Self-Catalyzed Intramolecular Hydrogen Atom Transfer (SCI-HAT) – A New Class of Reactions in Combustion Chemistry

The current paradigm of low-T combustion and autoignition of hydrocarbons is based on the sequential two-step oxygenation of fuel radicals. The addition of the first oxygen molecule forms a peroxy radical RO2, which isomerizes to a hydroperoxyalkyl radical (QOOH). The key chain-branching occurs when the second oxygenation adduct (OOQOOH) is isomerized releasing an OH radical and forming a key ketohydroperoxide (KHP) intermediate, O=POOH. Subsequent homolytic dissociation of relatively weak O-O bond in KHP generates two more radicals in the oxidation chain leading to ignition/explosion. Thus, the formation and consumption of KHPs is a key controlling process.We recently introduced a new type of intramolecular isomerization mechanism involving self-catalyzed migration of H-atoms relevant to keto-enol and other isomerization processes designated as “catalytic hydrogen atom transfer - CHAT” (J. Phys. Chem. 2024, 128, 2169), more adequately abbreviated here as SCI-HAT. On this basis, we have identified a new general unimolecular decomposition channel for the formation of enol hydroperoxides (EHP) - the classical isomers of KHPs using first-principles modeling and potential energy surface analysis. Even though the enols are currently involved in various combustion/flame chemistry models, their actual contribution in combustion processes mostly remains neglected, due to the high computed barriers for classical (“direct”) keto-enol tautomerization. Remarkably, the novel SCI-HAT mechanism dramatically reduces activation barriers for such a conversion in the case of EHPs. Here, we present detailed mechanistic and kinetic analysis of the SCI-HAT-facilitated pathways involving some models of n-hexane, n-heptane, and specifically n-pentane as a prototype molecule for gasoline, diesel and hybrid rocket fuels (HRF). We particularly examined the formation and subsequent dissociation kinetics of γ-enol-hydroperoxide (γ-EHP) isomer of the γ-KHP (γ-C5-KHP), the most abundant isomer of the pentane-derived ketohydroperoxides observed experimentally. The novel self-catalyzed bond-exchange mechanism can be regarded as an intramolecular version of the intermolecular relay transfer of H-atoms mediated by an external molecule (molecular catalyst), such as dihydrogen, water, acids, and even radicals. Earlier, we proposed a general systematization of such intermolecular processes illustrated in the simplest case of the H2-mediated reactions termed “dihydrogen catalysis” (Catal. Rev. - Sci. Eng. 2014, 56, 403). Following this systematization, the SCI-HAT catalysis can be assigned to the category of relay-transfer of H-atoms. To gain molecular level insight into the SCI-HAT catalysis, we have additionally explored the role of the catalytic moiety on SCI-HAT reactivity using selected small models. All applied models demonstrated significant reduction of the H-transfer barriers, primarily due to the decreased ring strain in transition states. The electronic and steric factors affecting reactivity and allowing this path to circumvent the geometric disadvantages of the uncatalyzed (direct) H-transfer processes, are also discussed. Depending on the dimensions and specific molecular parameters of the SCI-HAT catalytic moieties, the longer-range and sequential H-migration processes are also identified to extend the role of the new mechanism in combustion of large alkanes and paraffin-wax hybrid rocket fuels. Such processes are particularly illustrated by a combined double keto-enol conversion of heptane-2,6-diketo-4-hydroperoxide introducing a long-range H-migration as a potential chain-branching model.To assess the possible impact of the SCI-HAT channels on global fuel combustion characteristics, we present a detailed kinetic analysis of isomerization and decomposition of pentane 2,4-ketohydroperoxide comparing SCI-HAT with key alternative reactions, including direct dissociation and Korcek channels. Calculated rate parameters were implemented into a modified version of the n-pentane kinetic model developed earlier using RMG automated model generation software (ACS Omega, 2023, 8, 4908). Simulation of ignition delay times using such models revealed significant effects of the new pathways suggesting an important role of the SCI-HAT pathway in low-temperature combustion of large alkanes.