Experimental and Computational Studies of the Kinetics and Mechanisms for C6H5 Reactions with Acetone-h6 and -d6

Abstract

Kinetics and mechanisms for the C6H5 + CH3C(O)CH3 and CD3C(O)CD3 reactions have been investigated by cavity ring-down spectrometry (CRDS) and hybrid density functional theory (DFT) calculations. The rate constants measured for the two reactions at the constant pressure of 45 Torr using Ar as a carrier gas can be represented by the following Arrhenius expressions in units of cm3 mol-1 s-1: kH = (4.2 ± 0.4) × 1011 exp[-(955 ± 30)/T] and kD = (5.1 ± 0.6) × 1011 exp[-(1114 ± 43)/T] in the temperature ranges of 299−451 and 328−455 K, respectively. The significant kinetic isotope effect observed suggests that H-abstraction is the major path, according to their activation energies, with the C6H5 + CD3C(O)CD3 reaction being higher by 0.3 kcal/mol. DFT calculations at the B3LYP/aug-cc-PVTZ//B3LYP/cc-PVDZ level of theory indicate the reaction can in principle take place by three reaction paths, one H-abstraction and two addition reactions to the CO double bond at both C and O atom sites, with the latter two processes having significantly higher reaction barriers. The rate constants predicted by canonical variational transition state theory (CVT) with small curvature tunneling (SCT) corrections for the direct H- and D-abstraction reactions are in reasonable agreement with the experimental data after slightly decreasing the calculated barriers from 3.9 kcal/mol to 3.3 kcal/mol and from 4.7 kcal/mol to 4.1 kcal/mol, respectively. The predicted rate constants for C6H5 + CH3COCH3 in the temperature range of 298−1200 K can be represented reasonably by the expression, kH = (1.7 ± 0.6) × 10-1 T (4.2 ± 0.1) exp[-(466 ± 26)/T] cm3 mol-1 s-1.