ELECTRONIC-SPECTRUM OF ANTHRACENE - AN AB-INITIO MOLECULAR-ORBITAL CALCULATION COMBINED WITH A VALENCE-BOND INTERPRETATION

The properties of the two lowest excited singlet states of anthracene, S-1(B-1(1u)) and S-2(B-1(2u)), and the first triplet state, T-1(B-3(1u)), were calculated at the CIS/3-21C level of theory, The main structural changes are described, and the calculated vibrational levels are compared with available experimental data. The assignment of the two-photon observed transition at about 28 000 cm(-1) to the S-2 state is confirmed, as well as some recent argon matrix vibronic bands (Wolf, J.; Hohlneicher, G. Chem, Phys. 1994, 181, 185). The calculation suggests some changes in the vibrational assignment, and correlation with the ground state vibrational modes shows that one ba, mode undergoes a large frequency increase upon electronic excitation to St. This change is similar to that observed for Kekule-type modes of benzene, naphthalene, and their derivatives. It is compatible with the notion that the pi electrons of these molecules tend to distort the molecule to a Kekule structure.