The Attractive Gauche Effect of Ethylene Oxides

Abstract

Conformational energies of monomeric (1,2-dimethoxyethane, DME) and trimeric (triglyme) model compounds of poly(ethylene oxide) have been evaluated by accurate ab initio molecular orbital (MO) calculations at the MP2/6-311++G(3df, 3pd)//HF/6-31G(d) level. The first-order interaction energies (E σ 's) for gauche states around the C-C bonds of DME and the terminal repeating unit of triglyme are ca. +0.1 kcal mol -1 , whereas the central unit of triglyme has a slightly negative E σ value of ca. -0.1 kcal mol -1 . For the C-C bond conformations of triglyme, the MO calculations exactly agree with NMR observations using a nonpolar solvent of cyclohexane-d 22 . The attractive gauche effect of the ethylene oxides has been shown to exist independently of intramolecular (C-H)···O hydrogen bonds.