Mass-analyzed threshold ionization spectroscopy of pyrimidine: determining the geometry in the first excited and the ionic ground states

Vibrational spectra of the pyrimidine cation in the electronic ground state were measured via several intermediate states of the first excited state (0⁰,16a¹, 16a², 16a⁴, 16b¹, 10b¹, 6b², 6a¹, 1¹, 4¹, 4² and 12¹) by mass-analyzed threshold ionization spectroscopy. For the first time, several vibrational modes could be assigned in the first excited and the ionic ground states. Anharmonic coupling is shown to occur in the first excited state due to Fermi resonance between the 1¹ and the 16a⁴ vibrations. From the results of the measurements and calculations presented here, pyrimidine is predicted to be planar in the first excited and the ionic ground states, and it belongs to the C_2V point group.