Vibronic coupling effects in the low-energy 1 2B1g and 2 2B1g states of the C10H8+ radical Original Research Article

The vibrational structure of the 1 2Au (D0)→1 2B1g (D3) and 1 2Au (D0)→2 2B1g (D4) bands in the absorption spectrum of the naphthalene cation is studied in terms of vibronic coupling theory. The model calculations supported by the FORS MCSCF type computations show that the vibrational structure of the 1 2Au (D0)→1 2B1g (D3) band observed at ca. 21000 cm−1 is due to Franck–Condon and vibronic interactions introduced by the totally symmetric modes in the 1 2B1g (D3) and 2 2B1g (D4) states of the naphthalene cation. The strongest vibronic effect was found to arise in four modes with the ground state frequencies: ω1=514 cm−1 (CCC bending), ω2=725 cm−1 (skeletal breathing), ω5=1413 cm−1 (CC stretching) and ω7=1604 cm−1. On the other hand, the band-shape corresponding to the 1 2Au (D0)→2 2B1g (D4) transition located at ca. 25000 cm−1 is very slightly affected by the vibronic coupling. For this transition the vibrational structure is predominately due to FC activity of ω1=514 cm−1, ω2=725 cm−1 and ω3=1037 cm−1 modes of the naphthalene cation. The estimates suggest that certain anomalies observed in the (weak) absorption bands of many cations and anions might be attributed to a competition between the vibronic coupling and Franck–Condon effects.