Knowledge Bank

The ultraviolet absorption spectra of 1,3-diazabenzene-$d0$ and $d4$ vapors have been studied at high resolution between 2600 and $3500\backslash AA$. 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 to 10%. The two results are made consistent through the assignment of the two intense upperstate progressions to the totally-symmetric (ring-elongation) normal modes, $\nu 6a$ and $\nu 8a$: 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 $\nu 6a$ and $\nu 16b$ in each electronic state, are analyzed. These account for the complexity of vibrational structure. All strong bands are assigned to the single electronic transition $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B1(\pi \ast ,\pi )-1A1$.