SOLVENT EFFECTS .4. EFFECT OF SOLVENT ON THE E/Z ENERGY DIFFERENCE FOR METHYL FORMATE AND METHYL ACETATE

The effect of solvent on the difference in energy between the E and Z rotamers of methyl formate and methyl acetate has been studied using reaction field theory and the spherical cavity approximation. The calculated free energy difference for methyl formate was reduced from 5.2 kcal/mol in the ps phase to 1.6 kcal/mol for epsilon = 35.9 (acetonitrile). Similarly, with methyl acetate the difference was reduced from 8.5 to 5.2 kcal/mol. The changes in geometry, dipole moments, charge distributions, and vibrational frequencies on going from epsilon = 1 to 35.9 also were calculated and are in accord with increasing polar character of the carbonyl group in the more polar solvent. The change in E/Z energy difference for methyl formate was in very good agreement with that found previously using a more detailed treatment of the solvent interaction, and the change in energy difference for methyl acetate was also in good agreement with that calculated by statistical mechanics simulation. the change in equilibrium constant as a function of temperature in acetonitrile was calculated for methyl formate, and it was found that the change in dielectric constant of the solvent with temperature led to a much smaller than expected change in equilibrium constant with temperature. It is likely that the DELTAH-degrees and DELTAH(double dagger) values derived by measuring equilibrium or rate constants as a function of temperature in polar solvents will often be considerably in error because of the change in solvent dielectric constant with temperature.