Vibrational analysis and excited-state geometric changes of betaine-30 derived from Raman and infrared spectra combined with ab initio calculations

We present a combined experimental and theoretical study of vibrational modes and excited-state geometry changes of betaine-30 (B-30). Infrared and Raman spectra were recorded under electronic off-resonant excitation conditions and also in resonance with the charge transfer transition of B-30. Comparing these spectra with the corresponding vibrational patterns calculated by Hartree-Fock methods we obtained the assignments of the vibrational modes. Excited-state geometry changes were calculated for a smaller model system of B-30 using configuration interaction with single excitations. The calculations predict that the central phenoxide and pyridinium rings move from a twisted conformation in the electronic ground state into a perpendicular position in the first excited state. In addition, the pyridinium ring tilts and the nitrogen atom becomes pyramidalized. In correspondence, we measured two strong low-wavenumber Raman bands with large origin shifts at 291 and 133 cm^-1. They were assigned to N-inversion and torsional vibrational modes and are expected to mediate the excited state and back-electron transfer reaction of B-30. Copyright (C) 2000 John Wiley And Sons, Ltd.