Geometrical structure of the cis- and trans-isomers of 1,2-dihaloethylenes and the energetics of their chemical equilibrium in solution

Abstract

This work examines the isomeric cis—trans equilibrium of the 1,2-dihaloethylenes in several solvents (benzene, acetonitrile and water), by means of quantum chemical calculations involving the continuum model of solvation developed by Rivail's group. The need to take into account beyond the dipole moment to explain the negligible influence of the solvent on the equilibria is emphasized, in particular, if one realizes that the geometric isomer cis is a dipolar molecule whereas the trans is not. Geometrical changes induced by the solvent on both isomers are different: the CC bond distance and XCC bond angle show different trends with the solvent polarity for each isomer. These results are rationalized on the basis of the alternative role played by the dipole or quadrupole moment. The small influence of the long-range interactions on the energetics of the equilibrium can be only explained if electric multipole moments of higher order than quadrupole are considered. Cavitation and dispersion contributions have also been estimated in the case of the chloro derivative for which experimental information is available. Although these contributions are much larger than the electrostatic one, they do not discriminate between isomers, particularly when cavitation + dispersion is considered.