CONFORMATIONS OF 1,2-DIMETHOXYETHANE FROM AB-INITIO ELECTRONIC-STRUCTURE CALCULATIONS

The conformational properties of 1,2-dimethoxyethane (DME) have been analyzed in detail using ab initio electronic structure calculations in order to better understand the influence of oxygen gauche effects on the conformation energies and populations of DME. Our calculations indicate that the energy of the gauche conformation of the O-C-C-O bond relative to the trans conformation, which reflects the strength of the oxygen gauche effect in DME, depends strongly not only on the basis set size but also on electron correlation effects. Specifically, the energy of the tgt conformation of DME relative to the ttt conformation was determined for various basis sets at both the SCF and MP2 levels. MP2 level calculations with a D95+(2dfp) basis set, a Dunning double-zeta basis set of the form (10s6p2d1f/4s1p)/[5s2p2d1f/2s1p], yield an energy of ca.0.1 kcal/mol for the gauche conformation, as compared with 0.75 kcal/mol for MP2 level calculations with the smaller D95** basis set and 1.0 kcal/mol for SCF level calculations employing the D95+(2dfp) basis set. A salient result of our investigation is the determination that the energy of the tg+/-g-/+ conformation lies only ca. 0.2 kcal/mol above that of the ttt conformation, apparently due to strong O...H attractions. Finally, we find that the conformer populations of DME obtained from conformational energies determined at the MP2 level with a D95+(2dfp) basis set are in good agreement with those derived from electron diffraction experiments and can be used to successfully reproduce experimental values of NMR vicinal coupling constants and dipole moments for DME.