Ab initio studies of the exocyclic hydroxymethyl rotational surface in alpha-D-glucopyranose

The potential energy surface for rotation of the exocyclic hydroxymethyl group of alpha-D-glucopyranose has been studied using ab initio quantum mechanical methods. Relevant stationary points, including for the first time rotational transition states, have been characterized by full geometry optimization using basis sets ranging in quality from 6-31G(d) to 6-311(2d,1p). Effects of dynamical electron correlation on both the geometric structures and the energy surface are also investigated using second-order Moller-Plesset perturbation theory (MP2) and density functional methods (BLYP). A total of six stationary points along the hydroxymethyl rotational surface, including three minima and three transition states, were identified. The effects of basis set augmentation and electron correlation on the relative energies are small; the relative energies for each stationary point vary by less than 5 kJ mol(-1) for all levels of theory considered. Final energetic barriers to hydroxymethyl rotation ranged from 15 to 29 kJ mol(-1) Differences between these barriers and previously reported ab initio results on a carbohydrate model compound, 2-(hydroxymethyl)tetrahydropyran, as well as energies calculated using force field methods, are discussed.