THEORETICAL-STUDY OF THE REACTIONS OF OH RADICALS WITH SUBSTITUTED ACETALDEHYDES

Thermochemistry and transition-state properties for reactions of OH ra dicals with a series of halogen-substituted acetaldehydes have been ca lculated using semiempirical molecular orbital methods. In the case of the direct hydrogen abstraction, the calculations reproduce very well the sequence of rate constants observed experimentally at 300 K: k[OH + CH3CHO] > k[OH + CH2ClCHO] > k[OH + CHCl2CHO] > k[OH + CCl3CHO] > k [OH + CF3CHO]. It is shown that this trend is caused partly by the inc rease of the formyl C-H bond dissociation energies. Polarity effect is another possible factor which controls the reaction rates, as indicat ed by the electronic charge distributions calculated for the reactants and transition states of the abstraction reactions. A correlation is proposed between the activation energies and the polarity of the newly formed bond O-H. Comparison with recent ab initio calculations is dis cussed. The possibility of an addition-elimination mechanism is consid ered and seems less probable than abstraction.