ORIGIN AND QUANTITATIVE MODELING OF ANOMERIC EFFECT

The origin of the anomeric effect has been examined by decomposing ab initio wavefunctions of dihydroxymethane (1) and dimethoxymethane (2) with the Natural Bond Orbital (NBO) method. The NBO analysis suggests that the anomeric effect arises from stabilizing 2-electron n(O)-->sigma(co)(*) delocalizations. Geometry optimization of 1 and 2 in the absence of all n(O)-->sigma* interactions lengthens CO bond lengths by 0.06 Angstrom and reduces OCO bond angles. Optimization in the absence of all hyperconjugative interactions results in more dramatic changes in geometry (Delta r(co) = 0.13 Angstrom, Delta angle(OCO) = 1.5 degrees - 6.0 degrees). An energetic analysis of the optimized NOSTAR pure Lewis structures indicate that 2-g(+)a is destabilized relative to 2-g(+)g(+) by 0.12 kcal/mole. This implies that dipole-dipole destabilization of 2-g(+)a is an order of magnitude less important than n(O)-->sigma(co)(*) interactions. Geometry optimization of 1 and 2 at MP4SDTQ/6-31G* appear to approach limiting values, suggesting that these wavefunctions are suitable for quantitative modeling of the anomeric effect in 1 and 2.