BOND STRENGTH TRENDS IN HALOGENATED METHANOLS - EVIDENCE FOR NEGATIVEHYPERCONJUGATION

The hydroxyl bond energies in the series of fluorinated and chlorinate d methanols CFxH3-xOH and CClxH3-xOH have been examined using ab initi o molecular orbital theory. The O-H bond strength is found to increase with the degree of halogenation, with fluorine substituents having a greater effect than chlorine substituents. The bond strengthening is n ot, however, directly related to the degree of halogenation. Rather, t he bond energy appears to increase in a manner that coincides with the effectiveness of negative hyperconjugation between the methyl and hyd roxyl groups. The trends are consistent with the notion that bond stre ngthening arises from a decrease in effectiveness of negative hypercon jugation from the methanol to the methoxy radical, the difference bein g manifested in a modification of the O-H bond strength. Support for t his model is provided by the calculated bond length trends, conformati onal preferences, radical state energy separations, and orbital popula tions.