Relative reactivity and regioselectivity of halogen-substituted ethenes and propene toward addition of an OH radical or O (3P) atom: An ab initio study

This paper describes the results of ab initio investigations on the reactions of OH radical/O (3P) atom addition to halogen-substituted ethenes (CH2CHX, CH2CX2; X=F, Cl, Br) and propene. Two reaction pathways, addition to the substituted carbon atom (α-site) and addition to the non-substituted carbon atom (β-site), have been examined. The energetic of each reaction coordinate has been evaluated at the computational level of PMP2/aug-cc-pVTZ//MP2/6-311+G(2d,p). Our calculations suggested that the reactions of OH radical addition should be more exothermic than those of the O (3P) atom addition and that the α-addition products should be lower in energy than the corresponding β-addition products. Relative stabilities of the transition states for the α- and β-addition channels can be discussed in terms of the exothermicities of the reaction channels and the relative spin densities in the 3ππ* states of the reactant ethene molecules. For the reactions of the chloro- and bromo-substituted ethenes, the transition states for the α-addition were found to be higher in energy than those for the β-addition, but for the reactions of the fluoro-substituted ethenes and propene, the differences in energy between the transition states for the α- and β-addition channels were found to be rather small.