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 mod es and excited-state geometry changes of betaine-30 (B-30). Infrared and Ra man spectra were recorded under electronic off-resonant excitation conditio ns and also in resonance with the charge transfer transition of B-30, Compa ring these spectra with the corresponding vibrational patterns calculated b y 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 calcu lations predict that the central phenoxide and pyridinium rings move from a twisted conformation in the electronic ground state into a perpendicular p osition 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 mod es and are expected to mediate the excited state and back-electron transfer reaction of B-30, Copyright (C) 2000 John Wiley & Sons, Ltd.