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Resonance Raman spectra of the three isoelectronic proton transfer systems 2(2'-hydroxy-S-methylphenyl)-benzotriazole (TIN), 2-(2'-hydroxyphenyl)benzothiazole (HBT) and 2-(2'-bydroxyphenyl)benzoxazole (HBO) were measured with excitation in the range of the S o S , absorption band of the enol tautomers. In each compound, about 15 modes in the wavenumber range between 120 and 1650 cm-' show a strong coupling to the electronic transition. In TIN and HBT, combination tones of the strongest mode below 500 cm-' 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 So+ 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.

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