AB-INITIO MOLECULAR-ORBITAL CALCULATION OF CARBOHYDRATE MODEL COMPOUNDS .2. CONFORMATIONAL-ANALYSIS OF AXIAL AND EQUATORIAL 2-METHOXYTETRAHYDROPYRANS

An ab initio study of the conformational behavior of alpha- and beta-glycosidic linkages has been carried out on axial and equatorial 2-methoxytetrahydropyrans as models. The geometry of the conformers about the glycosidic C-O bond was determined by gradient optimization at the SCF level using the 4-21G and 6-31G* basis sets and at the second-order Moller-Plesset level using the MP2/6-31G* basis set. The potential of rotation has been calculated using 4-21G, 6-31G*, 6-31+G*, MP2/6-31G*, and 6-311++G** basis sets. At all levels of theory, both axial and equatorial forms prefer the GT conformation around the C-O glycosidic bond. Conformational changes in bond lengths and angles at the anomeric center also display significant variations with computational methods, but structural trends ate in fair agreement with experiment. The correction for the effect of zero-point energy, thermal energy, and entropy on the axial-equatorial energy difference at the 6-31G* level is -0.63 kcal/mol. After these corrections to the energy difference calculated at the 6-31G* level, the axial form is favored by 0.84 kcal/mol, in reasonable agreement with experimental values of Delta G = 0.7-0.9 kcal/mol estimated for nonpolar solvents. Solvent effects reduce this energy difference; in the extreme case of water, a value of 0.24 kcal/mol was obtained. Complete torsional profiles have been obtained for rotation about the glycosidic C-O bond in eleven solvents, and the calculated energy differences are in fair agreement with experimental data on 2-alkoxytetrahydropyrans in solutions. The MM3 (epsilon = 1.5) force field reproduces the 6-31G* ab initio energy difference reasonably well, but barrier heights are in poor agreement with the ab initio data. The calculated energies and geometries provide an essential set of data for the parametrization of the behavior of acetal fragments in molecular mechanical force fields for carbohydrates.