Hydrogen-bond vibrations in the S₁↔S₀ spectra of a nucleobase pair analog: a mixed dimer between 2-pyridone and formamide

Abstract

The vibronically resolved electronic spectra for S₁↔S₀ 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 S₀ and S₁ electronic states and also to calculate the normal vibrational modes. Assignments for the vibronic bands observed in the dispersed fluorescence spectrum of the 0⁰₀ 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 S₁ is significantly distorted from the geometry of the S₀ state.