Ab initio study of conjugative and negative hyperconjugative interactions in disubstituted radicals

Abstract

Electronic interactions in disubstituted radicals CHXY (X, Y is F, OH or NH₂) have been probed by examining the energetics of numerous conformations at the UMP3/6-31G(d)//ROHF/4-31G level. Negative hyperconjugative interactions in these radicals are indicated to be comparable in magnitude to π conjugation. Most of these disubstituted radicals are predicted to adopt unsymmetrical geometries in order to benefit from both types of stabilizing interactions. These results have interesting implications for reactions involving carbohydrate radicals, for which the simple systems serve as models. Thus, the structures and energetics of the dihydroxymethyl radical have been used to explain the observed stereoselectivity of a few reactions involving sugar-based radicals. The non-additivities of substituent effects in the disubstituted radicals have been compared with the magnitude of the anomeric effect in the corresponding disubstituted methanes. The inter-substituent interactions in the radicals should not be overlooked when using these systems as reference species in estimating other non-additive effects, such as the captodative effect.