contacts exert influence on the covalent bonding in the vicinity of the P-atom.\u003c/p\u003e\n\u003cp\u003eThe linear regression model shown in Fig. 4 is characterized by the\u0026nbsp;\u003cbr\u003eDIST6 = 1.19* DIST5 - 1.37 equation with an \u003cem\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/em\u003e = 0.723 giving an \u003cem\u003eR\u003c/em\u003e = 0.850 correlation coefficient. Further inspection revealed that the acceptor O\u003cstrong\u003e\u003csup\u003e\u0026ndash;\u003c/sup\u003e\u003c/strong\u003e.... H (DIST6) \u003cem\u003evs.\u003c/em\u003e the acceptor ... donor (O\u003cstrong\u003e\u003csup\u003e\u0026ndash;\u003c/sup\u003e\u003c/strong\u003e ... O, DIST5)\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003edistances are approaching \u003cem\u003efair\u003c/em\u003e correlation with an \u003cstrong\u003e\u003cem\u003eR\u003c/em\u003e\u003c/strong\u003e = 0.886, when one excludes metal-organic structures (see Suppl. Info Fig. SI-9). (In that case, the number of hits, as well as the fragment count numbers were reduced to somewhat less than the 2/3 of the counts presented above.) Considering that this search did not employ strong filtering, this correlation seems acceptable. In plain words, this correlation means nothing more than that these H-bonds drift towards linearity quite well. It is also clear that where the O ... O bridgehead distances are very short (\u0026lt; 2.5\u0026Aring;), the proton position may be expected closer to the midway. A notable feature of this diagram points to the unexpected long O\u003cstrong\u003e\u003csup\u003e\u0026ndash;.\u003c/sup\u003e\u003c/strong\u003e.. H distance originating from the CIRQAY X-ray model, too. This may stem from an elongated O -- H covalent bond, or possibly from fast switching between the two anchor atoms or maybe from the disorder \u0026ndash; like averaging of two opposing O -- H positions.\u003c/p\u003e\n\u003cp\u003eAs far as the thiophosphinate - thiophosphinic acid ensemble is concerned, the message from these data seems to be clear. It says that thiophosphinic acid salts are prone to make very strong electrostatic contacts. A few of these compounds may represent strong association ability, thus providing a bridge between molecular (covalent) and supramolecular chemistry. This also means that understanding covalent features, such as delocalization, one may have to consider effects transgressing molecular boundaries as well.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.3 Comparison of the geometry of the thiophosphinic and phosphinic scaffolds\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe obvious misplace of the H atom in the thiophosphinate - thiophosphinic acid H-bridge in the CIRQAY crystal structure and the unexpected bonding around the P-atom [16] requires further inspections. The parent diphenylthiosphospinic acid crystal structure was solved by Mattes \u0026amp; R\u0026uuml;hl in 1984 [29]. The related paper was not only an excellent demonstration of an experimental technique with competent results and with alike analysis, but it was also attesting relevant observations linking traditional organic chemistry (\u0026quot;covalent chemistry\u0026quot;) with the supramolecular chemistry. For host-guest chemistry audience, the statement \u0026quot;Molecular-weight measurements indicate that Ph\u003csub\u003e2\u003c/sub\u003eP(S)OH is also highly associated in solutions\u0026quot; may have escaped attention. It meant that a molecular association mostly attributed to crystalline state may be prevalent in such compounds even in solutions. This is in congruence with the picture we see emerging from Fig. 4, namely that the strongest interactions stem from phosphinic acid derivatives. Remarkably, thiophosphinic derivatives account more that the half of these shortest contacts. A recent publication mentions\u0026nbsp;Ph\u003csub\u003e2\u003c/sub\u003eP(S)OH as an intermittent product (CEJSIT01, [30]). Distances around the P-function were compared with those from theory (Table 4). This table compares basic dimensions of the neutral Ph\u003csub\u003e2\u003c/sub\u003eP(S)OH acid with those from the two examples selected for scrutiny (CIRQAY [16] and SORKOB [17] plus an additional one (PAHSOG [31]). This latter was chosen to exemplify another experimental issue of the failure of the charged entity assignment. The two independent experimental Ph\u003csub\u003e2\u003c/sub\u003eP(S)OH structures show essentially the same geometry. Additionally, three dipehenylthiophosphinate aninon structures were associated with the neutral forms. This table confirms that the experimental and theoretical data align in shorter-longer tendencies well. It may also be seen that the direct anion - acid contacts alter bonding dimensions as far as these lengths go. It is notable that these alterations may be observed primarily in the bond length of the P-O bond. It is definitely shorter both in the thiophosphonic and in the phosphonic acid examples, as those predicted by calculations. It may be also a sort of accumulated effects from the grand-average nature of the crystal structure, as well as its environment. Nevertheless, this shortening seems to be significant, showing effects of the strong association power of these kinds of molecules, as also attested in their recurrence amongst the shortest interactions in Fig. 4. \u0026nbsp;CIRQAY and SORKOB are also interesting as the misposition of the H atom in CIRQAY is attenuated by the SORKOB structure. Fig. 5 shows an excerpt from the latter. In SORKOB the acid P\u0026ndash;O length calculated by theory is 1.626 \u0026Aring;. It falls in the 1.61(2) \u0026Aring; statistical mean value (c.f. Table 1 and Suppl. Info Fig. SI-10). Thus, the theory P‑O distance is a typical P-O single bond length and does not show the shortening to 1.53 \u0026Aring; characteristic for the crystalline state model (c.f. Fig. 5). Moreover, the acid -OH bond distance is 0.964 \u0026Aring; in the calculated model while this size is 1.11 \u0026Aring; (1.107 \u0026Aring;) in the SORKOB X-ray model. This latter is close to the theory- calculated 1.05 \u0026Aring; of the CIRQAY. All in all this means that the single molecule approach in the calculations cannot account for the anionic acceptor role.\u003c/p\u003e\n\u003cp\u003eCrystal structure of the multi-functional SORKOB molecule supports that the thiophosphonate anion ... thiophosphonic acid contact is a decisive crystal engineering factor [32,33]. Indeed, a glance at the interaction energies as calculated by the CrystalExplorer program [34] using a HF-approach [35] shows that the cohesive forces along the {0 0 1} direction are an order of magnitude greater (at 161 kJ/mol) than all other interactions (c.f. Suppl. Info\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eTable SI-1, 2nd row). The almost extended SORKOB molecule (with two knicks at the P-atoms of its backbone) defines the crystallographic \u003cem\u003ec\u003c/em\u003e-axis \u003cem\u003evia\u003c/em\u003e the \u003cem\u003ez-1\u003c/em\u003e and \u003cem\u003ez+1\u003c/em\u003e translations (c.f. Suppl. Info\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eFig. SI-11). Multi-functional armory of the SORKOB molecule is also attested by the H-bridges graphs. The dominance and the importance of the anionic-neutral H-bridge connection is emphasized even in this environment.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 4.\u003c/strong\u003e P-function bond lengths in experiment-theory comparison for neutral Ph\u003csub\u003e2\u003c/sub\u003e-, S- or O-phosphonic acid molecules involved in H-bridges (CEJSIT and CEJSIT01) and with those involved in anionic - neutral acid crystalline associations. DIST1 is the P-OH bond, DIST2 is P=S (or P=O), DIST3 and DIST4 are P-C lengths, while DIST5 values are bridgehead atom distances (those from the -OH donor to the S (or O\u003csup\u003e\u0026ndash;\u003c/sup\u003e) acceptor or from HO to O\u003csup\u003e\u0026ndash;\u003c/sup\u003e donator ... acceptor atoms).\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"605\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 165px;\"\u003e\n \u003cp\u003eNAME\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003eDIST1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003eDIST2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003eDIST3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003eDIST4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003eDIST5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 165px;\"\u003e\n \u003cp\u003eCEJSIT/CEJSIT01*\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.583\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.955\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.803\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.801\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e3.139\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 165px;\"\u003e\n \u003cp\u003eCIRQAY-OH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.543\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.957\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.817\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.815\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e2.434\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 165px;\"\u003e\n \u003cp\u003eTheory-OH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.572\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.975\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.837\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.837\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e2.485\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 165px;\"\u003e\n \u003cp\u003eCIRQAY-O\u003cstrong\u003e-\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.531\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.960\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.821\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.821\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e2.434\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 165px;\"\u003e\n \u003cp\u003eTheory-O\u003cstrong\u003e-\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.528\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.998\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.848\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.848\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e2.485\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 165px;\"\u003e\n \u003cp\u003eSORKOB-OH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.532\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.499\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.794\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.828\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e2.419\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 165px;\"\u003e\n \u003cp\u003eTheory-OH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.626\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.485\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.813\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.823\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e**\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 165px;\"\u003e\n \u003cp\u003eSORKOB-O\u003cstrong\u003e-\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.525\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.505\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.795\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.832\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e2.419\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 165px;\"\u003e\n \u003cp\u003eTheory-O\u003cstrong\u003e-\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.515\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.509\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.815\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.837\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e**\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 165px;\"\u003e\n \u003cp\u003ePAHSOG***\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.508\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.933\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.804\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.825\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e2.338\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 165px;\"\u003e\n \u003cp\u003ePAHSOG***\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.505\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.934\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.789\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e1.801\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 88px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e* Mean distances averaged from Mattes \u0026amp; R\u0026uuml;hl\u0026nbsp;[29] and Shao et al., [30].\u003c/p\u003e\n\u003cp\u003e** N/A.\u003c/p\u003e\n\u003cp\u003e***\u0026nbsp;In the case of PAHSOG it was not possible to make clear assignment of neither the anionic Ph\u003csub\u003e2\u003c/sub\u003eP(S)O\u003csup\u003e-\u003c/sup\u003e, nor the acid form, yet another proof of association-related supramolecular effects.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2. A study of the geometry around the P- atom in Ph\u003csub\u003e2\u003c/sub\u003eP(O)OMe, Ph\u003csub\u003e2\u003c/sub\u003eP(O)SMe, Ph\u003csub\u003e2\u003c/sub\u003eP(S)OMe and Ph\u003csub\u003e2\u003c/sub\u003eP(S)SMe \u0026ndash; theoretical calculations vs. X-ray data base analysis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe computed gas-phase geometries of the four phosphorous compounds \u0026ndash; Ph₂P(O)OMe, Ph₂P(O)SMe, Ph₂P(S)OMe, and Ph₂P(S)SMe \u0026ndash; obtained at the B3LYP/6-311++G(2d,2p) level, show good agreement with the X-ray data obtained for similar scaffolds of crystalline compounds after a search in the data base (Table 5). The P=X (X = O, S) and P\u0026ndash;Y (Y = OMe, SMe) bond lengths differ from the experimental mean values by only 0.01\u0026ndash;0.05 \u0026Aring;, which is within the expected standard error range. Obviously, the absence of solid-state packing and/or solvent effects may also have affected the calculations. As such effects regularly appear in the X-ray diffraction experimental results, their effects may be estimated by comparing with the geometry of the molecule calculated \u003cem\u003ein vacuo\u003c/em\u003e. Instances, where such effects may become visible are just the subject of this paper. Similarly, the P\u0026ndash;C bond distances show deviations of less than 0.02 \u0026Aring;. The key bond angles, including C\u0026ndash;P=X and C\u0026ndash;P\u0026ndash;Y, exhibit differences of only 1\u0026ndash;3\u0026deg;, which may be attributed to crystal packing or intermolecular interactions absent in the vacuum model. As expected, the compounds containing a P=S bond show consistently longer bond lengths than their P=O analogues, both computationally and experimentally. Overall, the theoretical model accurately reproduces the molecular geometries, confirming the reliability of the chosen method for further mechanistic or electronic structure investigations.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 5\u003c/strong\u003e Selected bond lengths (in \u0026Aring;) and bond angles (\u0026deg;) for Ph2P(X)YMe esters computed at B3LYP/6-311++G(2d,2p) level of theory in vacuo. Mean bond distance (\u0026Aring;) and angle values (\u0026deg;) of the respective four different search fragments with their standard deviations (in parentheses) as derived from the search hits.\u0026nbsp;\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"633\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 78px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 68px;\"\u003e\n \u003cp\u003ePh\u003csub\u003e2\u003c/sub\u003eP(O)OMe\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u0026nbsp;C\u003csub\u003e2\u003c/sub\u003eP(O)O-C*\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003ePh\u003csub\u003e2\u003c/sub\u003eP(O)SMe\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003eC\u003csub\u003e2\u003c/sub\u003eP(O)S-C\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003ePh\u003csub\u003e2\u003c/sub\u003eP(S)OMe\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003eC\u003csub\u003e2\u003c/sub\u003eP(S)O-C\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003ePh\u003csub\u003e2\u003c/sub\u003eP(S)SMe\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003eC\u003csub\u003e2\u003c/sub\u003e-P(S)S-C\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 78px;\"\u003e\n \u003cp\u003eP=X\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 68px;\"\u003e\n \u003cp\u003e1.4814\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.47(1)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e1.4880\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.48(1)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e1.9557\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.94(2)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e1.9664\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.94(1)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 78px;\"\u003e\n \u003cp\u003eP\u0026ndash;Y\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 68px;\"\u003e\n \u003cp\u003e1.6168\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.59(2)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e2.1331\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e2.08(2)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e1.6263\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.61(2)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e2.1466\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e2.10(2)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 78px;\"\u003e\n \u003cp\u003eP\u0026ndash;C1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 68px;\"\u003e\n \u003cp\u003e1.8097\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.80(3)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e1.8205\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.83(3)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e1.8203\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.80(2)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e1.8320\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.81(2)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 78px;\"\u003e\n \u003cp\u003eP\u0026ndash;C2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 68px;\"\u003e\n \u003cp\u003e1.8202\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.80(3)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e1.8262\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.80(1)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e1.8308\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.80(2)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e1.8377\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.81(3)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 78px;\"\u003e\n \u003cp\u003eC1\u0026ndash;P=X\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 68px;\"\u003e\n \u003cp\u003e114.34\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e114(2)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e113.23\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e112(3)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e115.47\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e115(4)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e115.04\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e115(3)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 78px;\"\u003e\n \u003cp\u003eC1\u0026ndash;P\u0026ndash;Y\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 68px;\"\u003e\n \u003cp\u003e100.07\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e103(3)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e101.07\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e102(4)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e99.09\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e102(4)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e99.09\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e101(7)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 78px;\"\u003e\n \u003cp\u003eC2\u0026ndash;P=X\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 68px;\"\u003e\n \u003cp\u003e111.18\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e113(2)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e111.47\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e112(1)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e113.63\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e115(3)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e113.08\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e115(4)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 78px;\"\u003e\n \u003cp\u003eC2\u0026ndash;P\u0026ndash;Y\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 68px;\"\u003e\n \u003cp\u003e104.77\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e103(3)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e107.11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e105(3)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e103.62\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e104(3)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e106.84\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e102(6)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 78px;\"\u003e\n \u003cp\u003eC1\u0026ndash;P\u0026ndash;C2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 68px;\"\u003e\n \u003cp\u003e108.78\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e108(5)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e108.43\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e109(2)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e106.64\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e104(7)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e106.48\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e105(5)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 78px;\"\u003e\n \u003cp\u003eX=P\u0026ndash;Y\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 68px;\"\u003e\n \u003cp\u003e116.72\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e115(2)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e114.82\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e115(2)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e116.69\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e115(2)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e115.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e116(3)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe relatively high standard deviations of the X-ray database statistics stem from the wide range of chemistry these searches embrace. It is also obvious that many of these crystals may also contain solvent or other molecules. Apart from molecular diversity the crystalline environment variation also pose various attractive and repulsive interactions. It is also apparent that these values conform to the theoretical values within even single s.d. values.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3. The tautomerization of thiophosphinic acids\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe tautomerization of thiophosphinic or\u0026nbsp;\u0026ldquo;thionic acid\u0026rdquo; \u0026gt;P(S)OH (\u003cstrong\u003eA\u003c/strong\u003e) to \u0026ldquo;thiolic acid\u0026rdquo; \u0026gt;P(O)SH (\u003cstrong\u003eB\u003c/strong\u003e) \u0026ndash; typically involving equilibrium \u0026ndash; is a fundamental proton-transfer process that plays a crucial role in the reactivity and stability of these compounds. This isomerization influences the acidity, coordination behavior and biological activity of thiophosphinic derivatives, making them relevant in the synthetic and the related medicinal chemistry areas. Comprehension of the tautomeric preferences may promote the rational design of ligands for metal complexes, as well as the development of thiophosphinic-related biologically active agents. The position of the equilibrium and height of the activation barrier for the proton shift are dependent on the acid-base catalyst, solvent effects and nature of the substituents. Quantum chemical studies were planned to evaluate the tautomeric equilibrium involving the mechanism of the isomerisation. The tautomerism under discussion is a suitable model to study intra or intermolecular proton transfers.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.1.\u0026nbsp;The thermodynamic stability of thiophosphinic acid isomers (tautomers): \u0026gt;P(S)OH vs. \u0026gt;P(O)SH\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTheoretical calculations performed at the B3LYP/6-311++G(2d,2p) level of theory confirmed earlier literature findings by demonstrating that the \u0026gt;P(S)OH (thione-type) tautomer is significantly more stable than the \u0026gt;P(O)SH (thiol-type) form. This preference is valid for the series of both dialkyl- and diaryl-substituted thiophosphinic acids. For the different substituent combinations shown in Scheme 4, the calculated enthalpy change (\u0026Delta;H) fell in the range of +13.7 to +17.5 kJ mol\u003csup\u003e\u0026ndash;1\u003c/sup\u003e, indicating that the formation of the thiol form is endothermic, and hence thermodynamically unfavorable. It is clear that the \u0026gt;P(S)OH tautomer (the thione form) is the dominating component of the equilibrium. The value of \u0026Delta;H slightly depends on the electronic and steric properties of the substituents (Scheme 4).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.2.\u0026nbsp;Kinetics for the proton transfer isomerization (tautomerization)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn this section, the tautomerization of dimethylthiophosphinic acid (Me₂P(S)OH) was studied via an intramolecular proton transfer from the oxygen to the sulfur atom (Scheme 5) in a single elementary step. The reaction may proceed through a strained four-membered transition state. This monomolecular isomerization pathway is hindered kinetically, as suggested by a relatively high enthalpy of activation (\u0026Delta;H\u003csup\u003e#\u003c/sup\u003e) of 114.6 kJ mol⁻\u0026sup1;. Such a high barrier means that, under ambient conditions, in the absence of suitable catalysts or solvent assistance the tautomerization is negligible. The high \u0026Delta;H\u003csup\u003e#\u003c/sup\u003e value may be attributed to the strong hydrogen bond present and the geometric reorganization needed during the proton shift between the heteroatoms. The absence of an external proton shuttle or a solvent-network mediating the taking place in the gas phase limits the feasibility of a lower-energy mechanism. The kinetic barrier of a tautomerism determines the stability of thiophosphinic acid and its reactivity. In practical applications, such as ligand design, the kinetic stability of one tautomer over the other may significantly influence the binding modes, and hence the metal selectivity.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.3.\u0026nbsp;Isomerization in the presence of water as the base\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTheoretical modeling of the isomerisation by proton transfer in the presence of water involves a bimolecular mechanism, in which the water molecule abstracts the proton from Me₂P(S)OH to generate a thiophosphinate anion and a H\u003csub\u003e3\u003c/sub\u003eO\u003csup\u003e+\u003c/sup\u003e cation. Subsequently, the sulfur site of the ambident anion is protonated yielding the less stable \u0026gt;P(O)SH tautomer, as illustrated in Scheme 6. Despite the role of water as a proton shuttle, the overall transformation remains thermodynamically unfavorable due to the endothermic nature of this process characterized by an enthalpy change of 49.1 kJ mol⁻\u0026sup1;. In all, although water may assist the proton transfer, the process in whole is unfavorable.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.4.\u0026nbsp;Isomerization in the presence of water as the acid\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAn alternative mechanistic scenario may involve protonation of Me₂P(S)OH by a water molecule, leading to the formation of the protonated thiophosphinic acid, as outlined in Scheme 7. However, this transformation is highly unfavorable thermodynamically, as the calculated enthalpy change is 134.4 kJ mol⁻\u0026sup1;. Such a high degree of endothermicity excludes this pathway under standard conditions. The result is the consequence of the low acidity of water relative to Me₂P(S)OH, and confirms that water cannot act as a proton donor in this process. Therefore, this mechanism should be ruled.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.5.\u0026nbsp;Isomerization via autoprotolysis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAnother theoretically possible pathway for the tautomerization of Me₂P(S)OH involves autoprotolysis, in which a thiophosphinic acid molecule protonates another one forming a conjugate acid\u0026ndash;base pair as shown in Scheme 8. This bimolecular mechanism could, in principle, allow for proton transfer without the involvement of an external acid or base. However, the calculated enthalpy change for this process is 71.5 kJ mol\u003csup\u003e\u0026ndash;1\u003c/sup\u003e indicating that the transformation is rather endothermic, and thus thermodynamically unfavorable under ambient conditions. Hence, the autoprotolysis-related isomerization should be regarded negligible. This finding support the assumption that intramolecular or solvent-assisted pathways may be more plausible for the tautomerization of thiophosphinic acids than autoprotolysis.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.6.\u0026nbsp;Isomerization with the help of MeOH\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAs an alternative to water-mediated proton transfer, methanol may also act as a proton donor to promote the tautomerization of Me₂P(S)OH (Scheme 9). In this case, the proton transfer from \u003cstrong\u003ecomplex 1\u003c/strong\u003e proceeds with a low activation barrier of 40.2 kJ mol\u003csup\u003e\u0026ndash;1\u003c/sup\u003e indicating that this pathway is kinetically accessible under mild conditions. However, the overall transformation is endothermic, with an associated enthalpy change of 25.5 kJ mol\u003csup\u003e\u0026ndash;1\u003c/sup\u003e. One may conclude that methanol, due to its greater acidity, is more suitable to facilitate the proton transfer than water, and methanol has the ability to stabilize the transition-state structure via hydrogen bonding. Nevertheless, the unfavorable thermodynamics prevents the equilibrium to be shifted to \u003cstrong\u003ecomplex 2\u003c/strong\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.7.\u0026nbsp;Isomerization via a symmetric dimer\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA further mechanistic possibility involves isomerization starting from a symmetric hydrogen-bonded dimer of Me₂P(S)OH (\u003cstrong\u003edimer A\u003c/strong\u003e), as depicted in Scheme 10. In this scenario, one monomer acts as a proton donor, while the other accepts the proton enabling the proton transfer within the dimer without the involvement of an external species to afford \u003cstrong\u003edimer B\u003c/strong\u003e. Quantum chemical calculations indicate that the process is endothermic, with an enthalpy change of 46.7 kJ mol\u003csup\u003e\u0026ndash;1\u003c/sup\u003e, and proceeds via a transition state characterized by an activation barrier of 63.2 kJ mol\u003csup\u003e\u0026ndash;1\u003c/sup\u003e. Although this pathway is energetically more favorable than protonation by water or autoprotolysis, if the corresponding enthalpy changes (134.4 kJ mol\u003csup\u003e\u0026ndash;1\u003c/sup\u003e and 71.5 kJ mol\u003csup\u003e\u0026ndash;1\u003c/sup\u003e, respectively) are regarded, it represents a moderately hindered process. The results suggest that the dimer-assisted tautomerization may occur under appropriate conditions, but it is unlikely to be the dominant route at room temperature.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.8.\u0026nbsp;Isomerization via an asymmetric dimer\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAn alternative and more favorable pathway involves isomerization through an asymmetric hydrogen-bonded dimer of Me₂P(S)OH (\u003cstrong\u003edimer C\u003c/strong\u003e), as shown in Scheme 11. In this mechanism, one Me₂P(S)OH unit functions as a proton donor, while the other acts as a proton acceptor, leading eventually to the formation of an Me₂P(O)SH tautomer (as seen in \u003cstrong\u003edimer E\u003c/strong\u003e) from only one of the two starting molecules. This concerted, intermolecular proton shift proceeds through a well-defined transition state (\u003cstrong\u003eTS4\u003c/strong\u003e), and is characterized by a low activation barrier of 37.0 kJ mol\u003csup\u003e\u0026ndash;1\u003c/sup\u003e. Moreover, the overall transformation is slightly exothermic indicating that it is both kinetically and thermodynamically feasible under mild conditions. These results suggest that the asymmetric dimer pathway may represent a realistic route for tautomerization in condensed-phase environments, where such hydrogen-bonding interactions may occur and stabilize the transition structure.\u003c/p\u003e"},{"header":"Experimental","content":"\u003cp\u003e\u003cstrong\u003eTheoretical calculations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll computations were carried out with the Gaussian16 program package (G16C1) [36], using standard convergence criteria for the gradients of the root mean square (RMS) Force, Maximum Force, RMS displacement and maximum displacement vectors (3.0 \u0026times; 10\u003csup\u003e\u0026ndash;4\u003c/sup\u003e, 4.5 \u0026times; 10\u003csup\u003e\u0026ndash;4\u003c/sup\u003e, 1.2 \u0026times; 10\u003csup\u003e\u0026ndash;3\u003c/sup\u003e and 1.8 \u0026times; 10\u003csup\u003e\u0026ndash;3\u003c/sup\u003e). Computations were carried out at B3LYP level of theory [37], with the basis set of 6-311++G(2d,2p). The vibrational frequencies were computed at the same levels of theory. In some cases the IEFPCM method was also applied to model the solvent effect, by using the default settings of G16 [PCM(water)], \u0026epsilon; = 78 (for water) [38]. Thermodynamic functions U, H, G and S were computed at 298.15 K. See the Supplementary Information for details.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCSD data analysis\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIt was performed initially on the database CSD version 5.44 (updates Sep. 2023).consequentially repeated on the CSD version 5.46 Updates (Feb 2025) and 6.00 (April 2025) using ConQuest [39] and Mercury [40] for producing some molecular drawings and scatterplots as well as statistics. Search criteria were only adjusted where large enough sample number permitted this, for details consult Supplementary. Otherwise only general restriction was the omission of powder structures.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eSupplementary Information\u003c/strong\u003e The online version contains supplementary material belonging to the searches in the Cambridge Structure Database and to the theoretical calculations available at https://doi.org/\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eMC thanks Dr. P. Bombicz (HUN-REN Res. Cent. Nat. Sci.) for her support in the CSD use. ZM is grateful for the possibility of using HUN-REN Cloud[REFX] within the \u0026ldquo;Szerves-Biochem\u0026rdquo; project, which helped to achieve the results described in this paper. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAll authors contributed to the study conception and design. GK initiated and supervised the project. CSD analyses and critical appraisal were performed by MC. ZM performed the theoretical calculations. JD placed the project in literature context. The manuscript was written by MC and GK incorporating the remarks of the other authors. All authors read and approved the final manuscript.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis project was supported by the National Research, Development and Innovation Office (NKKP-ADVANCED 149447). \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability:\u003c/strong\u003e All data can be found in the article and the Supplementary information.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical approval:\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests:\u003c/strong\u003e The authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eVassiliou S (2011) Thiophosphinic acids: Historic overview and recent advances in their synthesis and applications. Curr Org Chem 15:2469\u0026ndash;2480. https://doi.org/10.2174/138527211796150642\u003c/li\u003e\n\u003cli\u003eWang F, Polavarapu PL, Drabowicz J, Mikolajczyk M, Lyzwa P (2001) Absolute configurations, predominant conformations, and tautomeric structures of enantiomeric \u003cem\u003etert\u003c/em\u003e-butylphenylphosphinothioic acid. J Org Chem 66:9015\u0026ndash;9019. https://doi.org/10.1021/jo0107406\u003c/li\u003e\n\u003cli\u003ePokora-Sobczak P, Mielniczak G, Krasowska D, Chrzanowski J, Zajac A, Drabowicz J (2015) \u003cem\u003et\u003c/em\u003e-Butylphenyl-(1-naphthyl)phosphinothioic acids and their selenium analogs: synthesis of the racemic mixtures and attempts to isolate the enantiomers of \u003cem\u003et\u003c/em\u003e-butylphenyl-1-naphthylphosphinothioic acid. 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Russ J Gen Chem 91:1667\u0026ndash;1673. https://doi.org/10.1134/S1070363221090097\u003c/li\u003e\n\u003cli\u003eMurugavel R, PothirajaR, Gogoi N, Cl\u0026eacute;rac R, Lecren L, Butcher RJ, Nethaji M (2007) Synthesis, magnetic behaviour, and X-ray structures of dinuclear copper complexes with multiple bridges. Efficient and selective catalysts for polymerization of 2,6-dimethylphenol. \u003cstrong\u003eDalton Trans\u003c/strong\u003e 2405\u0026ndash;2410. https://doi.org/10.1039/b618559b\u003c/li\u003e\n\u003cli\u003eKoucky F, Kotek J, Cisarova I, Havlickova J, Kubicek V, Herman P (2023) Transition metal complexes of cyclam with two 2,2,2-trifluoroethylphosphinate pendant arms as probes for \u003csup\u003e19\u003c/sup\u003eF magnetic resonance imaging. Dalton Trans 52:12208\u0026ndash;12223. http://doi.org/10.1039/D3DT01420G\u003c/li\u003e\n\u003cli\u003eMattes R, R\u0026uuml;hl D (1984) Structure of diphenylthiophosphinic acid, (C\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e5\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eP(=S)OH, at 140 K. Acta Cryst C40:106\u0026ndash;108. https://doi.org/10.1107/S0108270184003188\u003c/li\u003e\n\u003cli\u003eShao CW, Wan PF, Xu Q, Yang Z-N, Geng M-Y, Zhang Y, Zhang X-H, Li X-W (2024) Phosphinothio(seleno)ation of alkynes/olefins and application on the late-stage functionalization of natural products. Commun Chem 7:290.https://doi.org/10.1038/s42004-024-01326-9\u003c/li\u003e\n\u003cli\u003ePilkington MJ, Slawin AMZ, Williams DJ, Woollins JD (1992) Facile chalcogenide elimination reactions: the crystal structures of [Pt{(Ph\u003csub\u003e2\u003c/sub\u003ePO)\u003csub\u003e2\u003c/sub\u003eH}{(Ph\u003csub\u003e2\u003c/sub\u003ePSeO)\u003csub\u003e2\u003c/sub\u003eH}]\u0026middot;1.25CHCl\u003csub\u003e3\u003c/sub\u003e and [Pt\u003csub\u003e3\u003c/sub\u003e(Ph\u003csub\u003e2\u003c/sub\u003ePCH\u003csub\u003e2\u003c/sub\u003eCH\u003csub\u003e2\u003c/sub\u003ePPh\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003e][(Ph\u003csub\u003e2\u003c/sub\u003ePSO)\u003csub\u003e2\u003c/sub\u003eH][OH]\u0026middot;0.5CH\u003csub\u003e2\u003c/sub\u003eCl\u003csub\u003e2\u003c/sub\u003e. \u003cstrong\u003eJ Chem Soc Dalton Trans\u003c/strong\u003e 2425\u0026ndash;2426.https://doi.org/10.1039/DT9920002425\u003c/li\u003e\n\u003cli\u003eDesiraju GR (1997). 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Materials Science Monographs 54, Amsterdam: Elsevier ISBN 0444874577, 9780444874573.\u003c/li\u003e\n\u003cli\u003eProgram CrystalExplorer V24.11, Build a03138e, built 2024-11-18-03-43: Spackman PR, Turner MJ, McKinnon JJ, Wolff SK, Grimwood DJ, Jayatilaka D, Spackman MA (2021) CrystalExplorer: a program for Hirshfeld surface analysis, visualization and quantitative analysis of molecular crystals. \u003cem\u003eJ Appl Cryst\u003c/em\u003e\u003cem\u003e \u003c/em\u003e54:1006\u0026ndash;1011. https://doi.org/10.1107/S1600576721002910\u003c/li\u003e\n\u003cli\u003eMackenzie CF, Spackman PR, Jayatilaka D, Spackman MA (2017) CrystalExplorer model energies and energy frameworks: extension to metal coordination compounds, organic salts, solvates and open-shell systems. 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III. The role of exact exchange. J Chem Phys 98:5648\u0026ndash;5652. https://doi.org/10.1063/1.464913\u003c/li\u003e\n\u003cli\u003eTomasi J, Mennucci B, Cammi R (2005) Quantum mechanical continuum solvation models. Chem Rev 105:2999\u0026ndash;3093. https://doi.org/10.1021/cr9904009\u003c/li\u003e\n\u003cli\u003eBruno IJ, Cole JC, Edgington PR, Kessler M, Macrae CF, McCabe P, Pearson J, Taylor R (2002) New software for searching the Cambridge Structural Database and visualising crystal structures. Acta Cryst B58:389\u0026ndash;397. https://doi.org/10.1107/S0108768102003324\u003c/li\u003e\n\u003cli\u003eMacrae CF, Sovago I, Cottrell SJ, Galek PTA, McCabe P, Pidcock E, Platings M, Shields GP, Stevens JS, Towler M, Wood PA (2020) Mercury 4.0: from visualization to analysis, design and prediction. J Appl Cryst \u003cstrong\u003e53\u003c/strong\u003e:226\u0026ndash;235. https://doi.org/10.1107/S1600576719014092\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTable 1 and 2 are available in the Supplementary Files section.\u003c/p\u003e"},{"header":"Scheme ","content":"\u003cp\u003eScheme 1 to 11 are available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"structural-chemistry","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"stuc","sideBox":"Learn more about [Structural Chemistry](https://www.springer.com/journal/11224)","snPcode":"11224","submissionUrl":"https://submission.nature.com/new-submission/11224/3","title":"Structural Chemistry","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"thiophosphinic acids, bond lengths, delocalization, supramolecular interactions, tautomerism","lastPublishedDoi":"10.21203/rs.3.rs-7212964/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7212964/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eModel series of thiophosphinic acid derivatives were scrutinized at the B3LYP/6-311\u0026thinsp;+\u0026thinsp;+\u0026thinsp;G(2d,2p) level of theory, and the resulting geometries were compared with experimental crystal structure data obtained from the Cambridge Structure Database (CSD). Typical bond length and bond angle values were selected for a series of similar fragments from CSD, and mean dimensions were also calculated from the experimental data accumulated. Linking the results of theoretical calculations and the evidences of data mining allowed the reconsideration of a few past experimental models with (thio)phosphinate - anion - (thio)phosphinic acid type associations. It seems also from this survey that thiophosphinic acid derivatives may be suitable vehicles for transgressing boundaries from a single molecule to supramolecular relations and \u003cem\u003evice versa\u003c/em\u003e. The study shows that a few intramolecular distances, such as the P-O bond lengths both in the anionic and in the neutral forms may significantly depend on intermolecular contacts as well. The tautomerism involving the thionic acid and thiolic acid forms was studied by theoretical methods at the B3LYP/6-311\u0026thinsp;+\u0026thinsp;+\u0026thinsp;G(2d,2p) level of theory assuming monomolecular and different bimolecular mechanisms. The shift toward the thionic actor was supported by the kinetic and thermodynamic energetics.\u003c/p\u003e","manuscriptTitle":"Delocalization and geometries in the P-function of thiophosphinic derivatives; Tautomerism and supramolecular interactions","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-31 11:20:57","doi":"10.21203/rs.3.rs-7212964/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-08-09T05:16:44+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-07-29T18:26:23+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"34281630323716092305036553804462543824","date":"2025-07-29T18:08:50+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-07-29T17:54:20+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-07-29T16:50:31+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-07-29T13:42:25+00:00","index":"","fulltext":""},{"type":"submitted","content":"Structural Chemistry","date":"2025-07-25T09:48:37+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"structural-chemistry","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"stuc","sideBox":"Learn more about [Structural Chemistry](https://www.springer.com/journal/11224)","snPcode":"11224","submissionUrl":"https://submission.nature.com/new-submission/11224/3","title":"Structural Chemistry","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"5e4c1a78-a3aa-4238-8380-b5ada1ee9304","owner":[],"postedDate":"July 31st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-03-30T16:16:51+00:00","versionOfRecord":{"articleIdentity":"rs-7212964","link":"https://doi.org/10.1007/s11224-025-02596-2","journal":{"identity":"structural-chemistry","isVorOnly":false,"title":"Structural Chemistry"},"publishedOn":"2026-03-24 16:09:00","publishedOnDateReadable":"March 24th, 2026"},"versionCreatedAt":"2025-07-31 11:20:57","video":"","vorDoi":"10.1007/s11224-025-02596-2","vorDoiUrl":"https://doi.org/10.1007/s11224-025-02596-2","workflowStages":[]},"version":"v1","identity":"rs-7212964","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7212964","identity":"rs-7212964","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}