Ab Initio Molecular Orbital Calculation of Carbohydrate Model Compounds. 5. Anomeric, Exo-Anomeric, and Reverse Anomeric Effects in C-, N-, and S-Glycosyl Compounds

An ab initio study of the conformational behavior of α- and β-anomeric linkages in C-, N-, and S-glycosyl compounds has been carried out on axially and equatorially 2-substituted derivatives (2-ethyl, 2-methylamino, 2-thiomethyl, and 2-methylammonio) of tetrahydropyran as models. The geometry of the conformers about the anomeric C−X bond was determined by gradient optimization at the SCF level using the 6-31G* basis set. The potential of rotation has been calculated using the 6-31G* and 6-31+G* basis sets. Vibrational frequencies were calculated at the 6-31G* level and used to evaluate zero-point energies, thermal energies, and entropies for minima. Variations in calculated valence geometries for the compounds, display structural changes distinctive for the anomeric and exo-anomeric effects. Differences between bond lengths and bond angles for different conformers correlate with the importance of the lone pair delocalization interactions. The calculated conformational equilibria have been used to estimate...