New mechanistic insights into the gas-phase formation of methylzinc iodide via methyl iodide C-I activation with atomic zinc in excited triplet state as offered by the CCSD(T) study

New mechanistic insights into the gas-phase formation of methylzinc iodide via methyl iodide C-I activation with atomic zinc in excited triplet state as offered by the CCSD(T) study | 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 New mechanistic insights into the gas-phase formation of methylzinc iodide via methyl iodide C-I activation with atomic zinc in excited triplet state as offered by the CCSD(T) study Jerzy Moc This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6330328/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Activation of C-X bonds (X = halogen) by transition metal species is pivotal in many catalytic processes of industrial significance such as cross-coupling reactions. The relatively simple gas-phase system for studying the activation of the C-I bond (X = iodine), which was examined experimentally (J Phys Chem A 118:11204–11210) and is amenable to a high-level ab initio theoretical study, involves the reaction of methyl iodide (CH 3 I) with atomic zinc to form the methylzinc iodide monomer [IZnCH 3 (X 1 A 1 )]. Although a qualitative explanation of the underlying formation mechanism was given by the original authors, the pathways that play a role in generating the product molecule have not been well investigated in previous research. An important task here was to determine the ways by which the C-I bond of methyl iodide is activated by atomic zinc, and to suggest a plausible mechanism of formation of IZnCH 3 (X 1 A 1 ). This task was performed using the CCSD(T) method (the coupled cluster singles and doubles with perturbative triples procedure) along with the correlation-consistent basis sets (through aug-cc-pV-5Z) and relativistic pseudopotentials on the I and Zn atoms. C-I bond activation. methylzinc iodide monomer. CCSD(T). CH3I-Zn van der Waals complex. ZnI radical Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction Activation of C-X bonds (X = halogen) by transition metal (TM) compounds is of central importance for catalytic processes of industrial interest, such as, for instance, cross-coupling reactions [1]. A few examples of the C-I activation (X = iodine) at TM complexes or TM clusters documented in the literature that can be considered as representative of the experimental and theoretical research follow: the gas-phase reactions of iodobenzene with the mononuclear gold cations (R 3 P) n Au + (R = Me, Ph; n = 1,2) and ligated gold clusters [Au 3 L n ] + (L = Ph 2 P(CH 2 ) n PPh 2 ; n = 3–6) examined using mass spectrometry and density functional theory (DFT) [2]; the electrochemically induced activation of C-I bonds in diverse R-I substrates by the Pd 3 (dppm) 3 CO 2+ cluster as observed via cyclic voltammetry, coulometry and 31 P NMR spectroscopy in THF [3]; the C-I bond cleavage in iodobenzene, iodoethane and iodoethene over the Au n clusters (n = 3,4,14,20) and their cations investigated with DFT [4]. Over the past few decades, there has been an increased interest in studying the C-I bond activation and photodissociation of methyl iodide (CH 3 I) [5–10]. An inspiring example from this research activity is the reaction of methyl iodide vapour with zinc vapour in a DC discharge, reported by the Ziurys group [10], where the composition and molecular structure of the product were analyzed by means of rotational spectra. This analysis revealed the formation of the methylzinc iodide monomer, IZnCH 3 (X 1 A 1 ) [10]. Based on the experimental data, the authors proposed that the insertion of the activated zinc atom into the C-I bond of methyl iodide was responsible for the formation of the methylzinc iodide molecule; however, the exact mechanisms have not been fully elucidated. Inspired by the work of Ziurys and co-workers on a ″model system″ to investigate the C-I activation mentioned above, in this paper, we explore portions of the potential surface of this system which are relevant to the formation of methylzinc iodide by using the coupled-cluster CCSD(T) method (see next section for more details). The results of these calculations allow us to identify various modes of the atomic zinc mediated methyl iodide C-I activation, and present a plausible mechanism for the formation of IZnCH 3 (X 1 A 1 ). In addition, the availability of the gas-phase structure of the methylzinc iodide molecule that was derived from the rotational spectra [10], provide the opportunity for comparison with that obtained from the calculations. 2. Computational details Stationary points on the potential energy surface were located with the coupled cluster singles and doubles with perturbative triples method, CCSD(T) [11]. The open-shell systems were treated using the spin restricted CCSD(T) formalism based on an ROHF reference [12] (and denoted hereafter as RCCSD(T)). Two kinds of the basis set/pseudopotential (PP) combinations were utilized. First, the small-core energy-consistent relativistic PP was applied only for I with the corresponding aug-cc-pVnZ-PP basis sets [13] along with the all-electron aug-cc-pVnZ basis sets for the remaining atoms [14–15] - the resulting combination is designated simply as aug-cc-pVnZ, where n = T and Q. The nature of all stationary points was confirmed by harmonic vibrational frequency analysis using the aug-cc-pVTZ basis set. Next, to be able to estimate the scalar relativistic effects in Zn, the small-core energy-consistent relativistic PPs were employed for both the I and Zn atoms with the corresponding aug-cc-pVnZ-PP basis sets [13,16] in conjunction with the aug-cc-pVnZ basis sets for the C and H atoms [14] - the resulting combination is designated as aug-cc-pVnZ-PP, where n = Q and 5. Note that the PPs replace the inner-core I (1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 ) and Zn (1s 2 2s 2 2p 6 ) electrons. The quantum chemistry code MOLPRO2012.1 [17] was exploited in all CC calculations. 3. Results and discussion 3.1. Reactants The energy difference between the ground singlet state, 1 S (3d 10 4s 2 ), and the lowest excited triplet state, 3 P (3d 10 4s 1 4p 1 ), of atomic zinc (Zn) has been found to be 93.0 kcal/mol at the (R)CCSD(T)/aug-cc-pV5Z-PP level (‶(R)″ indicates that the RCCSD(T) method was used for the electronic triplet state). This result is consistent with the | 1 S – 3 P| energy separation of Zn provided by the recent DK-CCSD(T)/aug-cc-pV5Z-DK calculations (93.2 kcal/mol) [18] and the experimental value (93.5 kcal/mol) [19] (DK refers to the second-order Douglas-Kroll Hamiltonian by which scalar relativistic effects were included with the all-electron basis set). Relevant data regarding the description of the equilibrium structure of the methyl iodide 1 reactant, including the available experimental information [20], are shown in Fig. 1a. Therein, the CCSD(T) calculated equilibrium geometry of 1 obtained using the aug-cc-pVQZ (aug-cc-pVQZ-PP) basis set should be compared with the experimental structure. 3.2. The insertion of ground state atomic zinc into the C-I bond of methyl iodide We have first considered the insertion of ground state atomic zinc into the C-I bond of 1 to form a product molecule, IZnCH 3 (X 1 A 1 ) 3 (Fig. 1d). Two pre-reaction van der Waals complexes, Zn … H 2 ICH 2 (Fig. 1b) and CH 3 I … Zn 2a (Fig. 2), have been found, showing comparable stability (Table 1). Starting from 2 , the Zn insertion into the C-I bond would yield 3 . It is important to underline that in contrast to the insertion of ground state atomic zinc into the C-H bond of methane which is known to be endothermic (by 11.4 kcal/mol [18]), the Zn insertion into the C-I bond of 1 is predicted to be exothermic by a substantial 40.3 kcal/mol on the basis of CCSD(T)/aug-cc-pV5Z-PP calculations (Table 1). A transition state, TS1 , located for the latter reaction step involves the dissociation of the C-I bond and the formation of the Zn-C and Zn-I bonds, and it has the highly-bent C-Zn-I moiety (Fig. 1c). The most significant Zn relativistic effect on the TS1 geometry is observed for the Zn-C bond distance, which is reduced by 0.054 Å (when passing from aug-cc-pVQZ to aug-cc-pVQZ-PP). It is the energy barrier of the transition state TS1 of 45.5 kcal/mol (Table 1) that prevents the formation of 3 from ground state atomic zinc and 1 , consistent with the gas-phase experimental study by the Ziurys group [10]. In the latter study, DC discharge was used to make the reaction between atomic zinc and methyl iodide happen, possibly bringing about the electronic transition in the Zn atom to the excited triplet state. The activation of the H-H, C-H, Si-H, and C-Cl bonds by the Zn atom in the 3 P excited state has indeed been reported [18,21–23]. We note here that two distinct modes of the methyl iodide C-I activation with triplet atomic zinc are discussed in sections 3.5 and 3.6. The predicted equilibrium geometry of IZnCH 3 (X 1 A 1 ) 3 contains the linear C-Zn-I skeleton (Fig. 1d), in line with the structure derived from the rotational spectra [10]. Taking into account the relativistic effects in Zn (on top of those in I) is required to reproduce with good accuracy the Zn-C and Zn-I bond distances of IZnCH 3 (X 1 A 1 ) determined from the experimental rotational constants [10] (the relevant experimental r o parameters are indicated in Fig. 1d). Indeed, in going from the aug-cc-pVQZ to the aug-cc-pVQZ-PP basis set, the CCSD(T) calculated distances decrease by 0.018 and 0.015 Å, respectively, thereby giving better concordance with the experimental data. It is likely that the correspondence between the theoretical and experimental geometrical parameters involving hydrogen atoms (Fig. 1d) would have improved with the addition of more deuterium-substituted isotopologues [24] in the derivation of the molecular structure of methylzinc iodide from the rotational spectra [10]. 3.3. Theoretical and experimental vibrational frequencies for methylzinc iodide Table 2 lists the observed [25] and calculated vibrational frequencies for methylzinc iodide - the theoretical results including both the harmonic (ω i ) and anharmonic (ν i ) values - along with the harmonic intensities. The harmonic frequencies were evaluated numerically at the CCSD(T)/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ-PP levels. In turn, the anharmonic frequencies, which were determined by adding the anharmonic corrections from second-order vibrational perturbation theory [26–28] to the CCSD(T)/aug-cc-pVTZ-PP harmonic frequencies, are shown in column ′Hybrid′ of Table 2 (other computational details are provided in the footnotes of this table). For comparison, the harmonic frequencies of IZnCH 3 (X 1 A 1 ) predicted by DFT [10] are given in column ′Literature′. Table 1. Relative energies a (kcal/mol) of species during the insertion reaction of a ground state atomic zinc into the C-I bond of methyl iodide calculated by the CCSD(T) method with the aug-cc-pVQZ and aug-cc-pVnZ-PP [n=Q,5] basis sets System aug-cc-pVQZ b aug-cc-pVQZ-PP c aug-cc-pV5Z-PP d Zn( 1 S) + CH 3 I ( 1 ) 0.0 0.0 0.0 Zn … H 2 ICH ( 2 ) -1.1 -1.2 -1.2 Zn … ICH 3 ( 2a ) -1.2 -1.2 -1.3 TS1 44.4 45.7 45.5 IZnCH 3 (X 1 A 1 ) ( 3 ) -40.9 -39.7 -40.3 a Relative to the reactants. b CCSD(T)/aug-cc-pVQZ//CCSD(T)/aug-cc-pVQZ, including the zero-point energy (ZPE) correction. c CCSD(T)/aug-cc-pVQZ-PP//CCSD(T)/aug-cc-pVQZ-PP, including the ZPE correction. d CCSD(T)/aug-cc-pV5Z-PP//CCSD(T)/aug-cc-pVQZ-PP, including the ZPE correction. Table 2 Theoretical and experimental vibrational frequencies a for methylzinc iodide Description of Mode b Sym. of Vib. CCSD(T)/aug-cc-pVTZ (ω i ) CCSD(T)/aug-cc-pVTZ-PP c (ω i ) "Hybrid" d (ν i ) "Literature" e (ω i ) Exp. f CH 3 a-stretch E 3118 3119 2917 (4) 3118 g CH 3 s-stretch A 1 3031 3033 2923 (9) 3036 g CH 3 a-deform E 1466 1470 1455 (1) 1458 g CH 3 s-deform A 1 1208 1217 1172 (1) 1205 g CH 3 rock E 695 715 698 (26) 711 640 h , 654 i ZnC stretch A 1 577 585 578 (28) 562 530 h , 523 i ZnI stretch A 1 239 240 238 (14) 230 g CZnI a-deform E 99 106 88 (3) 99 g a In cm − 1 . b " s " stands for symmetric and " a " stands for asymmetric . c The CCSD(T)/aug-cc-pVTZ-PP equilibrium geometry of IZnCH 3 (X 1 A 1 ) is: r(Zn-C) = 1.924 Å, r(Zn-I) = 2.421 Å, r(C-H) = 1.093 Å, and < H-C-H = 108.6 o . d Obtained in this work by adding the anharmonic corrections [26] (based on the MP2/aug-cc-pVTZ calculations [27,28]) to the CCSD(T)/aug-cc-pVTZ-PP harmonic frequencies; the values in parentheses are harmonic intensities (in km/mol) determined at the MP2/aug-cc-pVTZ level. e The B3LYP harmonic frequencies taken from Ref. [10]. f Taken from Ref. [25]. g Not observed in Ref. [25]. h Measured in THF solution. i Measured in DME solution. Table 3. Relative energies a (kcal/mol) of species during the methyl iodide C-I activation by triplet atomic zinc calculated by the CCSD(T) method with the aug-cc-pVQZ and aug-cc-pVnZ-PP [n=Q,5] basis sets (for open-shell species, CCSD(T) implies RCCSD(T)) System aug-cc-pVQZ b aug-cc-pVQZ-PP c aug-cc-pV5Z-PP d Zn( 3 P) + CH 3 I ( 1 ) 88.7 ( 0.0 ) 93.0 ( 0.0 ) 93.0 ( 0.0 ) IZn … CH 3 ( 3 A¢) ( 7 ' ) f 27.7 ( -60.9 ) 30.0 (- 63.0 ) 29.6 (- 63.5 ) IZn … CH 3 ( 3 A¢) ( 7 ) f 27.7 ( -60.9 ) 30.0 (- 63.0 ) 29.6 (- 63.5 ) Zn … H 3 CI( 3 A¢) ( 8 ) 86.9 ( -1.8 ) 90.8 ( -2.1 ) 91.0 ( -2.0 ) TS2 ( 3 A¢) 86.3 (- 2.3) 90.0 (- 2.9 ) 90.2 (- 2.8 ) ZnCH 3 …I( 3 A 1 ) ( 9 ) 44.1 ( -44.6 ) 45.3 ( -47.6 ) 45.6 ( -47.4 ) ZnI( 2 S + ) ( 4 ) + CH 3 ( 2 A 2 ") ( 5 ) 29.0 ( -59.7 ) 31.2 ( -61.8 ) 30.7 ( -62.3 ) ZnCH 3 ( 2 A 1 ) ( 6 ) + I( 2 P) 45.4 ( -43.2 ) 46.6 ( -46.4 ) 46.8 ( -46.2 ) a Relative to the ground-state reactants, except for the energies given in parentheses in italics which are relative to the triplet reactants. b CCSD(T)/aug-cc-pVQZ//CCSD(T)/aug-cc-pVQZ, including the zero-point energy (ZPE) correction. c CCSD(T)/aug-cc-pVQZ-PP//CCSD(T)/aug-cc-pVQZ-PP, including the ZPE correction. d CCSD(T)/aug-cc-pV5Z-PP//CCSD(T)/aug-cc-pVQZ-PP, including the ZPE correction. f 7 is confirmed as a minimum structure (see the text). The following points are evident from Table2: 1. The CCSD(T) harmonic frequencies of methylzinc iodide are consistent with the DFT [10] results. 2. The CCSD(T) harmonic frequencies increase when the Zn relativistic effects are taken into consideration, by 2–20 cm -1 . 3. The CH 3 rock and ZnC stretch vibrations are predicted to have most significant IR intensity, in agreement with the experimental evidence - the two fundamental frequencies were observed in the IR spectra of methylzinc iodide in solution [25]. Finally, the incorporation of the correction for anharmonicity lowered the frequencies to various extents, with the most pronounced effect seen for the CH 3 a-stretch , CH 3 s-stretch , and CH 3 s-deform vibrations, being of the order of 202, 107 and 45 cm -1 , respectively. The theoretical results presented in this section can be used for interpreting the vibrational spectrum of the methylzinc iodide molecule. 3.4. Radicals that make up intermediates in the methyl iodide C-I activation by triplet atomic zinc As will be discussed in detail below, intermediate complexes consisting of zinc-bearing and methyl radicals are predicted in the methyl iodide C-I activation by triplet atomic zinc. Hence, we shall briefly report on the findings pertaining to the structures of these radicals, prior to analyzing the activation pathways. Thus, the equilibrium geometries obtained for ZnI(X 2 Σ + ) 4 , ZnCH 3 (X 2 A 1 ) 5 , and CH 3 (X 2 A 2 ") 6 at the RCCSD(T)/aug-cc-pVnZ (n = T, Q) and RCCSD(T)/aug-cc-pVQZ-PP levels are compared with the available experimental [29–31] structures in Fig. 3. For the zinc-bearing radicals 4 and 5 , a decrease in the Zn-I and Zn-C bond distances of 0.011 and 0.014 Å, respectively, can be observed from this figure after accounting for the scalar relativistic effects in Zn (in moving from aug-cc-pVQZ to aug-cc-pVQZ-PP). The magnitude of the effect is therefore consistent with that indicated above for both bonds in IZnCH 3 (X 1 A 1 ) 3 . In addition, we notice that the experimental equilibrium structure of 5 [30], and possibly that of 4 [29], require updating to permit a meaningful comparison with the theoretical counterparts. 3.5. The insertion of triplet atomic zinc into the C-I bond of methyl iodide We next turned to an examination of the pathway of insertion of triplet atomic zinc into the C-I bond of 1 (in an overall 3 A' electronic state). The unsuccessful attempts to locate the relevant insertion transition state at the RCCSD(T)/aug-cc-pVTZ level have suggested that this reaction occurs through a barrierless triplet potential surface. To gain additional insight into this question, we also carried out the relaxed-potential energy scan along the Zn-I bond (with the methyl group being staggered with respect to the Zn atom), starting at a Zn-I distance of 16.0 Å and decreasing the distance to 2.5 Å. For this purpose, the UB3LYP-D3/aug-cc-pVTZ method [28,32–34] was utilized. The latter UDFT calculations have indicated that the insertion of triplet atomic zinc into the C-I bond of 1 is energetically downhill process, affording the intermediate IZnCH 3 ( 3 A′) 7' (Figure S1). Such result is consistent with the prediction that the energy of 7' is lower than that of the triplet reactants by over 60 kcal/mol, according to both (U)B3LYP-D3/aug-cc-pVTZ (Figure S1) and (R)CCSD(T)/aug-cc-pV5Z (Table 3) calculations. Similar to the UB3LYP-D3 structure of 7' (Figure S1), its RCCSD(T) analogue (Fig. 4a) exhibits the heavy-atom arrangement with the less than 90 o bond angle. The staggered structure 7 ' appears to produce a small imaginary frequency associated with the rotation of the methyl group, leading to the eclipsed structure 7 with a somewhat shorter Zn-C distance (Fig. 4b). It can be noticed from Fig. 4 that the insertion intermediate can be represented as a complex, IZn…CH 3 ( 3 A′), with the Zn-C distance of around 3.2 Å. Clearly, this complex shows insignificant stability against dissociation (1.2 kcal/mol) (Table 3). 3.6. S N 2-like mechanism of the reaction of triplet atomic zinc with methyl iodide In this section, we present an alternative mode of the C-I activation that involves an attack of the triplet zinc atom on the methyl group of 1 along with the substitution and inversion of configuration at carbon. This so-called S N 2-like mechanism starts with the formation of van der Waals complex, Zn…HCH 2 I( 3 A′) 8 , showing a non-linear arrangement of the heavy atoms (Fig. 5a). In addition, one of the Zn … H-C contacts in 8 appears to be relatively short (2.148 Å at the RCCSD(T)/aug-cc-pVQZ-PP level) with the C-H bond being lengthened by 0.020 Å relative to free 1 , suggestive of the interaction similar to the agostic one [35]. Next, when the Zn atom moves closer to the carbon atom to make the Zn-C bond, the I atom departs, breaking the C-I bond, which atom displacement entails the inversion transition state TS2 ( 3 A′) (characterized by the imaginary frequency of 485 i cm -1 , Fig. 5b). The C-I (Zn-C) bond distance for the transition state is elongated (shortened) by 0.084 (0.305 Å) relative to 8 ; also, both the non-linear heavy-atom arrangement and agostic-type interaction are preserved in TS2 ( 3 A′). The resulting intermediate is a weakly bound complex with the linear positioning of heavy atoms, ZnCH 3 …I ( 3 A′) 9 (Fig. 5c). On the basis of CCSD(T)/aug-cc-pV5Z-PP calculations, the S N 2-like mechanism is found to have a submerged energy barrier (Fig. 6). Due to the lower stability of the triplet intermediate 9 in the S N 2-like pathway compared to the triplet intermediate 7 in the C-I insertion route (by 16 kcal/mol), the former pathway is predicted to be energetically less efficient than the latter. 3.7. A plausible mechanism of formation of IZnCH 3 (X 1 A 1 ) We now suggest a plausible mechanism for the formation of the IZnCH 3 (X 1 A 1 ) 3 molecule that is relevant to the experimental conditions described in Ref. 10. This mechanism requires the prerequisite Zn 3 P ← 1 S electronic excitation, followed by the activation of the C-I bond of 1 by triplet atomic zinc. As elaborated above, the latter leads to the appearance of a weakly bound intermediate, IZn…CH 3 ( 3 A′) 7 or ZnCH 3 …I ( 3 A 1 ) 9 , which dissociates into radicals, as indicated in Fig. 6. The next steps would include the recombination of the radical fragments (via zinc) and the relaxation to 3 (Fig. 6) (the expected stabilization of the product by the argon carrier gas [10] ought to be mentioned here). But, a mechanism involving the radicals was not supported by Ziurys and co-workers because the zinc-bearing radicals ( 4 and/or 5 ) had not been detected in the discharge mixture [10]. It should be pointed out, though, that our scenario for the formation of IZnCH 3 (X 1 A 1 ) is not inconsistent with theirs proposed to be the insertion of activated zinc atom into the C-I bond of methyl iodide. That is, the steps of (1) the triplet atomic zinc mediated methyl iodide C-I activation, including the insertion of triplet atomic zinc into the C-I bond, and (2) the radical recombination, might have occurred fast enough that made the detection of the radical species difficult. Besides, Ziurys and co-workers admitted that an additional experimental investigation would be needed to clarify the underlying mechanism [10]. 4. Conclusions This CCSD(T) study has been motivated by experiments revealing the formation of the methylzinc iodide molecule [IZnCH 3 (X 1 A 1 )] in the reaction between vaporized methyl iodide (CH 3 I) and atomic zinc reactants [10]. We have found that the insertion of a ground state zinc atom into the C-I bond of methyl iodide to yield IZnCH 3 (X 1 A 1 ) has a negative value of the reaction energy (-40.3 kcal/mol) but a high energy barrier (45.5 kcal/mol) suggesting no reaction. This result is consistent with the experiments where an external stimulus was applied that made the reaction occur in the gas phase. We have predicted the equilibrium geometry of IZnCH 3 (X 1 A 1 ) containing the linear C-Zn-I skeleton, in harmony with the structure derived from the rotational spectra [10]. In order to obtain good agreement with the measurements for the Zn-C and Zn-I bond distances of IZnCH 3 (X 1 A 1 ), it was found necessary to include the scalar relativistic effects in Zn (on top of those in I) in the calculations. Two modes of activation of the C-I bond of methyl iodide by a zinc atom in the excited triplet state have been identified: the direct insertion of triplet atomic zinc into the C-I bond, and S N 2-like displacement, generating weakly bound intermediates, IZn…CH 3 ( 3 A′) and ZnCH 3 …I ( 3 A 1 ), respectively. We have suggested the recombination of the radical fragments (as resulting from the dissociation of the triplet intermediates) followed by the relaxation to IZnCH 3 (X 1 A 1 ) as its possible formation scenario. More work is needed to examine the feasibility of the IZnCH 3 (X 1 A 1 ) formation that involves atomic Zn in the excited 1 P state [10], which is beyond the scope of this study. 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J Chem Phys 122:014108 Møller C, Plesset MS (1934) Note on an approximation treatment for many-electron systems. Phys Rev 46:618-622 Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone V, Mennucci B, Petersson GA, Nakatsuji H, Caricato M, Li X, Hratchian HP, Izmaylov AF, Bloino J, Zheng G, Sonnenberg JL, Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao O, Nakai H, Vreven T, Montgomery Jr JA, Peralta JE, Ogliaro F, Bearpark M, Heyd JJ, Brothers E, Kudin KN, Staroverov VN, Kobayashi R, Normand J, Raghavachari K, Rendell A, Burant J, Iyengar SS, Tomasi J, Cossi M, Rega N, Millam JM, Klene M, Knox JE, Cross JB, Bakken V, Adamo C, Jaramillo J, Gomperts R, Stratmann RE, Yazyev O, Austin AJ, Cammi R, Pomelli C, Ochterski JW, Martin RL, Morokuma K, Zakrzewski VG, Voth GA, Salvador P, Dannenberg JJ, Dapprich S, Daniels AD, Farkas Ö, Foresman JB, Ortiz JV, Cioslowski J, Fox DJ (2009) Gaussian09 Revision C.01. Gaussian Inc., Wallingford CT Elmoussaoui S, Korek M (2015) Electronic structure with dipole moment calculation of the low-lying electronic states of the molecule ZnI. Journal of Quantitative Spectroscopy and Radiative Transfer 161:131-135 Cerny TM, Tan XQ, Williamson JM, Robles ESJ, Ellis AM, Miller TAJ (1993) High resolution electronic spectroscopy of ZnCH 3 and CdCH 3 . J Chem Phys 99:9376-9388 Herzberg G (1967) Electronic spectra of polyatomic molecules. Van Nostrand, Princeton Becke AD (1993) Density-functional thermochemistry. III. The role of exact exchange. J Chem Phys 98:5648-5652 Stephens PJ, Devlin FJ, Chabalowski CF, Frisch MJ (1994) Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields. J Phys Chem 98:11623-11627 Grimme S, Antony J, Ehrlich S, Krieg HA (2010) A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J Chem Phys 132:154104 Q. Lu, F. Neese, G. Bistoni, Formation of Agostic Structures Driven by London Dispersion. Angew. Chem., Int. Ed. 57 (2018) 4760-4764. Additional Declarations No competing interests reported. Supplementary Files SuppZnI.doc Cite Share Download PDF Status: Posted Version 1 posted 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. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-6330328","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":440259325,"identity":"77cd86f3-28a0-4934-8e13-ad2aeb152637","order_by":0,"name":"Jerzy Moc","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAtklEQVRIiWNgGAWjYBACxgYg8YGBgYdPghQtjDOAWtiI1gICzDxAgngtzNMOH/xsm2MnwybdfEyCoeKeXQNBh81OS5bO3ZbMwyZzLNmA4UxxMhFacsyYc7cxA/2SY/iAsS0hmaDDwFost9WDtBgcIF4L47bDcFvsiNCSlizZu+04xC8JZxISCGoxnJ188MPPbdX2/KAQ+1CRYE9YSwMyD2hFYgNWdUhAHl2AsC2jYBSMglEw4gAAR1Qz8XRO2yMAAAAASUVORK5CYII=","orcid":"","institution":"","correspondingAuthor":true,"prefix":"","firstName":"Jerzy","middleName":"","lastName":"Moc","suffix":""}],"badges":[],"createdAt":"2025-03-28 18:38:13","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6330328/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6330328/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":81111910,"identity":"4eb70497-f77f-4632-934d-fdbced8630c7","added_by":"auto","created_at":"2025-04-22 10:40:28","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":389616,"visible":true,"origin":"","legend":"\u003cp\u003e(a) methyl iodide (CH\u003csub\u003e3\u003c/sub\u003eI) (\u003cstrong\u003e1\u003c/strong\u003e), (b) pre-reactive van der Waals complex Zn\u003cstrong\u003e…\u003c/strong\u003eH\u003csub\u003e2\u003c/sub\u003eICH (\u003cstrong\u003e2\u003c/strong\u003e), (c) transition state (\u003cstrong\u003eTS1\u003c/strong\u003e) for the insertion of a ground state zinc atom into the C-I bond of methyl iodide (with the imaginary frequency magnitude, in cm\u003csup\u003e-1\u003c/sup\u003e, given in brackets []), and (d) insertion product IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) (\u003cstrong\u003e3\u003c/strong\u003e). Values in bold are the available experimental data taken from: Ref.[20] (CH\u003csub\u003e3\u003c/sub\u003eI) and Ref.[10] [IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e)]. Bond lengths are in angstroms and bond angles are in degrees.\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-6330328/v1/7f1609fb4cc88b19bc28ef17.png"},{"id":81111909,"identity":"f5fbc7c6-b26f-43d4-8fd1-9df1e635c04e","added_by":"auto","created_at":"2025-04-22 10:40:28","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":87199,"visible":true,"origin":"","legend":"\u003cp\u003eAlternative van der Waals complex CH\u003csub\u003e3\u003c/sub\u003eI\u003cstrong\u003e…\u003c/strong\u003eZn (\u003cstrong\u003e2a\u003c/strong\u003e) arising from the interaction of a ground state zinc atom with methyl iodide (bond lengths are in angstroms).\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-6330328/v1/9e3d9bdabce773b6c0ea83d5.png"},{"id":81111912,"identity":"06b9ff4f-1e34-43ec-bbcf-e19ecb268552","added_by":"auto","created_at":"2025-04-22 10:40:28","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":171303,"visible":true,"origin":"","legend":"\u003cp\u003eRadicals that make up intermediates in the methyl iodide C-I activation by triplet atomic zinc: ZnI(X\u003csup\u003e2\u003c/sup\u003eΣ\u003csup\u003e+\u003c/sup\u003e) (\u003cstrong\u003e4\u003c/strong\u003e), ZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e2\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e)\u003csub\u003e \u003c/sub\u003e(\u003cstrong\u003e5\u003c/strong\u003e), and CH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e2\u003c/sup\u003eA\u003csub\u003e2\u003c/sub\u003e\") (\u003cstrong\u003e6\u003c/strong\u003e). Values in bold are the available experimental data taken from: Ref.[29] [ZnI(X\u003csup\u003e2\u003c/sup\u003eΣ\u003csup\u003e+\u003c/sup\u003e)], Ref.[30] [ZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e2\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e)] (an asterisk indicates that the structural parameters were obtained in Ref. [30] assuming that r(C-H) = 1.10 Å), and Ref.[31] [CH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e2\u003c/sup\u003eA\u003csub\u003e2\u003c/sub\u003e\")] (bond lengths are in angstroms and bond angles are in degrees).\u003c/p\u003e","description":"","filename":"Figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-6330328/v1/a5b5712337edbe4486d75746.png"},{"id":81111914,"identity":"05ecf08c-0050-4dcc-83e6-f309a7cf44cc","added_by":"auto","created_at":"2025-04-22 10:40:28","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":183712,"visible":true,"origin":"","legend":"\u003cp\u003e(a) staggered (\u003cstrong\u003e7'\u003c/strong\u003e) and (b) eclipsed (\u003cstrong\u003e7\u003c/strong\u003e)\u003cstrong\u003e \u003c/strong\u003estructures\u003cstrong\u003e \u003c/strong\u003eof the intermediate IZnCH\u003csub\u003e3\u003c/sub\u003e(\u003csup\u003e3\u003c/sup\u003eA¢) in the insertion of triplet atomic zinc into the C-I bond of methyl iodide (bond lengths are in angstroms and bond angles are in degrees).\u003c/p\u003e","description":"","filename":"Figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-6330328/v1/9b2e00fede935fa938f20d21.png"},{"id":81112144,"identity":"8c1d5817-bda2-4d20-80d0-4014810cfd1a","added_by":"auto","created_at":"2025-04-22 10:48:29","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":371204,"visible":true,"origin":"","legend":"\u003cp\u003e(a) pre-reactive van der Waals complex Zn…HCH\u003csub\u003e2\u003c/sub\u003eI(\u003csup\u003e3\u003c/sup\u003eA′) (\u003cstrong\u003e8\u003c/strong\u003e), (b) inversion transition state [\u003cstrong\u003eTS2\u003c/strong\u003e(\u003csup\u003e3\u003c/sup\u003eA′)] (with the imaginary frequency magnitude, in cm\u003csup\u003e-1\u003c/sup\u003e, given in brackets []), and (c) ensuing intermediate ZnCH\u003csub\u003e3\u003c/sub\u003e…I (\u003csup\u003e3\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) (\u003cstrong\u003e9\u003c/strong\u003e) belonging to S\u003csub\u003eN\u003c/sub\u003e2-like pathway of the reaction of triplet atomic zinc with methyl iodide (bond distances are in Å, and bond angles are in degrees).\u003c/p\u003e","description":"","filename":"Figure5.png","url":"https://assets-eu.researchsquare.com/files/rs-6330328/v1/6a8a4db69a8f3ed77550540a.png"},{"id":81111923,"identity":"375ed0d5-770d-496c-9685-15051ef2da02","added_by":"auto","created_at":"2025-04-22 10:40:29","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":247976,"visible":true,"origin":"","legend":"\u003cp\u003eProfiles for reaction of atomic zinc with methyl iodide. CCSD(T)/aug-cc-pV5Z-PP//CCSD(T)/aug-cc-pVQZ-PP, corrected for zero-point energy (ZPE). For open-shell species, CCSD(T) implies RCCSD(T). \"S\" and \"T\" refer to the singlet and triplet electronic states.\u003c/p\u003e","description":"","filename":"Figure6.png","url":"https://assets-eu.researchsquare.com/files/rs-6330328/v1/79f17189d22070b32d850ff4.png"},{"id":104403979,"identity":"dbdf1a17-bba1-4954-ad5f-7b1dd80c978f","added_by":"auto","created_at":"2026-03-11 12:19:31","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2626957,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6330328/v1/732542ea-dae2-4282-96d7-da17c6b6586b.pdf"},{"id":81111908,"identity":"8cca864c-7289-443c-b957-958041d775b8","added_by":"auto","created_at":"2025-04-22 10:40:28","extension":"doc","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":56320,"visible":true,"origin":"","legend":"","description":"","filename":"SuppZnI.doc","url":"https://assets-eu.researchsquare.com/files/rs-6330328/v1/cc48782ecf2b59f824c8da8e.doc"}],"financialInterests":"No competing interests reported.","formattedTitle":"New mechanistic insights into the gas-phase formation of methylzinc iodide via methyl iodide C-I activation with atomic zinc in excited triplet state as offered by the CCSD(T) study","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eActivation of C-X bonds (X\u0026thinsp;=\u0026thinsp;halogen) by transition metal (TM) compounds is of central importance for catalytic processes of industrial interest, such as, for instance, cross-coupling reactions [1]. A few examples of the C-I activation (X\u0026thinsp;=\u0026thinsp;iodine) at TM complexes or TM clusters documented in the literature that can be considered as representative of the experimental and theoretical research follow: the gas-phase reactions of iodobenzene with the mononuclear gold cations (R\u003csub\u003e3\u003c/sub\u003eP)\u003csub\u003en\u003c/sub\u003eAu\u003csup\u003e+\u003c/sup\u003e (R\u0026thinsp;=\u0026thinsp;Me, Ph; n\u0026thinsp;=\u0026thinsp;1,2) and ligated gold clusters [Au\u003csub\u003e3\u003c/sub\u003eL\u003csub\u003en\u003c/sub\u003e]\u003csup\u003e+\u003c/sup\u003e (L\u0026thinsp;=\u0026thinsp;Ph\u003csub\u003e2\u003c/sub\u003eP(CH\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003en\u003c/sub\u003ePPh\u003csub\u003e2\u003c/sub\u003e; n\u0026thinsp;=\u0026thinsp;3\u0026ndash;6) examined using mass spectrometry and density functional theory (DFT) [2]; the electrochemically induced activation of C-I bonds in diverse R-I substrates by the Pd\u003csub\u003e3\u003c/sub\u003e(dppm)\u003csub\u003e3\u003c/sub\u003eCO\u003csup\u003e2+\u003c/sup\u003e cluster as observed via cyclic voltammetry, coulometry and \u003csup\u003e31\u003c/sup\u003eP NMR spectroscopy in THF [3]; the C-I bond cleavage in iodobenzene, iodoethane and iodoethene over the Au\u003csub\u003en\u003c/sub\u003e clusters (n\u0026thinsp;=\u0026thinsp;3,4,14,20) and their cations investigated with DFT [4].\u003c/p\u003e \u003cp\u003eOver the past few decades, there has been an increased interest in studying the C-I bond activation and photodissociation of methyl iodide (CH\u003csub\u003e3\u003c/sub\u003eI) [5\u0026ndash;10]. An inspiring example from this research activity is the reaction of methyl iodide vapour with zinc vapour in a DC discharge, reported by the Ziurys group [10], where the composition and molecular structure of the product were analyzed by means of rotational spectra. This analysis revealed the formation of the methylzinc iodide monomer, IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) [10]. Based on the experimental data, the authors proposed that the insertion of the \u003cem\u003eactivated\u003c/em\u003e zinc atom into the C-I bond of methyl iodide was responsible for the formation of the methylzinc iodide molecule; however, the exact mechanisms have not been fully elucidated.\u003c/p\u003e \u003cp\u003eInspired by the work of Ziurys and co-workers on a \u0026Prime;model system\u0026Prime; to investigate the C-I activation mentioned above, in this paper, we explore portions of the potential surface of this system which are relevant to the formation of methylzinc iodide by using the coupled-cluster CCSD(T) method (see next section for more details). The results of these calculations allow us to identify various modes of the atomic zinc mediated methyl iodide C-I activation, and present a plausible mechanism for the formation of IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e). In addition, the availability of the gas-phase structure of the methylzinc iodide molecule that was derived from the rotational spectra [10], provide the opportunity for comparison with that obtained from the calculations.\u003c/p\u003e"},{"header":"2. Computational details","content":"\u003cp\u003eStationary points on the potential energy surface were located with the coupled cluster singles and doubles with perturbative triples method, CCSD(T) [11]. The open-shell systems were treated using the spin restricted CCSD(T) formalism based on an ROHF reference [12] (and denoted hereafter as RCCSD(T)). Two kinds of the basis set/pseudopotential (PP) combinations were utilized. First, the small-core energy-consistent relativistic PP was applied only for I with the corresponding aug-cc-pVnZ-PP basis sets [13] along with the all-electron aug-cc-pVnZ basis sets for the remaining atoms [14\u0026ndash;15] - the resulting combination is designated simply as aug-cc-pVnZ, where n\u0026thinsp;=\u0026thinsp;T and Q. The nature of all stationary points was confirmed by harmonic vibrational frequency analysis using the aug-cc-pVTZ basis set. Next, to be able to estimate the scalar relativistic effects in Zn, the small-core energy-consistent relativistic PPs were employed for both the I and Zn atoms with the corresponding aug-cc-pVnZ-PP basis sets [13,16] in conjunction with the aug-cc-pVnZ basis sets for the C and H atoms [14] - the resulting combination is designated as aug-cc-pVnZ-PP, where n\u0026thinsp;=\u0026thinsp;Q and 5. Note that the PPs replace the inner-core I (1s\u003csup\u003e2\u003c/sup\u003e2s\u003csup\u003e2\u003c/sup\u003e2p\u003csup\u003e6\u003c/sup\u003e3s\u003csup\u003e2\u003c/sup\u003e3p\u003csup\u003e6\u003c/sup\u003e3d\u003csup\u003e10\u003c/sup\u003e) and Zn (1s\u003csup\u003e2\u003c/sup\u003e2s\u003csup\u003e2\u003c/sup\u003e2p\u003csup\u003e6\u003c/sup\u003e) electrons. The quantum chemistry code MOLPRO2012.1 [17] was exploited in all CC calculations.\u003c/p\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec4\"\u003e\n \u003ch2\u003e3.1. Reactants\u003c/h2\u003e\n \u003cp\u003eThe energy difference between the ground singlet state, \u003csup\u003e1\u003c/sup\u003eS (3d\u003csup\u003e10\u003c/sup\u003e4s\u003csup\u003e2\u003c/sup\u003e), and the lowest excited triplet state, \u003csup\u003e3\u003c/sup\u003eP (3d\u003csup\u003e10\u003c/sup\u003e4s\u003csup\u003e1\u003c/sup\u003e4p\u003csup\u003e1\u003c/sup\u003e), of atomic zinc (Zn) has been found to be 93.0 kcal/mol at the (R)CCSD(T)/aug-cc-pV5Z-PP level (‶(R)″ indicates that the RCCSD(T) method was used for the electronic triplet state). This result is consistent with the |\u003csup\u003e1\u003c/sup\u003eS – \u003csup\u003e3\u003c/sup\u003eP| energy separation of Zn provided by the recent DK-CCSD(T)/aug-cc-pV5Z-DK calculations (93.2 kcal/mol) [18] and the experimental value (93.5 kcal/mol) [19] (DK refers to the second-order Douglas-Kroll Hamiltonian by which scalar relativistic effects were included with the all-electron basis set).\u003c/p\u003e\n \u003cp\u003eRelevant data regarding the description of the equilibrium structure of the methyl iodide \u003cstrong\u003e1\u003c/strong\u003e reactant, including the available experimental information [20], are shown in Fig.\u0026nbsp;1a. Therein, the CCSD(T) calculated equilibrium geometry of \u003cstrong\u003e1\u003c/strong\u003e obtained using the aug-cc-pVQZ (aug-cc-pVQZ-PP) basis set should be compared with the experimental structure.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\"\u003e\n \u003ch2\u003e3.2. The insertion of ground state atomic zinc into the C-I bond of methyl iodide\u003c/h2\u003e\n \u003cp\u003eWe have first considered the insertion of ground state atomic zinc into the C-I bond of \u003cstrong\u003e1\u003c/strong\u003e to form a product molecule, IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) \u003cstrong\u003e3\u003c/strong\u003e (Fig.\u0026nbsp;1d). Two pre-reaction van der Waals complexes, Zn\u003cstrong\u003e…\u003c/strong\u003eH\u003csub\u003e2\u003c/sub\u003eICH \u003cstrong\u003e2\u003c/strong\u003e (Fig.\u0026nbsp;1b) and CH\u003csub\u003e3\u003c/sub\u003eI\u003cstrong\u003e…\u003c/strong\u003eZn \u003cstrong\u003e2a\u003c/strong\u003e (Fig.\u0026nbsp;2), have been found, showing comparable stability (Table\u0026nbsp;1). Starting from \u003cstrong\u003e2\u003c/strong\u003e, the Zn insertion into the C-I bond would yield \u003cstrong\u003e3\u003c/strong\u003e. It is important to underline that in contrast to the insertion of ground state atomic zinc into the C-H bond of methane which is known to be endothermic (by 11.4 kcal/mol [18]), the Zn insertion into the C-I bond of \u003cstrong\u003e1\u003c/strong\u003e is predicted to be exothermic by a substantial 40.3 kcal/mol on the basis of CCSD(T)/aug-cc-pV5Z-PP calculations (Table\u0026nbsp;1). A transition state, \u003cstrong\u003eTS1\u003c/strong\u003e, located for the latter reaction step involves the dissociation of the C-I bond and the formation of the Zn-C and Zn-I bonds, and it has the highly-bent C-Zn-I moiety (Fig.\u0026nbsp;1c). The most significant Zn relativistic effect on the \u003cstrong\u003eTS1\u003c/strong\u003e geometry is observed for the Zn-C bond distance, which is reduced by 0.054 Å (when passing from aug-cc-pVQZ to aug-cc-pVQZ-PP).\u003c/p\u003e\n \u003cp\u003eIt is the energy barrier of the transition state \u003cstrong\u003eTS1\u003c/strong\u003e of 45.5 kcal/mol (Table\u0026nbsp;1) that prevents the formation of \u003cstrong\u003e3\u003c/strong\u003e from ground state atomic zinc and \u003cstrong\u003e1\u003c/strong\u003e, consistent with the gas-phase experimental study by the Ziurys group [10]. In the latter study, DC discharge was used to make the reaction between atomic zinc and methyl iodide happen, possibly bringing about the electronic transition in the Zn atom to the excited triplet state. The activation of the H-H, C-H, Si-H, and C-Cl bonds by the Zn atom in the \u003csup\u003e3\u003c/sup\u003eP excited state has indeed been reported [18,21–23]. We note here that two distinct modes of the methyl iodide C-I activation with triplet atomic zinc are discussed in sections 3.5 and 3.6.\u003c/p\u003e\n \u003cp\u003eThe predicted equilibrium geometry of IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) \u003cstrong\u003e3\u003c/strong\u003e contains the linear C-Zn-I skeleton (Fig.\u0026nbsp;1d), in line with the structure derived from the rotational spectra [10]. Taking into account the relativistic effects in Zn (on top of those in I) is required to reproduce with good accuracy the Zn-C and Zn-I bond distances of IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) determined from the experimental rotational constants [10] (the relevant experimental r\u003csub\u003eo\u003c/sub\u003e parameters are indicated in Fig.\u0026nbsp;1d). Indeed, in going from the aug-cc-pVQZ to the aug-cc-pVQZ-PP basis set, the CCSD(T) calculated distances decrease by 0.018 and 0.015 Å, respectively, thereby giving better concordance with the experimental data. It is likely that the correspondence between the theoretical and experimental geometrical parameters involving hydrogen atoms (Fig.\u0026nbsp;1d) would have improved with the addition of more deuterium-substituted isotopologues [24] in the derivation of the molecular structure of methylzinc iodide from the rotational spectra [10].\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\"\u003e\n \u003ch2\u003e3.3. Theoretical and experimental vibrational frequencies for methylzinc iodide\u003c/h2\u003e\n \u003cp\u003eTable 2 lists the observed [25] and calculated vibrational frequencies for methylzinc iodide - the theoretical results including both the harmonic (ω\u003csub\u003ei\u003c/sub\u003e) and anharmonic (ν\u003csub\u003ei\u003c/sub\u003e) values - along with the harmonic intensities. The harmonic frequencies were evaluated numerically at the CCSD(T)/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ-PP levels. In turn, the anharmonic frequencies, which were determined by adding the anharmonic corrections from second-order vibrational perturbation theory [26–28] to the CCSD(T)/aug-cc-pVTZ-PP harmonic frequencies, are shown in column ′Hybrid′ of Table 2 (other computational details are provided in the footnotes of this table). For comparison, the harmonic frequencies of IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) predicted by DFT [10] are given in column ′Literature′.\u003c/p\u003e\n \u003cp\u003eTable 1. Relative energies\u003csup\u003ea\u0026nbsp;\u003c/sup\u003e(kcal/mol) of species during the insertion reaction of a ground state atomic zinc into the C-I bond of methyl iodide calculated by the CCSD(T) method with the aug-cc-pVQZ and aug-cc-pVnZ-PP [n=Q,5] basis sets\u0026nbsp;\u003c/p\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"546\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eSystem\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp;aug-cc-pVQZ\u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp;aug-cc-pVQZ-PP\u003csup\u003ec\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eaug-cc-pV5Z-PP\u003csup\u003ed\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eZn(\u003csup\u003e1\u003c/sup\u003eS) + CH\u003csub\u003e3\u003c/sub\u003eI \u0026nbsp;(\u003cstrong\u003e1\u003c/strong\u003e)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;0.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;0.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;0.0\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eZn\u003cstrong\u003e…\u003c/strong\u003eH\u003csub\u003e2\u003c/sub\u003eICH\u0026nbsp;(\u003cstrong\u003e2\u003c/strong\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; -1.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; -1.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; -1.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eZn\u003cstrong\u003e…\u003c/strong\u003eICH\u003csub\u003e3\u003c/sub\u003e\u0026nbsp; (\u003cstrong\u003e2a\u003c/strong\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; -1.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; -1.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; -1.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eTS1\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;44.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;45.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; 45.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eIZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) \u0026nbsp;(\u003cstrong\u003e3\u003c/strong\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; -40.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;-39.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; -40.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003csup\u003ea\u003c/sup\u003eRelative to the reactants. \u0026nbsp;\u003csup\u003eb\u003c/sup\u003eCCSD(T)/aug-cc-pVQZ//CCSD(T)/aug-cc-pVQZ, including the zero-point energy (ZPE) correction. \u0026nbsp;\u003csup\u003ec\u003c/sup\u003eCCSD(T)/aug-cc-pVQZ-PP//CCSD(T)/aug-cc-pVQZ-PP, including the ZPE correction. \u0026nbsp;\u003csup\u003ed\u003c/sup\u003eCCSD(T)/aug-cc-pV5Z-PP//CCSD(T)/aug-cc-pVQZ-PP, including the ZPE correction.\u003c/p\u003e\n \u003cdiv\u003e\n \u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv\u003eTable 2\u003c/div\u003e\n \u003cdiv\u003e\n \u003cp\u003eTheoretical and experimental vibrational frequencies\u003csup\u003ea\u003c/sup\u003e for methylzinc iodide\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"7\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eDescription\u003c/p\u003e\n \u003cp\u003eof Mode\u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSym. of Vib.\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCCSD(T)/aug-cc-pVTZ\u003c/p\u003e\n \u003cp\u003e(ω\u003csub\u003ei\u003c/sub\u003e)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCCSD(T)/aug-cc-pVTZ-PP\u003csup\u003ec\u003c/sup\u003e\u003c/p\u003e\n \u003cp\u003e(ω\u003csub\u003ei\u003c/sub\u003e)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\"Hybrid\"\u003csup\u003ed\u003c/sup\u003e\u003c/p\u003e\n \u003cp\u003e(ν\u003csub\u003ei\u003c/sub\u003e)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\"Literature\"\u003csup\u003ee\u003c/sup\u003e\u003c/p\u003e\n \u003cp\u003e(ω\u003csub\u003ei\u003c/sub\u003e)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eExp.\u003csup\u003ef\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eCH\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e \u003cem\u003ea-stretch\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eE\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3118\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3119\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2917 (4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3118\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003csup\u003eg\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eCH\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e \u003cem\u003es-stretch\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eA\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3031\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3033\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2923 (9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3036\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003csup\u003eg\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eCH\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e \u003cem\u003ea-deform\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eE\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1466\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1470\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1455 (1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1458\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003csup\u003eg\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eCH\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e \u003cem\u003es-deform\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eA\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1208\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1217\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1172 (1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1205\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003csup\u003eg\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eCH\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e \u003cem\u003erock\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eE\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e695\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e715\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e698 (26)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e711\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e640\u003csup\u003eh\u003c/sup\u003e, 654\u003csup\u003ei\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eZnC stretch\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eA\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e577\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e585\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e578 (28)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e562\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e530\u003csup\u003eh\u003c/sup\u003e, 523\u003csup\u003ei\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eZnI stretch\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eA\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e239\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e240\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e238 (14)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e230\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003csup\u003eg\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eCZnI a-deform\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eE\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e99\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e106\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e88 (3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e99\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003csup\u003eg\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003ctfoot\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"7\"\u003e\u003csup\u003ea\u003c/sup\u003eIn cm\u003csup\u003e− 1\u003c/sup\u003e. \u003csup\u003eb\u003c/sup\u003e\"\u003cem\u003es\u003c/em\u003e\" stands for \u003cem\u003esymmetric\u003c/em\u003e and \"\u003cem\u003ea\u003c/em\u003e\" stands for \u003cem\u003easymmetric\u003c/em\u003e. \u003csup\u003ec\u003c/sup\u003eThe CCSD(T)/aug-cc-pVTZ-PP equilibrium geometry of IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) is: r(Zn-C) = 1.924 Å, r(Zn-I) = 2.421 Å, r(C-H) = 1.093 Å, and \u0026lt; H-C-H = 108.6\u003csup\u003eo\u003c/sup\u003e. \u003csup\u003ed\u003c/sup\u003eObtained in this work by adding the anharmonic corrections [26] (based on the MP2/aug-cc-pVTZ calculations [27,28]) to the CCSD(T)/aug-cc-pVTZ-PP harmonic frequencies; the values in parentheses are harmonic intensities (in km/mol) determined at the MP2/aug-cc-pVTZ level. \u003csup\u003ee\u003c/sup\u003eThe B3LYP harmonic frequencies taken from Ref. [10]. \u003csup\u003ef\u003c/sup\u003eTaken from Ref. [25]. \u003csup\u003eg\u003c/sup\u003eNot observed in Ref. [25]. \u003csup\u003eh\u003c/sup\u003eMeasured in THF solution. \u003csup\u003ei\u003c/sup\u003eMeasured in DME solution.\u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tfoot\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003eTable 3.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eRelative energies\u003csup\u003ea\u0026nbsp;\u003c/sup\u003e(kcal/mol) of species during the methyl iodide C-I\u0026nbsp;\u003c/p\u003e\n \u003cp\u003eactivation by triplet atomic zinc calculated by the CCSD(T) method with the\u003c/p\u003e\n \u003cp\u003eaug-cc-pVQZ and aug-cc-pVnZ-PP [n=Q,5] basis sets (for open-shell species,\u0026nbsp;\u003c/p\u003e\n \u003cp\u003eCCSD(T) implies RCCSD(T))\u003c/p\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"539\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;System\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp;aug-cc-pVQZ\u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp;aug-cc-pVQZ-PP\u003csup\u003ec\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eaug-cc-pV5Z-PP\u003csup\u003ed\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eZn(\u003csup\u003e3\u003c/sup\u003eP) + CH\u003csub\u003e3\u003c/sub\u003eI (\u003cstrong\u003e1\u003c/strong\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;88.7\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;(\u003cem\u003e0.0\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;93.0\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;(\u003cem\u003e0.0\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; 93.0\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; (\u003cem\u003e0.0\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eIZn\u003cstrong\u003e…\u003c/strong\u003eCH\u003csub\u003e3\u003c/sub\u003e(\u003csup\u003e3\u003c/sup\u003eA¢) (\u003cstrong\u003e7\u003c/strong\u003e\u003cstrong\u003e'\u003c/strong\u003e)\u003csup\u003ef\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;27.7\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;(\u003cem\u003e-60.9\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;30.0\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;(-\u003cem\u003e63.0\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; 29.6 \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;(-\u003cem\u003e63.5\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eIZn\u003cstrong\u003e…\u003c/strong\u003eCH\u003csub\u003e3\u003c/sub\u003e(\u003csup\u003e3\u003c/sup\u003eA¢) (\u003cstrong\u003e7\u003c/strong\u003e)\u003csup\u003ef\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;27.7\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;(\u003cem\u003e-60.9\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;30.0\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;(-\u003cem\u003e63.0\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; 29.6 \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;(-\u003cem\u003e63.5\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eZn\u003cstrong\u003e…\u003c/strong\u003eH\u003csub\u003e3\u003c/sub\u003eCI(\u003csup\u003e3\u003c/sup\u003eA¢)\u0026nbsp;\u0026nbsp;(\u003cstrong\u003e8\u003c/strong\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;86.9\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; (\u003cem\u003e-1.8\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;90.8\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; (\u003cem\u003e-2.1\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; 91.0\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;(\u003cem\u003e-2.0\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eTS2\u003c/strong\u003e(\u003csup\u003e3\u003c/sup\u003eA¢)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;86.3\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; (-\u003cem\u003e2.3)\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;90.0\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; (-\u003cem\u003e2.9\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; 90.2 \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;(-\u003cem\u003e2.8\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eZnCH\u003csub\u003e3\u003c/sub\u003e…I(\u003csup\u003e3\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) (\u003cstrong\u003e9\u003c/strong\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;44.1\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; (\u003cem\u003e-44.6\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;45.3\u003c/p\u003e\n \u003cp\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e(\u003cem\u003e-47.6\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; 45.6\u003c/p\u003e\n \u003cp\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e(\u003cem\u003e-47.4\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eZnI(\u003csup\u003e2\u003c/sup\u003eS\u003csup\u003e+\u003c/sup\u003e) (\u003cstrong\u003e4\u003c/strong\u003e) + CH\u003csub\u003e3\u003c/sub\u003e(\u003csup\u003e2\u003c/sup\u003eA\u003csub\u003e2\u003c/sub\u003e\") (\u003cstrong\u003e5\u003c/strong\u003e)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp;29.0\u003c/p\u003e\n \u003cp\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e(\u003cem\u003e-59.7\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; 31.2 \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e(\u003cem\u003e-61.8\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; 30.7\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;(\u003cem\u003e-62.3\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eZnCH\u003csub\u003e3\u003c/sub\u003e (\u003csup\u003e2\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) (\u003cstrong\u003e6\u003c/strong\u003e) + I(\u003csup\u003e2\u003c/sup\u003eP)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;45.4\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; (\u003cem\u003e-43.2\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; 46.6\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; (\u003cem\u003e-46.4\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;46.8\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;(\u003cem\u003e-46.2\u003c/em\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003csup\u003ea\u003c/sup\u003eRelative to the ground-state reactants, except for the energies given in parentheses in italics which are relative to the triplet reactants.\u0026nbsp;\u003csup\u003eb\u003c/sup\u003eCCSD(T)/aug-cc-pVQZ//CCSD(T)/aug-cc-pVQZ, including the zero-point energy (ZPE) correction. \u003csup\u003ec\u003c/sup\u003eCCSD(T)/aug-cc-pVQZ-PP//CCSD(T)/aug-cc-pVQZ-PP, including the ZPE correction. \u003csup\u003ed\u003c/sup\u003eCCSD(T)/aug-cc-pV5Z-PP//CCSD(T)/aug-cc-pVQZ-PP, including the ZPE correction. \u003csup\u003ef\u003c/sup\u003e\u003cstrong\u003e7\u003c/strong\u003e is confirmed as a minimum structure (see the text).\u003c/p\u003e\n \u003cp\u003eThe following points are evident from Table2: 1. The CCSD(T) harmonic frequencies of methylzinc iodide are consistent with the DFT [10] results. 2. The CCSD(T) harmonic frequencies increase when the Zn relativistic effects are taken into consideration, by 2–20 cm\u003csup\u003e-1\u003c/sup\u003e. 3. The \u003cem\u003eCH\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e \u003cem\u003erock\u003c/em\u003e and \u003cem\u003eZnC stretch\u003c/em\u003e vibrations are predicted to have most significant IR intensity, in agreement with the experimental evidence - the two fundamental frequencies were observed in the IR spectra of methylzinc iodide in solution [25]. Finally, the incorporation of the correction for anharmonicity lowered the frequencies to various extents, with the most pronounced effect seen for the \u003cem\u003eCH\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e \u003cem\u003ea-stretch\u003c/em\u003e, \u003cem\u003eCH\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e \u003cem\u003es-stretch\u003c/em\u003e, and \u003cem\u003eCH\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e \u003cem\u003es-deform\u003c/em\u003e vibrations, being of the order of 202, 107 and 45 cm\u003csup\u003e-1\u003c/sup\u003e, respectively. The theoretical results presented in this section can be used for interpreting the vibrational spectrum of the methylzinc iodide molecule.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\"\u003e\n \u003ch2\u003e3.4. Radicals that make up intermediates in the methyl iodide C-I activation by triplet atomic zinc\u003c/h2\u003e\n \u003cp\u003eAs will be discussed in detail below, intermediate complexes consisting of zinc-bearing and methyl radicals are predicted in the methyl iodide C-I activation by triplet atomic zinc. Hence, we shall briefly report on the findings pertaining to the structures of these radicals, prior to analyzing the activation pathways. Thus, the equilibrium geometries obtained for ZnI(X\u003csup\u003e2\u003c/sup\u003eΣ\u003csup\u003e+\u003c/sup\u003e) \u003cstrong\u003e4\u003c/strong\u003e, ZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e2\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) \u003cstrong\u003e5\u003c/strong\u003e, and CH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e2\u003c/sup\u003eA\u003csub\u003e2\u003c/sub\u003e\") \u003cstrong\u003e6\u003c/strong\u003e at the RCCSD(T)/aug-cc-pVnZ (n = T, Q) and RCCSD(T)/aug-cc-pVQZ-PP levels are compared with the available experimental [29–31] structures in Fig.\u0026nbsp;3. For the zinc-bearing radicals \u003cstrong\u003e4\u003c/strong\u003e and \u003cstrong\u003e5\u003c/strong\u003e, a decrease in the Zn-I and Zn-C bond distances of 0.011 and 0.014 Å, respectively, can be observed from this figure after accounting for the scalar relativistic effects in Zn (in moving from aug-cc-pVQZ to aug-cc-pVQZ-PP). The magnitude of the effect is therefore consistent with that indicated above for both bonds in IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) \u003cstrong\u003e3\u003c/strong\u003e. In addition, we notice that the experimental equilibrium structure of \u003cstrong\u003e5\u003c/strong\u003e [30], and possibly that of \u003cstrong\u003e4\u003c/strong\u003e [29], require updating to permit a meaningful comparison with the theoretical counterparts.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec8\"\u003e\n \u003ch2\u003e3.5. The insertion of triplet atomic zinc into the C-I bond of methyl iodide\u003c/h2\u003e\n \u003cp\u003eWe next turned to an examination of the pathway of insertion of triplet atomic zinc into the C-I bond of \u003cstrong\u003e1\u003c/strong\u003e (in an overall \u003csup\u003e3\u003c/sup\u003eA' electronic state). The unsuccessful attempts to locate the relevant insertion transition state at the RCCSD(T)/aug-cc-pVTZ level have suggested that this reaction occurs through a barrierless triplet potential surface. To gain additional insight into this question, we also carried out the relaxed-potential energy scan along the Zn-I bond (with the methyl group being staggered with respect to the Zn atom), starting at a Zn-I distance of 16.0 Å and decreasing the distance to 2.5 Å. For this purpose, the UB3LYP-D3/aug-cc-pVTZ method [28,32–34] was utilized. The latter UDFT calculations have indicated that the insertion of triplet atomic zinc into the C-I bond of \u003cstrong\u003e1\u003c/strong\u003e is energetically downhill process, affording the intermediate IZnCH\u003csub\u003e3\u003c/sub\u003e(\u003csup\u003e3\u003c/sup\u003eA′) \u003cstrong\u003e7'\u003c/strong\u003e (Figure S1). Such result is consistent with the prediction that the energy of \u003cstrong\u003e7'\u003c/strong\u003e is lower than that of the triplet reactants by over 60 kcal/mol, according to both (U)B3LYP-D3/aug-cc-pVTZ (Figure S1) and (R)CCSD(T)/aug-cc-pV5Z (Table\u0026nbsp;3) calculations.\u003c/p\u003e\n \u003cp\u003eSimilar to the UB3LYP-D3 structure of \u003cstrong\u003e7'\u003c/strong\u003e (Figure S1), its RCCSD(T) analogue (Fig.\u0026nbsp;4a) exhibits the heavy-atom arrangement with the less than 90\u003csup\u003eo\u003c/sup\u003e bond angle. The staggered structure 7\u003cstrong\u003e'\u003c/strong\u003e appears to produce a small imaginary frequency associated with the rotation of the methyl group, leading to the eclipsed structure 7 with a somewhat shorter Zn-C distance (Fig.\u0026nbsp;4b). It can be noticed from Fig.\u0026nbsp;4 that the insertion intermediate can be represented as a complex, IZn…CH\u003csub\u003e3\u003c/sub\u003e (\u003csup\u003e3\u003c/sup\u003eA′), with the Zn-C distance of around 3.2 Å. Clearly, this complex shows insignificant stability against dissociation (1.2 kcal/mol) (Table\u0026nbsp;3).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\"\u003e\n \u003ch2\u003e3.6. S\u003csub\u003eN\u003c/sub\u003e2-like mechanism of the reaction of triplet atomic zinc with methyl iodide\u003c/h2\u003e\n \u003cp\u003eIn this section, we present an alternative mode of the C-I activation that involves an attack of the triplet zinc atom on the methyl group of \u003cstrong\u003e1\u003c/strong\u003e along with the substitution and inversion of configuration at carbon. This so-called S\u003csub\u003eN\u003c/sub\u003e2-like mechanism starts with the formation of van der Waals complex, Zn…HCH\u003csub\u003e2\u003c/sub\u003eI(\u003csup\u003e3\u003c/sup\u003eA′) \u003cstrong\u003e8\u003c/strong\u003e, showing a non-linear arrangement of the heavy atoms (Fig.\u0026nbsp;5a). In addition, one of the Zn\u003cstrong\u003e…\u003c/strong\u003eH-C contacts in \u003cstrong\u003e8\u003c/strong\u003e appears to be relatively short (2.148 Å at the RCCSD(T)/aug-cc-pVQZ-PP level) with the C-H bond being lengthened by 0.020 Å relative to free \u003cstrong\u003e1\u003c/strong\u003e, suggestive of the interaction similar to the agostic one [35]. Next, when the Zn atom moves closer to the carbon atom to make the Zn-C bond, the I atom departs, breaking the C-I bond, which atom displacement entails the inversion transition state \u003cstrong\u003eTS2\u003c/strong\u003e(\u003csup\u003e3\u003c/sup\u003eA′) (characterized by the imaginary frequency of 485\u003cem\u003ei\u003c/em\u003e cm\u003csup\u003e-1\u003c/sup\u003e, Fig.\u0026nbsp;5b). The C-I (Zn-C) bond distance for the transition state is elongated (shortened) by 0.084 (0.305 Å) relative to \u003cstrong\u003e8\u003c/strong\u003e; also, both the non-linear heavy-atom arrangement and agostic-type interaction are preserved in \u003cstrong\u003eTS2\u003c/strong\u003e(\u003csup\u003e3\u003c/sup\u003eA′).\u003c/p\u003e\n \u003cp\u003eThe resulting intermediate is a weakly bound complex with the linear positioning of heavy atoms, ZnCH\u003csub\u003e3\u003c/sub\u003e…I (\u003csup\u003e3\u003c/sup\u003eA′) \u003cstrong\u003e9\u003c/strong\u003e (Fig.\u0026nbsp;5c). On the basis of CCSD(T)/aug-cc-pV5Z-PP calculations, the S\u003csub\u003eN\u003c/sub\u003e2-like mechanism is found to have a submerged energy barrier (Fig.\u0026nbsp;6). Due to the lower stability of the triplet intermediate \u003cstrong\u003e9\u003c/strong\u003e in the S\u003csub\u003eN\u003c/sub\u003e2-like pathway compared to the triplet intermediate \u003cstrong\u003e7\u003c/strong\u003e in the C-I insertion route (by 16 kcal/mol), the former pathway is predicted to be energetically less efficient than the latter.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\"\u003e\n \u003ch2\u003e3.7. A plausible mechanism of formation of IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e)\u003c/h2\u003e\n \u003cp\u003eWe now suggest a plausible mechanism for the formation of the IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) \u003cstrong\u003e3\u003c/strong\u003e molecule that is relevant to the experimental conditions described in Ref. 10. This mechanism requires the prerequisite Zn \u003csup\u003e3\u003c/sup\u003eP ← \u003csup\u003e1\u003c/sup\u003eS electronic excitation, followed by the activation of the C-I bond of \u003cstrong\u003e1\u003c/strong\u003e by triplet atomic zinc. As elaborated above, the latter leads to the appearance of a weakly bound intermediate, IZn…CH\u003csub\u003e3\u003c/sub\u003e (\u003csup\u003e3\u003c/sup\u003eA′) \u003cstrong\u003e7\u003c/strong\u003e or ZnCH\u003csub\u003e3\u003c/sub\u003e…I (\u003csup\u003e3\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) \u003cstrong\u003e9\u003c/strong\u003e, which dissociates into radicals, as indicated in Fig.\u0026nbsp;6. The next steps would include the recombination of the radical fragments (via zinc) and the relaxation to \u003cstrong\u003e3\u003c/strong\u003e (Fig.\u0026nbsp;6) (the expected stabilization of the product by the argon carrier gas [10] ought to be mentioned here). But, a mechanism involving the radicals was not supported by Ziurys and co-workers because the zinc-bearing radicals (\u003cstrong\u003e4\u003c/strong\u003e and/or \u003cstrong\u003e5\u003c/strong\u003e) had not been detected in the discharge mixture [10]. It should be pointed out, though, that our scenario for the formation of IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) is not inconsistent with theirs proposed to be the insertion of \u003cem\u003eactivated\u003c/em\u003e zinc atom into the C-I bond of methyl iodide. That is, the steps of (1) the triplet atomic zinc mediated methyl iodide C-I activation, including the insertion of triplet atomic zinc into the C-I bond, and (2) the radical recombination, might have occurred fast enough that made the detection of the radical species difficult. Besides, Ziurys and co-workers admitted that an additional experimental investigation would be needed to clarify the underlying mechanism [10].\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eThis CCSD(T) study has been motivated by experiments revealing the formation of the methylzinc iodide molecule [IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e)] in the reaction between vaporized methyl iodide (CH\u003csub\u003e3\u003c/sub\u003eI) and atomic zinc reactants [10]. We have found that the insertion of a ground state zinc atom into the C-I bond of methyl iodide to yield IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) has a negative value of the reaction energy (-40.3 kcal/mol) but a high energy barrier (45.5 kcal/mol) suggesting no reaction. This result is consistent with the experiments where an external stimulus was applied that made the reaction occur in the gas phase. We have predicted the equilibrium geometry of IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) containing the linear C-Zn-I skeleton, in harmony with the structure derived from the rotational spectra [10]. In order to obtain good agreement with the measurements for the Zn-C and Zn-I bond distances of IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e), it was found necessary to include the scalar relativistic effects in Zn (on top of those in I) in the calculations. Two modes of activation of the C-I bond of methyl iodide by a zinc atom in the excited triplet state have been identified: the direct insertion of triplet atomic zinc into the C-I bond, and S\u003csub\u003eN\u003c/sub\u003e2-like displacement, generating weakly bound intermediates, IZn\u0026hellip;CH\u003csub\u003e3\u003c/sub\u003e (\u003csup\u003e3\u003c/sup\u003eA\u0026prime;) and ZnCH\u003csub\u003e3\u003c/sub\u003e\u0026hellip;I (\u003csup\u003e3\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e), respectively. We have suggested the recombination of the radical fragments (as resulting from the dissociation of the triplet intermediates) followed by the relaxation to IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) as its possible formation scenario. More work is needed to examine the feasibility of the IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e) formation that involves atomic Zn in the excited \u003csup\u003e1\u003c/sup\u003eP state [10], which is beyond the scope of this study.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eSupplementary information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSupplementary Information associated with this article is available.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the Wroclaw Centre for Networking and Supercomputing, WCSS.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eCrespo M, Mart\u0026iacute;nez M, Nabavizadeh SM, Rashidi M (2014) Kinetico-mechanistic studies on C-X (X=H, F, Cl, Br, I) bond activation reactions on organoplatinum (II) complexes. 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Ed. 57 (2018) 4760-4764.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"C-I bond activation. methylzinc iodide monomer. CCSD(T). CH3I-Zn van der Waals complex. ZnI radical","lastPublishedDoi":"10.21203/rs.3.rs-6330328/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6330328/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eActivation of C-X bonds (X\u0026thinsp;=\u0026thinsp;halogen) by transition metal species is pivotal in many catalytic processes of industrial significance such as cross-coupling reactions. The relatively simple gas-phase system for studying the activation of the C-I bond (X\u0026thinsp;=\u0026thinsp;iodine), which was examined experimentally (J Phys Chem A 118:11204\u0026ndash;11210) and is amenable to a high-level ab initio theoretical study, involves the reaction of methyl iodide (CH\u003csub\u003e3\u003c/sub\u003eI) with atomic zinc to form the methylzinc iodide monomer [IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e)]. Although a qualitative explanation of the underlying formation mechanism was given by the original authors, the pathways that play a role in generating the product molecule have not been well investigated in previous research. An important task here was to determine the ways by which the C-I bond of methyl iodide is activated by atomic zinc, and to suggest a plausible mechanism of formation of IZnCH\u003csub\u003e3\u003c/sub\u003e(X\u003csup\u003e1\u003c/sup\u003eA\u003csub\u003e1\u003c/sub\u003e). This task was performed using the CCSD(T) method (the coupled cluster singles and doubles with perturbative triples procedure) along with the correlation-consistent basis sets (through aug-cc-pV-5Z) and relativistic pseudopotentials on the I and Zn atoms.\u003c/p\u003e","manuscriptTitle":"New mechanistic insights into the gas-phase formation of methylzinc iodide via methyl iodide C-I activation with atomic zinc in excited triplet state as offered by the CCSD(T) study","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-22 10:40:24","doi":"10.21203/rs.3.rs-6330328/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"767b6985-81c2-4519-920d-9bf8567e4467","owner":[],"postedDate":"April 22nd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-03-07T07:54:39+00:00","versionOfRecord":[],"versionCreatedAt":"2025-04-22 10:40:24","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6330328","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6330328","identity":"rs-6330328","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}