Ab initio calculations of S1 excited state vibrational spectra of benzene, naphthalene and anthracene Original Research Article

Normal-mode calculations are presented for the lowest singlet excited states S1 of benzene, naphthalene and anthracene. Optimized geometries and Cartesian harmonic force constants of the excited states are obtained 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/6-31G∗ levels for naphthalene and anthracene. Normal-mode analysis is performed in internal coordinates, yielding vibrational frequencies and forms of normal modes for the parent molecules and their perdeuterated derivatives. The results are compared with corresponding properties of the ground state S0 calculated at the Hartree-Fock level and with available experimental data. The overall changes in molecular geometry and vibrational spectra upon S0 → S1 excitation are small. For benzene we find excellent agreement of the entire calculated S1 vibrational spectrum with experimental results. For naphthalene and anthracene the calculated vibrational frequencies are in good agreement with the limited available experimental data. Our calculations provide predictions of frequencies and types of normal modes for the complete S1 vibrational spectra of these molecules, and suggest reassignments of several normal-mode frequencies compared to the limited experimental results that are currently available.