SOLVENT EFFECT ON THE CONFORMATION OF 1,2-DIMETHOXYPROPANE

Conformational energies for the central C-C bond of 1,2-dimethoxypropane (1,2-DMP), a monomer model compound of poly(propyrene oxide), placed in the gas phase as well as in solutions have been evaluated from the temperature dependence of H-1 NMR vicinal coupling constants. The conformation was found to exhibit remarkable solvent dependence; as the dielectric constant of the medium increases from 1.0 (the vapor phase) to 45.0 (dimethyl sulfoxide), [E(g)+], the mean energy difference between the gauche+ and trans states, decreases from -0.06 +/- 0.05 to -0.30 +/- 0.05 kcal mol(-1), and [E(g)-] from +0.50 +/- .0.10 to -0.09 +/- 0.05 kcal mol(-1) Here the definition of the conformers is based on the (R)-optical antipode. Thus the 1,2-DMP molecule was shown to become more polar with increasing solvent polarity.