Nuclear coordinate dependence of electronic transition moments involving torsional modes: the Franck–Condon/symmetry forbidden m03+ band in the allowed S1←S0 transition of toluene

A novel theory of vibronic intensity borrowing by large-amplitude vibrations was developed in a form to modify the conventional Herzberg–Teller theory. The ingredients are: (1) Crude Born–Oppenheimer wavefuntions (CBO Wf’s) at more-than-one (2×2×3=12) conformations were used as building blocks of the adiabatic BO Wf. (2) The latter was made to belong to the molecular symmetry group (G12) by Fourier-series expansion terminated at the low (ninth) order. (3) The non-Born–Oppenheimer effect was shown to be less significant. The experimental observation of the subtitle was successfully interpreted with this formalism, endorsing the previous assertion by Walker et al. [J. Chem. Phys. 102 (1995) 8718].