Electronic transitions in cis- and trans-dichloroethylenes and tetrachloroethylene.

Electronic structures of trans- and cis-dichloroethylenes and tetrachloroethylene were studied using symmetry-adapted cluster configuration interaction theory. Basis sets up to the aug-cc-pVTZ of Dunning, Jr., augmented with appropriate Rydberg functions were used for the calculations. The results derived in the present study show good agreement with the available experimental values. In all cases, the main bright excitation was the pi-->pi( *) transition. The other vertical excitations, pi-->sigma( *), n-->sigma( *), and n-->pi( *), which have not been studied before, were also investigated. First Rydberg series involving transitions from the pi orbitals to one 3s, three 3p, and five 3d orbitals were identified clearly. Several new assignments and reassignments of features in the experimental spectra were suggested. Contrary to earlier prediction, two n-sigma( *) states, along with a pi-sigma( *) state in the dichloroethylenes, were calculated to be located above the main pi-pi( *) state. Accordingly, crossing between both the n-sigma( *) states with the bright pi-pi( *) state is highly likely, unlike conclusions made in the earlier studies. This indicates that the photodissociation mechanism proposed by the earlier calculations warrants revision. Several low-lying triplet excited states were also studied. Electronic spectra of trans-1-chloro-2-fluoroethylene and cis-1-chloro-2-fluoroethylene were also calculated. The pi-->pi( *) transitions of these haloethylenes are compared and interpreted in terms of the inductive and resonance effects.