Hydrogen-bond vibrations in the S(1)<-->S(0) spectra of a nucleobase pair analog: a mixed dimer between 2-pyridone and formamide.

The vibronically resolved electronic spectra for S(1)<-->S(0) transitions of a mixed dimer between 2-pyridone (2PY) and formamide have been measured in a supersonic free jet expansion using laser-induced fluorescence spectroscopy. Quantum chemistry method at different levels of theory has been used to optimize the geometries of the dimer for the S(0) and S(1) electronic states and also to calculate the normal vibrational modes. Assignments for the vibronic bands observed in the dispersed fluorescence spectrum of the 0(0) (0) band have been suggested with the aid of the ground state frequencies calculated by density functional theoretical method. Spectral analysis reveals that electronic excitation causes extensive mixing of the low-frequency intermolecular vibrational modes of the dimer with some of the intramolecular modes of the 2PY moiety. This spectral behavior is consistent with the complete active space self-consistent field theoretical prediction that with respect to a number of geometrical parameters the dimer geometry in S(1) is significantly distorted from the geometry of the S(0) state.