RESONANCE RAMAN STUDIES OF HETEROCYCLIC AROMATIC-COMPOUNDS SHOWING ULTRAFAST INTRAMOLECULAR PROTON-TRANSFER

Resonance Raman spectra of the three isoelectronic proton transfer systems 2(2'-hydroxy-5'-methylphenyl)-benzotriazole (TIN), 2(2'-hydroxyphenyl)beozothiazole (HBT) and 2(2'-hydroxyplhenyl)benzoxazole (HBO) were measured with excitation in the range of the S0S1 absorption band of tbe enol tautomers. In each compound, about 15 modes in the wavenumber range between 120 and 1650 cm(-1) show a strong coupling to the electronic transition. In TIN and HBT, combination tones of the strongest mode below 500 cm(-1) and the vibrations at higher wavenumbers point to the strong anharmonicity of the low wavenumber motion. The experimental results are analysed by normal-mode calculations, which provide insight into the microscopic elongations in the molecules. The coupling strengths derived from the spectra are used to calculate the envelope of the S0S1 absorption bands of TIN and HBT, reproducing the measured spectra very well. Theoretical simulations of the combination tone spectra give information on the vibrational modes involved in excited-state proton transfer.