AB-INITIO AND DENSITY-FUNCTIONAL THEORY STUDY OF STRUCTURES AND ENERGIES FOR DIMETHOXYMETHANE AS A MODEL FOR THE ANOMERIC EFFECT

Ab initio molecular orbital theory and density functional theory calcu lations have been carried out on dimethoxymethane as a model for the a nomeric effect. We optimized various conformations of dimethoxymethane using Gaussian 92 at the MP2/6-311++G*, MP2/DZP + Diffuse, MP2/6-31G *, and Becke3LYP/631G** levels of theory. These methods were evaluate d based on their performance in reproducing structures and energies of dimethoxymethane when compared to experiment. This study also examine d the structure and energy of dimethoxymethane as a function of dihedr al angles for examining the anomeric effect at the MP2/6-31G* and Bec ke3LYP/6-31G* levels of theory. These calculations are qualitatively consistent with the anomeric effect observations in carbohydrates and with earlier calculations. Quantitative comparisons with earlier resul ts reveal that dimethoxymethane has lower total energies, smaller rota tional barriers, and shorter bond lengths than was previously determin ed. The Becke3LYP calculations were also compared to the MP2 results. The density functional theory findings show that the minimum energy st ructures correspond well with experimental and MP2 data. The total and relative energies from molecular orbital theory and density functiona l theory vary to some extent. Contour plots of the relative energies o f dimethoxymethane were evaluated and compared to a relative energy co ntour plot determined by MM3. The contour plots were similar, showing slightly larger changes in energies for the MP2 results than for the B ecke3LYP results, which in turn were slightly larger than the MM3 resu lts. Density functional theory calculations are an excellent alternati ve method of calculation due to increased speed and reliable accuracy of the density functional calculations. These results will serve as a benchmark for modeling the anomeric effect in carbohydrates. (C) 1996 by John Wiley & Sons, Inc.