Atomic charges and charge fluxes in cis and trans C2H2X2 (X = F, Cl): an ab-initio study.

Electro-optical parameters have been determined in terms of equilibrium charges and charge fluxes for cis- and trans-C2H2X2 (X = F,Cl) using atomic polar tensors (APTs) obtained from 6-31G* ab initio calculations. On the whole, these results point to a greater similarity between the electronic structures of cis- and trans-C2H2Cl2 than between those of the fluorine-containing isomers, the major difference in the latter being due to their equilibrium atomic charges. These conclusions are supported by the better agreement between experimental infrared intensities of the trans-C2H2Cl2 isomer and its predicted values evaluated from the experimental APTs of the cis isomer. Finally, the basis set dependence of the standard and corrected Mulliken charges of cis-C2H2F2 has been analysed. The similarity between these is only reached when diffuse functions are used.