A CASSCF and CASPT2 study of the photochemistry of 1,1- and 1,2-difluoroethylenes

Multiconfigurational complete active space self-consistent field (CASSCF) calculations corrected with second order perturbation theory (CASPT2) have been employed to characterize the valence and 3s and 3p Rydberg states of 1,1- and 1,2-cis and trans difluoroethylenes. The calculated energies show a good agreement with the available experimental values. Potential energy curves along the torsion around the double bond and the pyramidalization at both carbons have been computed to gain insight into the possible crossing points between the valence and Rydberg states. Whereas few changes are found in the Rydberg states along these two coordinates, a number of degenerated points are found between the valence states; they serve as starting points for the optimization of conical intersections. Conical intersections for torsion, pyramidalization and hydrogen migration have been located. Based on these stationary points, the photodissociation mechanisms of difluoroethylenes is discussed.