AB-INITIO STUDIES OF THE EXOCYCLIC HYDROXYMETHYL ROTATIONAL SURFACE IN ALPHA-D-GLUCOPYRANOSE

The potential energy surface for rotation of the exocyclic hydroxymeth yl group of alpha-D-glucopyranose has been studied using ab initio qua ntum mechanical methods. Relevant stationary points, including for the first time rotational transition states, have been characterized by f ull geometry optimization using basis sets ranging in quality from 6-3 1G(d) to 6-311(2d,1p). Effects of dynamical electron correlation on bo th the geometric structures and the energy surface are also investigat ed using second-order Moller-Plesset perturbation theory (MP2) and den sity functional methods (BLYP). A total of six stationary points along the hydroxymethyl rotational surface, including three minima and thre e transition states, were identified. The effects of basis set augment ation and electron correlation on the relative energies are small; the relative energies for each stationary point vary by less than 5 kJ mo l(-1) for all levels of theory considered. Final energetic barriers to hydroxymethyl rotation ranged from 15 to 29 kJ mol(-1) Differences be tween these barriers and previously reported ab initio results on a ca rbohydrate model compound, 2-(hydroxymethyl)tetrahydropyran, as well a s energies calculated using force field methods, are discussed.