Reappraisal of cis effect in 1,2-dihaloethenes: an improved virtual orbital multireference approach.

Computed relative stabilities for isomers of 1,2-difluoroethene and 1,2-dichloroethene isomers are compared with predictions based on chemical hardness (eta) and electrophilicity (omega) using the principles of maximum hardness and minimum electrophilicity. The chemical hardness and electrophilicity deduced either from improved virtual orbital (IVO) energies or from correlated treatments correctly predict that cis 1,2-difluoroethene and 1,2-dichloroethene are energetically more stable than the corresponding trans isomers, and the ground state energies from multireference perturbation theory with IVO orbitals agree with these predictions. However, when the same quantities are computed using Hartree-Fock orbitals, serious inconsistencies between the two approaches emerge in predicting the stability of the isomers of the 1,2-dihaloethenes. The present study clearly demonstrates that the IVO energies are appropriate for the computation of hardness related parameters, notably the chemical hardness and electrophilicity. Moreover, the IVO methods also provide smooth potential energy curves for the cis-trans isomerization of the two 1,2-dihaloethenes.