CAN SIMPLE QUANTUM-CHEMICAL CONTINUUM MODELS EXPLAIN THE GAUCHE EFFECT IN POLY(ETHYLENE OXIDE)

We report ab initio quantum-chemical calculations at the level of seco nd-order many-body perturbation theory aimed at the equilibrium betwee n the all-trans (ttt) and the trans-gauche-trans (tgt) conformations o f dimethoxyethane, which is the smallest analog of poly(ethylene oxide ). Solvent effects are accounted for by two continuum models: one assu mes statistically fluctuating electric fields around the dimethoxyetha ne molecule caused by the solvent, and the other is an Onsager reactio n-field model. We find the ttt conformer to be more stable in the gas phase. Use of the stochastic field model has no effect on the relative stabilities, whereas the reaction-field method brings the energy of t he tgt conformer down very dose to the ttt energy. We conclude that th e gauche effect in dimethoxyethane and, by analogy poly(ethylene oxide ), is mainly due to the presence of a polarizable environment and not to some intrinsic conformational preference.