Vibronic structure of magnetic circular dichroism (MCD) spectra. II. Perturbational treatment of vibronic coupling effects in molecules with nondegenerate electronic states

Abstract

Vibronic coupling effects in the MCD spectra of polyatomic molecules with nondegenerate electronic states are studied theoretically. The construction of vibronic wave functions for a molecule in which vibronic coupling can be treated by perturbation theory is discussed in detail. These wave functions are used directly in the calculation of the B term that governs the MCD spectra of the molecules studied. The approach allows the straightforward inclusion of differences in geometries and force fields between magnetically coupled electronic states (non-Condon effects, part I), it takes into account nonadiabatic effects and treats electric and magnetic transition dipole moments on an equal footing. Most of these contributions are neglected in previous treatments. The paper discusses different schemes of magnetic and vibronic couplings between the excited electronic states of a planar molecule for in-plane polarized electric dipole transitions. The theory is used to interpret the vibronic structure of the MCD spectra of anthracene, 9,10-dichloroanthracene and 2,3-dimethylanthracene in the region of the 1La and 1Lb electronic states. This analysis locates the zero-point levels of the hidden 1Lb states in these molecules and yields values for the displacement parameters of the 1400 cm−1 totally symmetric C–C stretching mode in the 1Lb states. It is shown that vibronic interactions via the 1462 cm−1 b1g mode between the 1La and 1Lb and between the 1La and 1Bb states play an important role in shaping the vibronic structure of these MCD spectra, although their main features are determined by non-Condon effects resulting from slightly different C–C bond lengths in the 1La and 1Lb states.