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Glycolaldehyde + OH Gas Phase Reaction:  A Quantum Chemistry + CVT/SCT Approach

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journal contribution
posted on 13.01.2005, 00:00 by Annia Galano, J. Raúl Alvarez-Idaboy, Ma. Esther Ruiz-Santoyo, Annik Vivier-Bunge
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 koverall = 7.76 × 10-13 e1328/RT cm3·molecule-1·s-1 and koverall = 1.09 × 10-21T3.03 e3187/RT cm3 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.