THEORETICAL CALCULATIONS ON 1,2-ETHANEDIOL - GAUCHE TRANS EQUILIBRIUM IN GAS-PHASE AND AQUEOUS-SOLUTION

The gauche-trans equilibrium of 1,2-ethanediol was investigated in the gas phase and in aqueous solution with ab initio quantum chemical calculations and Monte Carlo simulations. MP2/6-31G*//6-31G* energy calculations and HF/6-31G* normal frequency analyses were carried out for the tGg', gGg', g'Gg' (gauche), and tTt (trans) isomers. The free energy of the most stable tGg' isomer with an intramolecular hydrogen bond is more negative by 2.8 kcal/mol than the trans form at T = 298 K. This coincides with the experimental finding of no trans rotamer in the gas phase. The effect of hydration on the equilibrium ratio was followed by statistical perturbation calculations using the Monte Carlo method at T = 298 K and P = 1 atm. In solution only three conformers were considered. On the basis of the gas-phase rigid-rotation potential map, two different paths were considered for the tTt to tGg' transformation and a single path for the tGg' to gGg' change. The solvation free energy for the trans rotamer is more negative by 1.2 kcal/mol than that for tGg'. Also gGg' is stabilized by about 1.0 kcal/mol upon solvation. Considering both total internal and hydration free energies, the solution contains about 97% gauche conformers. This is basically in accordance with recent NMR results. The solution structure was analyzed with use of energy pair and radial distribution functions obtained after taking 6000K configurations. Positions of the water molecules around the solutes were determined by statistical averaging of about 4500 snapshots of the solution configurations. The first hydration shell of the 1,2-ethanediol solute contains approximately six water molecules. There are, on average, four water molecules strongly hydrogen bonded to the solute in the tTt conformation. This number decreases for gauche conformers with intramolecular hydrogen bonds in solution. The water-solute hydrogen bonds are slightly bent with a favorable O...H distance of 1.8-1.9 angstrom. Water molecule acceptors in these bonds are more localized than those acting as proton donors.