VIBRONIC COUPLING EFFECTS IN THE LOW-ENERGY 1(2)B(1G) AND 2(2)B(1G) STATES OF THE C10H8+ RADICAL

The vibrational structure of the 1 (2)A(u)(D-0) --> 1 B-2(1g)(D-3) and 1 (2)A(u)(D-0) --> 2 B-2(1g)(D-4) bands in the absorption spectrum of the naphthalene cation is studied in terms of vibronic coupling theor y. The model calculations supported by the FORS MCSCF type computation s show that the vibrational structure of the 1 (2)A(u)(D-0) --> 1 B-2( 1g)(D-3) band observed at ca. 21000 cm(-1) is due to Franck-Condon and vibronic interactions introduced by the totally symmetric modes in th e 1 B-2(1g)(D-3) and 2 B-6(1g)(D-4) states of the naphthalene cation. The strongest vibronic effect was found to arise in four modes with th e ground state frequencies: omega(1) = 514 cm(-1) (CCC bending), omega (2) = 725 cm(-1) (skeletal breathing), omega(5) = 1413 cm(-1) (CC stre tching) and omega(7) = 1604 cm(-1). On the other hand, the band-shape corresponding to the 1 (2)A(u)(D-0) --> 2 B-2(1g)(D-4) transition loca ted at ca. 25000 cm(-1) is very slightly affected by the vibronic coup ling. For this transition the vibrational structure is predominately d ue to FC activity of omega(1), = 514 cm(-1), omega(2) = 725 cm(-1) and omega(3) = 1037 cm(-1) modes of the naphthalene cation. The estimates suggest that certain anomalies observed in the (weak) absorption band s of many cations and anions might be attributed to a competition betw een the vibronic coupling and Franck-Condon effects. (C) 1998 Elsevier Science B.V. All rights reserved.