Features of the potential energy surface for the reaction of OH radical with acetone

Abstract

The mechanism of the reaction of OH with acetone has been studied by quantum chemical computations. 21 stationary points (among them reactant complexes, reaction transition states, intermediate complexes and product complexes) have been characterised on the potential energy surface of the reaction. The MP2 method with 6-31G(d,p) basis set was employed for geometry optimisation. Electronic energies were obtained at the CCSD(T)/6-311G(d,p) level of theory. Hydrogen abstraction was found to occur through two complex mechanisms; no transition state for direct abstraction could be located. Minimum energy path analyses have revealed two distinct pathways which lead to CH₃ (+CH₃COOH) formation. One of them sets out the abstraction channel and proceeds via intermolecular complexes and the other one involves addition of OH to the carbonyl double bond and subsequent decomposition of the adduct hydroxy-alkoxy radical. The rate limiting steps involve large energy barriers and, consequently, these pathways do not explain the high methyl yields observed experimentally at and below room temperature. Characteristic for the reaction of OH with acetone is the existence of numerous hydrogen-bridged complexes on the potential energy surface that are stabilised by as much as 3.2–26.6 kJ mol⁻¹ binding energy. Some properties of these complexes and their possible role in the molecular mechanism of the reaction are discussed.