Glycolaldehyde + OH gas phase reaction: a quantum chemistry + CVT/SCT approach.

We present a theoretical study of the mechanism and kinetics of the OH hydrogen abstraction from glycolaldehyde. Optimum geometries, frequencies, and gradients have been computed at the BHandHLYP/6-311++G(d,p) level of theory for all stationary points, as well as for additional points along the minimum energy path (MEP). Energies are 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 are 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 involving the formation of a reactant complex in the entrance channel and a product complex in the exit channel, for all the modeled paths. The overall agreement between the calculated and experimental kinetic data that are available at 298 K is very good. This agreement supports the reliability of the parameters obtained for the temperature dependence of the glycolaldehyde + OH reaction. The expressions that best describe the studied reaction are k(overall) = 7.76 x 10(-13) e(1328/)(RT) cm(3).molecule(-1).s(-1) and k(overall) = 1.09 x 10(-21)T(3.03) e(3187/)(RT) cm(3) molecule(-1) s(-1), for the Arrhenius and Kooij approaches, respectively. The predicted activation energy is (-1.36 +/- 0.03) kcal/mol, at about 298 K. The agreement between the calculated and experimental branching ratios is better than 10%. The intramolecular hydrogen bond in OO-s-cis glycolaldehyde is found to be responsible for the discrepancies between SAR and experimental rate coefficients.