Quantum Chemical Conformational Analysis of 1, 2-Ethanediol: Correlation and Solvation Effects on the Tendency To Form Internal Hydrogen Bonds in the Gas Phase and in Aqueous Solution

Correlated ab initio calculations with very large (correlation-consistent polarized valence triple-ζ) basis sets predict that 1, 2-ethanediol adopts a gas-phase population of conformers at 298 K comprised of rotamers 98% gauche and 2% trans about the C-C bond. Gauche conformers that have internal hydrogen bonds make up 83% of the total population. Changes in relative energy of up to 0.6 kcal/mol are observed upon decreasing the size of the basis set to correlation-consistent polarized double-ζ (which is still larger than commonly used polarized double-ζ basis sets), illustrating the difficulty of even gas-phase conformational analysis in this seemingly simple molecule; the extra variational freedom and more complete polarization space in the larger basis stabilizes trans hydroxyl conformations and increases by a factor of 2 both the predicted fractional population of trans C-C rotamers and the predicted population of conformers with no internal hydrogen bond. Solvation effects were studied using the SMx series of quantum statistical aqueous solvation models. By adding calculated free energies of solvation to gas-phase free energies, it is found that the trans population increases from 2 to 12%, and the portion of conformers having no internal hydrogen bond increases from 17 to 25%. The calculated results are in reasonable agreement with experimental results both in the gas phase and in aqueous solution. The results provide a consistent picture of the competition between the various effects (electronic energies, zero point effects, thermal vibrational-rotational free energy components, and electric polarization and first hydration shell contributions to solvation free energies) that, when combined with the proper statistics, contribute to determining the populations of all possible isomers in aqueous solution. Calculated relative solvation free energies for gauche vs trans C-C torsion are also in good agreement with classical Monte Carlo and molecular dynamics simulation results.