Electrostatic effects on conformational equilibria. Solvent dependence

Abstract

The conformational equilibrium E ⇄ Z of methyl 2-fluorobenzoate was determined from the intensity of the carbonyl bands in 17 aprotic solvents of different polarity. The estimated values ΔG0 depend linearly on the reciprocal permittivity of the solvent as expected according to the electrostatic theory in combination with common cavity models. If a small correction is admitted for the unequal absorption coefficients of the two rotamers and/or for the steric effect of fluorine, the results agree very well with electrostatic calculations; the values extrapolated to vacuum agree also resonably with CNDO/2, INDO and PCILO, but not with STO-3G calculations. On the whole, the electrostatic theory seems to work better with conformational equilibria than with proton transfer reactions. This conclusion does not depend on the solvent used or on any particular cavity model.