ANALYSIS OF A-1B1-X-1A1 ELECTRONIC TRANSITIONS OF PYRIMIDINE-D0 AND D4 VAPORS

The ultraviolet absorption spectra of 1,3-diazabenzene-d0 and d4 vapors have been studied at high resolution between 2600 and 3500 Å. Each band, including the 0-0 band, shows the very strong and sharp Q-branch edge of a type C band. Thus, the electronic transition moment must lie perpendicular to the plane of the molecule. Rotational fine structure of the origin band of the d0 molecule indicates that the transition effects only about a 0.5% increase of the average distance of the atoms from the center of mass. However, interpretation of the vibrational intensities, on both sides of the origin band, by means of the Franck-Condon Principle, suggests that some bond distances in the aromatic ring change by 5-10%. The two results are made consistent through the assignment of the two intense upper-state progressions to the totally-symmetric (ring-elongation) normal modes, ν6a and ν8a: Atoms on the C2 symmetry axis of the molecule are little affected by the transition while the off-axis heavy atoms move only slightly away from the center of mass. Bond angles CNC open, but also only slightly. Extensive Fermi resonances, linking ν6a and ν16b in each electronic state, are analyzed. These account for the complexity of vibrational structure. All strong bands are assigned to the single electronic transition 1B1(π*, n)-1A1. New measurements of the Raman spectrum of pyrimidine-d4 are appended. © 1969.