THEORETICAL CALCULATIONS ON 1,2-ETHANEDIOL .2. EQUILIBRIUM OF THE GAUCHE CONFORMERS WITH AND WITHOUT AN INTRAMOLECULAR HYDROGEN-BOND IN AQUEOUS-SOLUTION

Ab initio calculations were carried out on the tGg, tGt, and gGg conformers of 1,2-ethanediol in the gas phase and then Monte Carlo simulations were carried out in aqueous solution. MP2/6-31G*//6-31G* results with zero-point and thermal corrections (T = 298 K) show that these conformers without intramolecular hydrogen bonding are higher in free energy by 3-4 kcal/mol than the most stable tGg' conformer. Hydration of the tGg conformer leads to stabilization by 5.2 kcal and is the most likely conformer in aqueous solution. It represents 64% of the conformer population considering tGg, gGg', tGg', tGt, tTt, and gGg conformers. This finding of the tGg-dominant conformer with a gauche O-C-C-O linkage and without intramolecular hydrogen bonding fits well with NMR and Raman experiments for the solution. The polar groups of the solute are hydrated by rive to seven water molecules and form about four hydrogen bonds with them. The most stable bonds are nearly linear O-H(alc)...O(water) arrangments, Hydrogen bonds to O(alc) with donor water molecules exhibit less defined geometry. Water molecules hydrating the gauche hydroxyl group are more strictly localized than those for a trans one. The number of water molecules in the nonpolar region is not effected by conformational changes of the solute considering rotation either about the C-C axes or about the C-O bonds.