On the applicability of the molecular-mechanical force field (MM2) scheme to the gauche oxygen effect characteristic of glycol ethers such as 1,2-dimethoxyethane, 1,2-dimethoxypropane and 1,2-dimethoxy-2-methylpropane

Abstract

MM2 calculations have been carried out for a series of glycol ethers such as 1,2-dimethoxyethane (DME), 1,2-dimethoxypropane (DME) and 1,2-dimethoxy-2-methylpropane (DMMP). For DME, in accordance with the results of Baldwin [11], the observed energy of the gauche state ( E σ = −0.5 kcal mol −1) for the OCCO bond is found to be approximately reproduced by the calculation when the dipole—dipole interaction term is suppressed by adopting a relatively large dielectric constant (ϵ eff = 7.5). With the same value of ϵ eff, the gauche energy of DMMP is calculated to be Eψ = —0.9 kcal mol −1, which is at variance with the observed value of 0.5 kcal mol −1. In DMP, the two gauche forms around the OCCO bond are non-equivalent, and the stability of the conformers varies in the order g α &>; t &>; g ß. These experimental observations were not produced by the calculation. The MM2 program failed, in the state of the art, to provide an adequate explanation for the gauche oxygen effect observed in the aboved-mentioned glycol ethers as well as in there polymer analogs. It is pointed out that inclusion of an empirical correction term in the torsional potential for the individual rotational states offers a simple amendment which serves for practical purposes.