Authors: Rubik Asatryan | Jason M. Hudzik | Venus Amiri | Mark T. Swihart
Source: Molecules (article, 2025-01-24)
Field: Chemistry — Organic Chemistry

Internally Catalyzed Hydrogen Atom Transfer (I-CHAT)—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 key chain-branching occurs when the second oxygenation adduct (OOQOOH) is isomerized releasing an OH radical and a key <i>ketohydroperoxide</i> (KHP) intermediate. The subsequent homolytic dissociation of relatively weak O-O bonds in KHP generates two more radicals in the oxidation chain leading to ignition. Based on the recently introduced intramolecular "catalytic hydrogen atom transfer" mechanism (<i>J. Phys. Chem.</i> 2024, <i>128</i>, 2169), abbreviated here as <i>I-CHAT</i>, we have identified a novel unimolecular decomposition channel for KHPs to form their classical isomers<i>-enol hydroperoxides</i> (EHP). The uncertainty in the contribution of enols is typically due to the high computed barriers for conventional ("direct") keto-enol tautomerization. Remarkably, the <i>I-CHAT</i> dramatically reduces such barriers. The novel mechanism can be regarded as an <i>intramolecular</i> version of the <i>intermolecular</i> relay transfer of H-atoms mediated by an external molecule following the general classification of such processes (<i>Catal. Rev.-Sci. Eng.</i> 2014, <i>56</i>, 403). Here, we present a detailed mechanistic and kinetic analysis of the <i>I-CHAT</i>-facilitated pathways applied to <i>n</i>-hexane, <i>n</i>-heptane, and <i>n</i>-pentane models as prototype molecules for gasoline, diesel, and hybrid rocket fuels. We particularly examined the formation kinetics and subsequent dissociation of the γ-enol-hydroperoxide isomer of the most abundant pentane-derived isomer γ-C5-KHP observed experimentally. To gain molecular-level insight into the <i>I-CHAT</i> catalysis, we have also explored the role of the internal catalyst moieties using truncated models. All applied models demonstrated a significant reduction in the isomerization barriers, primarily due to the decreased ring strain in transition states. In addition, the longer-range and sequential H-migration processes were also identified and illustrated via a <i>combined double</i> keto-enol conversion of heptane-2,6-diketo-4-hydroperoxide as a potential chain-branching model. To assess the possible impact of the <i>I-CHAT</i> channels on global fuel combustion characteristics, we performed a detailed kinetic analysis of the isomerization and decomposition of γ-C5-KHP comparing <i>I-CHAT</i> with key alternative reactions-direct dissociation and Korcek channels. Calculated rate parameters were implemented into a modified version of the <i>n</i>-pentane kinetic model developed earlier using RMG automated model generation tools (<i>ACS Omega,</i> 2023, <i>8,</i> 4908). Simulations of ignition delay times revealed the significant effect of the new pathways, suggesting an important role of the <i>I-CHAT</i> pathways in the low-T combustion of large alkanes. Chemistry ; Tautomer ; Intramolecular force ; Dissociation (chemistry) ; Photochemistry ; Radical ; Enol ; Catalysis ; Hydrogen atom ; Medicinal chemistry ; Stereochemistry ; Organic chemistry ; Alkyl Free Radicals and Antioxidants ; Advanced Combustion Engine Technologies ; Chemical Reactions and Mechanisms