Theoretical study on the reaction of tropospheric interest: hydroxyacetone + OH. Mechanism and kinetics.

A theoretical study of the mechanism and kinetics of the OH hydrogen abstraction from hydroxyacetone is presented. Optimum geometries and frequencies have been computed at the BH and HLYP/6-311++G(d,p) level of theory for all stationary points. Energy values have been improved by single-point calculations at the above geometries using CCSD(T)/ 6-311++G(d,p). The rate coefficients are calculated for the temperature range 280-500 K by using conventional transition state theory (TST), including tunneling corrections. Our analysis supports 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. Four experimental values of the rate constant at 298 K have been previously reported: three of them in great agreement (approximately 3 x 10(-12) cm(3) molecule(-1) s(-1)), and one of them twice larger. The calculations in the present work support the smaller value. A curved Arrhenius plot was found in the studied temperature range; thus the expression that best describes the obtained data is k(280-500)(overall) = 5.29 x 10(-23)T(3.4)e(1623/T) cm(3) molecule(-1) s(-1). The activation energy was found to vary with temperature from -1.33 to +0.15 kcal/mol.