GENERALIZED ANOMERIC EFFECTS AND HYPERCONJUGATION IN CH2(OH)2, CH2(SH)2, CH2(SEH)2, AND CH2(TEH)2

The origin of generalized anomeric effects (energy differences between gauche and anti structures) in methanediol and its sulfur, selenium, and tellurium derivatives has been examined in detail by ab initio calculations through the MP2 level and by NBO analyses of the Hartree-Fock wave functions. Gauche preferences of SH, SeH, and TeH substituents decrease but do not vanish. The X lone pairs as well as the CH, CX, and XH bonding electrons are slighly delocalized (X = O, S, Se, and Te). If the wave functions are expanded in terms of localized and orthogonal molecular orbitals, these delocalizations are represented by hyperconjugative orbital interactions. p(X) --> sigma*CX and sigma(CH) --> sigma*XH hyperconjugation strongly favors gauche structures. However, other orbital interactions such as p(X) --> sigma*CH, sp(X) --> sigma*CX, and sigma(XH) --> sigma*CX are stronger in the anti structures and compensate the effects of the p(X) --> sigma*CX and sigma(CH) --> sigma*XH interactions to a large extent. Nonhyperconjugative contributions to the energy difference between the gauche and anti structures are small for methanediol. Thus, dipole repulsions do not account for the anomeric effect. NBO analysis of the C-O bond rotation energies in methanediol and fluoromethanol reveals that the V1, V2, and V3 Fourier components of the potential functions do not represent steric and electrostatic effects and the p(X) --> sigma*CX hyperconjugation contribution adequately. In contrast to methanediol, nonhyperconjugative contributions are significant for the sulfur, selenium, and tellurium derivatives. These contributions favor the anti conformers and account for the decreased gauche preferences of CH2(SH)2, CH2(SeH)2, and CH2(TeH)2. Generalized anomeric effects (conformational preferences) and nonadditive bond separation energy behavior have complex origins (of which p(X) --> sigma*CX hyperconjugation is only one contribution) and are not directly related.