ABINITIO MOLECULAR-ORBITAL CONFORMATIONAL-ANALYSIS OF PROTOTYPICAL ORGANIC-SYSTEMS .1. ETHYLENE-GLYCOL AND 1,2-DIMETHOXYETHANE

Ab initio calculations have been used to study the changes in energy of ethylene glycol and 1,2-dimethoxyethane as a function of rotation around the central C-C bond, Geometries have been fully optimized at the 3-21G and 6-31G* levels, and single-point calculations have been carried out at higher levels (up to 6-311++G** for ethylene glycol and 6-31+G* for 1,2-dimethoxyethane), including electron correlation up to MP4(SDTQ). For ethylene glycol, the H-O-C-C angles were started in a trans orientation to prevent intramolecular hydrogen bonding. In 1,2-dimethoxyethane, the C-O-C-C dihedral angles also were started in the trans orientation. At all levels of theory, both ethylene glycol and 1,2-dimethoxyethane slightly prefer a trans O-C-C-O orientation. For both molecules, the 3-21G relative energies are quite different from those calculated at the 6-31G* level, but all larger basis sets give relative energies which agree fairly well with the 6-31G* results. Electron correlation is shown to have a significant effect on the relative energies. The highest-level calculations for both ethylene glycol and 1,2-dimethoxyethane indicate that the trans-gauche energy difference is 0.4-0.5 kcal/mol. However, these values decrease as the basis set is increased, and, in the limits of infinite basis set and complete treatment of electron correlation, the trans-gauche difference for both molecules should be somewhat lower. Vibrational frequencies have been calculated for all conformers of both ethylene glycol and 1,2-dimethoxyethane; the effect of zero-point energies and vibrational enthalpies on the trans-gauche energy difference are quite small, but there is a more significant lowering of the barrier heights. To judge the importance of intramolecular hydrogen bonding in ethylene glycol, several lower-energy gauche O-C-C-O conformers which do possess intramolecular hydrogen bonds also were located. The global minimum has one H-O-C-C angle gauche and the other H-O-C-C angle trans, in agreement with experiment. The trans-trans-gauche conformer of 1,2-dimethoxyethane, with one gauche C-O-C-C angle, also was studied and was found to be approximately 1.5 kcal/mol above the all-trans global minimum. For both ethylene glycol and 1,2-dimethoxyethane, the MM2 force field does a reasonable job of reproducing the trans-gauche energy differences but is in poor agreement with the ab initio syn barriers to rotation. However, the MM3 barrier heights are in much better agreement with the ab initio data. Further, most of the other conformational energy differences also are better reproduced by MM3, which in many ways appears to provide a superior treatment for these 1,2-dioxy-substituted ethane derivatives.