(702d) Intramolecular Catalytic Hydrogen Atom Transfer (CHAT): A Novel Mechanism Relevant to the Combustion of Traditional Fuels

Intramolecular catalysis (IntraCat) is the acceleration of a process at one site of a molecule catalyzed by a functional group in the same molecule. Here, we report a novel IntraCat mechanism involving intramolecular catalytic transfer of hydrogen atoms, when a reactive part of a molecule (chat-catalyst moiety or chat-agent, which may include peroxyl, carboxyl, and other ambivalent H-donor/acceptor groups) catalyzes an interconversion process, for instance, keto-enol tautomerization and cyclization in the same molecule, while recovering itself. In contrast to traditional IntraCat processes, however, CHAT is a unimolecular catalysis since the process occurs within a single molecule with no external agent being involved.

The CHAT bond-exchange mechanism can be regarded as an intramolecular version of the intermolecular relay transfer of H-atoms, which is mediated by a separate molecule (molecular catalyst), e.g., dihydrogen, water, various inorganic and carboxylic acids. A general systematization of such processes has been proposed earlier, as illustrated in the simplest case of the H₂-mediated processes termed dihydrogen catalysis [1]. Following this systematization, the CHAT catalysis is assigned to the category of relay-transfer of H-atoms.

Here, we provide a characteristic example of the chat-catalysis reactions involving keto-enol tautomerization of the pentane 2,4-ketohydroperoxide - a key intermediate in combustion of common hydrocarbon fuels, based on first-principles modeling and potential energy surface analyses. A significant reduction of the H-transfer barrier (by nearly a factor of 2) occurs due to the decreased ring strain in the transition state. The enol product is shown to feasibly decompose to form H₂O₂ - a mid-temperature chain branching agent.

The rate parameters for the model reaction are calculated and implemented in a kinetic model to analyze its influence upon global combustion characteristics.

1. Asatryan, E. Ruckenstein, Dihydrogen Catalysis: A Remarkable Avenue in the Reactivity of Molecular Hydrogen, Catal. Rev.–Sci. Eng. 2014, 56, 403-475