Mechanisms for the Reactions of Hydroxyl Radicals with Acrolein: A Theoretical Study.

Three low-energy pathways for the reaction of HO(•) with acrolein, a key reaction in atmospheric environments, have been investigated by means of quantum-mechanical electronic structure methods (UQCISD and RQCISD(T)). The first step of all the reaction pathways studied involves the barrierless formation of a prereaction loosely bound complex in the entrance channel, lying a few kcal/mol below the energy of the reactants. The lowest-energy barrier pathway at 0 K is found to be the HO(•) abstraction of the aldehydic H-atom through a transition-state structure lying 1.1 kcal/mol below the energy of the reactants. The addition of HO(•) to the terminal carbon atom of the C═C double bond proceeds via a transition-state structure lying 0.7 kcal/mol below the energy of reactants at 0 K, whereas the HO(•) addition to the central carbon atom takes place via a transition-state structure lying 0.8 kcal/mol above the energy of the reactants at 0 K. On the basis of conventional transition-state theory calculations at 298 K, it is predicted that 74.5% of the HO(•) reaction with acrolein proceeds via abstraction of the aldehydic H-atom, 24.2% via HO(•) addition to the terminal carbon atom of the double bond, and 1.3% via HO(•) addition to the central carbon atom of the double bond. These results are in close agreement with available experimental data.