Conformational studies of ethylene glycol and its two methyl ether derivatives

Abstract

Vicinal CH₂-CH₂ coupling constants were determined for the two compounds at 28°C in solvents with dielectric constants ranging from 2 to 44, and over a wide range of temperatures. The average coupling constants were analysed in terms of a trans-gauche conformational equilibrium to obtain a ΔE for this equilibrium. Solvent interactions were taken into account by using a simple dielectric theory, considering the solvent as a continuous dielectric medium interacting with the dipoles and the quadrupoles of the solute molecule. In non-polar solvents, the gauche form is preferred for MOE (ΔE = -0·9 to -1·0 kcal/mol), presumably because of intramolecular hydrogen bond formation, while the preference is weaker for DME (ΔE = -0·5 to -0·6 kcal/mol). The data are in reasonable agreement with the theoretical analysis in the preceding paper. For molecules of the type treated here, corrections of the simple dielectric theory are necessary to account for internal hydrogen bonding, the existence of several rotational isomers in each of the gauche and trans forms, and for temperature and solvent dependence of the molecular geometry in the individual rotamers (variations of the dihedral angles). This restricts the use of the vapour phase as a state of reference.