AB-INITIO CALCULATIONS OF S-1 EXCITED-STATE VIBRATIONAL-SPECTRA OF BENZENE, NAPHTHALENE AND ANTHRACENE

Normal-mode calculations are presented for the lowest singlet excited states S-1 of benzene, naphthalene and anthracene. Optimized geometrie s and Cartesian harmonic force constants of the excited states are obt ained from ab initio calculations at the CIS/6-31G, CIS/6-31G, CIS/6- 311G and CIS/6-311G levels for benzene, and at CIS/6-31G and CIS/6-31 G levels for naphthalene and anthracene. Normal-mode analysis is perf ormed in internal coordinates, yielding vibrational frequencies and fo rms of normal modes for the parent molecules and their perdeuterated d erivatives. The results are compared with corresponding properties of the ground state S-0 calculated at the Hartree-Fock level and with ava ilable experimental data. The overall changes in molecular geometry an d vibrational spectra upon S-0 --> S-1 excitation are small. For benze ne we find excellent agreement of the entire calculated S-1 vibrationa l spectrum with experimental results. For naphthalene and anthracene t he calculated vibrational frequencies are in good agreement with the l imited available experimental data. Our calculations provide predictio ns of frequencies and types of normal modes for the complete S-1 vibra tional spectra of these molecules, and suggest reassignments of severa l normal-mode frequencies compared to the limited experimental results that are currently available.