Mechanism and kinetics of the reaction of OH radicals with glyoxal and methylglyoxal: a quantum chemistry + CVT/SCT approach.

A theoretical study of the mechanism and kinetics of the OH hydrogen abstraction from glyoxal and methylglyoxal is presented. Optimum geometries, frequencies, and gradients have been computed at the BHandHLYP/6-311++G(d,p) level of theory for all the stationary points, as well as for 12 additional points along the minimum energy path (MEP). Energies were obtained by single-point calculations at the above geometries using CCSD(T)/ 6-311++G(d,p) to produce the potential energy surface. The rate coefficients were calculated for the temperature range 200-500 K by using canonical variational theory (CVT) with small-curvature tunneling (SCT) corrections. Our analysis suggests a stepwise mechanism, which involves the formation of a reactant complex. The overall agreement between the calculated and experimental kinetic data is very good. This agreement supports the reliability of the Arrhenius parameters of the glyoxal + OH reaction that are proposed in this work for the first time. The Arrhenius expressions that best describe the studied reactions are k1 = (9.63 +/- 0.23) x l0(-13)exp[(517 +/- 7)/T] and k2 = (3.93 +/- 0.11) x 10(-13)exp[(1060 +/- 8)/T]cm3 molecule(-1)s(-1) for glyoxal and methylglyoxal, respectively.