GLOBAL CONFORMATIONAL-ANALYSIS OF 1,2-ETHANEDIOL

Ab initio molecular orbital methods have been employed to study the global conformational structure of 1,2-ethanediol. The global torsional potential energy surface was obtained by scanning through the three torsional angles of the molecule with the remaining of the nuclear coordinates being energy minimized. Two types of minimum-energy paths for the conformational transformation along the torsional angle of the carbon-carbon bond were specifically calculated. All the geometries were optimized at the MP2(fu)/6-31** level, and the energies were calculated up to the MP2(fu)/6-311+G(2d,p) level. An analytical functional form which is quite adequate in representing the full three-dimensional torsional potential of 1,2-ethanediol was reported. Comparison was also made with the potential obtained by MM3 molecular mechanics method. The gas-phase thermodynamic functions of 1,2-ethanediol were calculated along the general line of separation of internal rotations from the rest of the vibrational motions. The differences between the present calculations and the results of harmonic oscillator approximation are quite substantial, especially in the values of entropy. The theoretical thermodynamic functions are in agreement with the available gaseous phase experimental data.