Anharmonicity effects in the resonance Raman spectra of heterocyclic aromatic molecules showing photoinduced intramolecular proton transfer

The anharmonic vibrational coupling appearing for some combination bands in the resonance Raman spectra of 2-(2′-hydroxy-5′-methylphenyl)benzotriazole (TIN) is analyzed. The spectra show progressions in the range from 1600 to 3000 cm−1 in which an in-plane mode at 469 cm−1 gives rise to combination bands with the strongest fundamentals up to 4th order. The underlying anharmonic coupling in the ground state is derived from the observed reduction of vibrational frequency with order. The change in the intensity distribution in the combination tone pattern with excitation wavelength gives insight into the anharmonicity and the strong shift of the potential minimum in the first excited state. A qualitative potential energy surface for excited state proton transfer is presented which involves coherent motions along both the 469 cm−1 coordinate and a coupled high-frequency coordinate, resulting in a reaction channel with negligible barrier. This model is in qualitative agreement with the recent observation of vibrational coherence during the after excited state proton transfer in TIN.