Conformational characteristics of dimethoxymethane based upon ab initio electronic structure calculations

Abstract

The conformational properties of dimethoxymethane (DMM) have been determined from ab initio electronic structure calculations as a critical step in the characterization of the conformational statistics of poly(oxymethylene). The preference of the gauche (g) conformation of the C⋯O-C⋯O bond over the trans (t) conformation, as denoted by the energy ΔEgt of the gt conformer relative to the gg conformer, was found to depend strongly on basis set size, with the energy decreasing from 4.7 kcal/mol for a 4-31G MP2 calculation to 2.8 kcal/mol using a D95+(2df,p) basis set, also at the MP2 level. Electron correlation effects were also found to be important but were accounted for accurately at the MP2 level; calculations of correlation effects beyond the MP2 level resulted in only a slight decrease in ΔEgt. These effects led to a best estimate for ΔEgt of ca. 2.5 kcal/mol. Such a strong preference for the g conformation results in a very steep energy well for the DMM gg conformer relative to the remaining conformers. In terms of a rotational isomeric state (RIS) model, this necessitates the inclusion of preexponential factors for the statistical weights which deviate significantly from unity. The energies and preexponential factors which were estimated from the ab initio calculations yielded RIS conformer populations for DMM that reproduce the gas-phase dipole moment and NMR vicinal coupling constant as a function of temperature, in good agreement with experiment. © 1994 American Chemical Society.