REACTIVITY AND REGIOSELECTIVITY OF HYDROXYL RADICAL-ADDITION TO HALOGENATED ETHENES

The reactions of hydroxyl radical with ethene, fluoroethene, and chlor oethene have been studied by quantum chemical methods. Reactants, prer eaction complexes, transition-state structures, and products were opti mized and vibrational frequencies were calculated at the UMP2/6-311+G( 2d,p) level. Transition-state structures are significantly different f rom the prereaction complexes formed on the reactant side of the MEP. The convergence of barrier heights and reaction enthalpies has been sy stematically investigated with respect to the size and quality of basi s set and the treatment-of correlation energy. The best agreement with experimental results is found at the MP2/aug-cc-pVTZ level of theory. Regioselectivity is discussed in terms of two properties of the radic al and the investigated alkenes. The first factor is the relative spin density in the (3) pi pi state of the alkene. The second factor is t he relative strengths of the product C-O bond, i.e., relative stabilit y of the corresponding radical product. In the case of fluoroethene th ese two effects oppose each other and regioselectivity is negligible. In the case of chloroethene spin density is the dominant factor and th e addition of OH radicals to the unsubstituted carbon atom is preferre d.