Thermochemistry and transition-state properties for reactions of OH radicals with a series of halogen-substituted acetaldehydes have been calculated 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 increase of the formyl C-H bond dissociation energies. Polarity effect is another possible factor which controls the reaction rates, as indicated 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 discussed. The possibility of an addition-elimination mechanism is considered and seems less probable than abstraction.
|Number of pages||7|
|Journal||Journal of physical chemistry|
|Publication status||Published - Jan 1 1993|
ASJC Scopus subject areas
- Physical and Theoretical Chemistry