CONDENSED-PHASE EFFECTS ON THE CONFORMATIONAL EQUILIBRIUM OF ETHYLENE-GLYCOL

Density functional calculations for ethylene glycol (CH2OHCH2OH) in th e gas and in a dielectric medium are reported. The condensed-phase cal culations are based on the self-consistent reaction held approach and the environment has the dielectric constant of liquid methanol. NPT Mo nte Carlo simulations of ethylene glycol (ETG) in liquid methanol are also reported. The simulations were carried out for three conformers o f ETG (tGg', gGg', and tTt). Comparison between SCRF results for the c onformational equilibrium in the gas and in the dielectric suggests th at the tGg' conformer is slightly stabilized relative to the gGg' conf ormer in the solvent. However, the energy difference between them is l ess the 1.0 kJ/mol, which indicates that frequent interconversions bet ween the tGg' and gGg' conformers are expected in the condensed phase. The all-trans conformer (tTt) is higher than the most stable conforme r in the gas by 14 kJ/mol. Monte Carlo simulations predict that the tG g' and gGg' conformers have very similar energies in the solvent. Howe ver, the simulations also show, in agreement with experimental data, t hat the tTt conformer is stabilized in liquid methanol, relative to th e gas phase. The microscopic mechanism leading to the stabilization of the tTt conformer in the liquid is related to the differential hydrog en-bonding formation between the ETG conformers and the methanol molec ules. (C) 1996 John Wiley & Sons, Inc.