CNDO/S-CI SCF MO Study of the Lower States of Perfluoroacetic Anhydride. Singlet-Singlet and Singlet-Triplet Electronic Transitions

The CNDO/S-Cl spectral parametrization has been used to elucidate the lower electronic states of perfluoroacetic anhydride. The a2 (n*) unoccupied molecular orbital lies predominantly on the carbonyl group, while the b1 (no) is largely on the oxygen atoms. The lowest energy singlet excited state 1B2 is comprised almost entirely of the n n* configuration. Since the singlet and triplet 1B2 (no n*), 1A2 (no n*) result from a transition from molecular orbitals that are predominantly located on the oxygens to a molecular orbital containing about 80% C=O character, transitions to these states result in charge transfer from the oxygen to the carbon atom of the carbonyl group. Although the transition energies calculated in this study may not yield absolute comparisons with experimental values, it appears that the introduction of self-consistency, together with configuration interaction, leads to a fairly good interpretation of the singlet-singlet and singlet- triplet transitions. Various approximations were used to evaluate the two-center Coulomb repulsion integrals and the core Hamiltonian matrix elements