AB-INITIO STUDY OF CONJUGATIVE AND NEGATIVE HYPERCONJUGATIVE INTERACTIONS IN DISUBSTITUTED RADICALS

Electronic interactions in disubstituted radicals CHXY (X, Y is F, OH or NH2) have been probed by examining the energetics of numerous confo rmations at the UMP3/6-31G(d)//ROHF/4-31G level. Negative hyperconjuga tive interactions in these radicals are indicated to be comparable in magnitude to pi conjugation. Most of these disubstituted radicals are predicted to adopt unsymmetrical geometries in order to benefit from b oth 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 energetic s of the dihydroxymethyl radical have been used to explain the observe d stereoselectivity of a few reactions involving sugar-based radicals. The non-additivities of substituent effects in the disubstituted radi cals have been compared with the magnitude of the anomeric effect in t he corresponding disubstituted methanes. The inter-substituent interac tions in the radicals should not be overlooked when using these system s as reference species in estimating other non-additive effects, such as the captodative effect.