Ab initio and kinetic study of the reaction of ketones with ȮH for T = 500–2000 K. Part I: hydrogen-abstraction from H3CC(O)CH3–x(CH3)x, x = 0 ↦ 2

Abstract

A theoretical study is presented of the mechanism and kinetics of the reactions of the hydroxyl radical with three ketones: dimethyl (DMK), ethylmethyl (EMK) and iso-propylmethyl (iPMK) ketones. CCSD(T) values extrapolated to the basis set limit are used to benchmark the computationally less expensive methods G3 and G3MP2BH&H, for the DMK + OH reaction system. These latter methods are then used in computations involving the reactions of the larger ketones. All possible abstraction channels have been modeled. A stepwise mechanism involving the formation of a reactant complex in the entrance channel and a product complex in the exit channel has been recognized in part of the abstracting processes. High-pressure limit rate constants of the title reactions have been calculated in the temperature range of 500-2000 K using the Variflex code including Eckart tunneling corrections. Variable reaction coordinate transition state theory (VRC-TST) has been used for the rate constants of the barrier-less entrance channel. Calculated total rate constants (cm(3) mol(-1) s(-1)) are reported as follows: k(DMK) = 1.32 × 10(2)×T(3.30)exp(503/T), k(EMK) = 3.84 × 10(1)×T(3.51)exp(1515/T), k(iPMK) = 2.08 × 10(1)×T(3.58)exp(2161/T). Group rate constants (on a per H atom basis) for different carbon sites in title reactions have also been provided.