The reactions of hydroxyl radical with ethene, fluoroethene and chloroethene have been studied by quantum chemical methods. Reactants, pre-reaction complexes, transition-state structures, and products were optimized and vibrational frequencies were calculated at UMP2/6-311+G(2d, p) level. Transition-state structures are significantly different from the pre-reaction complexes formed on the reactant side of the MEP. The convergence of barrier heights and reaction enthalpies has been systematically investigated with respect to the size and quality of basis set and the treatment of correlation energy. The best agreement with experimental results is found at PMP2/aug-cc-pVTZ level of theory. Regioselectivity is discussed in terms of two properties of the radical and the investigated alkenes. The first factor is the relative spin density in the ^3pipi^* state of alkene. The second factor is the relative strengths of the product C-O bond, i.e. relative stability of the corresponding radical product. In the case of fluoroethene these two effects oppose each other and regioselectivity is negligable. In the case of chloroethene spin density is the dominant factor and the addition of OH radical to the unsubstituted carbon atom is preferred.

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