Near ultraviolet n - π transitions of the pyrazine molecule. I. The singlet n - π state and the ground state

The nature of the first n π* excited state of the pyrazine molecule has been investigated by studying the vibrational structure of the spectrum in the region of 3200 Å. The spectrum was observed in the vapor, in the pure crystal, and in crystalline solid solutions in cyclohexane and benzene. The phosphorescence and fluorescence spectra were used to determine ground state frequencies in the various media and to give the Franck-Condon factors of the various modes. Assignments of ν1, ν6a, ν9a, and ν10a in the excited state have been made. It was found that ν10a has a strongly quartic component in its potential function and that there is a large negative anharmonic coupling term between 6a and 10a. These results are interpreted qualitatively in terms of the expected electron redistribution in the n π* state.