ON THE VIBRONIC STRUCTURE OF THE ABSORPTION-SPECTRA OF RADICAL CATIONS OF SOME POLYCYCLIC AROMATIC-HYDROCARBONS

The vibronic structure of electronic absorption spectra of four polyacenic radical cations is investigated by means of quantum chemical calculations. We use the semiempirical quantum consistent force field/pi electron method (QCFF/PI) augmented by a configuration interaction (CI) scheme that includes all excited configurations having at most one electron in the originally empty molecular orbitals and the ab initio restricted open shell Hartree-Fock (ROHF) method. Displacement parameters for the totally symmetric modes are calculated for different electronic transitions and the resulting Franck-Condon structure is compared with the experimental spectra. The analysis reaffirms the initial interpretation of the absorption spectrum of the cation of naphthalene and reinterprets the recently observed spectrum of the cation of anthracene. First ever analysis of the absorption spectrum of the radical cation of pyrene and perylene is given. For the former,we demonstrate that the unusually large frequency value of the highest a(g) mode active in the strong absorption band of astronomical interest (443.8 nm) is due to vibronic coupling. For the latter, a resonance vibronic coupling mechanism between manifolds of the 1(2)B(3g) and 1(2)B(2g) states is suggested to explain the doublet structure of the origin of the transition to the latter state.