B3LYP/6-311++G** study of α- and β-D-glucopyranose and 1,5-anhydro-D-glucitol:: 4C1 and 1C4 chairs, 3,OB and B3,0 boats, and skew-boat conformations

Geometry optimization, at the B3LY-P/6-311++G(**) level of theory, was carried out on C-4(1) and C-1(4) chairs, B-3,B-O and B-3,B-O boats, and skew-boat conformations of alpha- and beta-D-glucopyranose. Similar calculations on 1,5-anhydro-D-glucitol allowed examination of the effect of removal of the 1-hydroxy group on the energy preference of the hydroxymethyl rotamers. Stable minimum energy boat conformers of glucose were found, as were stable skew boats, all having energies ranging from similar to4-15 kcal/mol above the global energy C-4(1) chair conformation. The C-1(4) chair electronic energies were similar to5-10 kcal/mol higher than the C-4(1) chair, with the C-1(4) alpha-anomers being lower in energy than the beta-anomers. Zero-point energy, enthalpy, entropy, and relative Gibbs free energies are reported at the harmonic level of theory. The a-anomer C-4(1) chair conformations were found to be similar to1 kcal/mol lower in electronic energy than the beta-anomers. The hydroxymethyl gt conformation was of lowest electronic energy for both the alpha- and beta-anomers. The glucose alpha/beta anomer ratio calculated from the relative free energies is 63/37%. From a numerical Hessian calculation, the tg conformations were found to be similar to0.4-0.7 kcal/mol higher in relative free energy than the gg or gt conformers. Transition-state barriers to rotation about the C-5-C-6 bond were calculated for each glucose anomer with resulting barriers to rotation of similar to3.7-5.8 kcal/ mol. No energy barrier was found for the path between the alpha-gt and alpha-gg B-3,B-O boat forms and the equivalent C-4(1) chair conformations. The alpha-tg conformation has an energy minimum in the S-1(3) twist form. Other boat and skew-boat forms are described. The beta-anomer boats retained their starting conformations, with the exception of the beta-tg-B-3,B-O boat that moved to a skew form upon optimization. Published by Elsevier Ltd.