Knowledge Bank

Among the better-resolved rotational band contours of electronic spectra of aromatic molecules are those of type C bands near the origin of the $1B1-1A1$ transition of 1,2-diazine. Because microwave studies have provided inertial constants for the $1A1(v-0)$ level, these bands offer interesting tests of maximum reliability for computer simulations of band contours of near-oblate asymmetric rotors. The most definitive result of such tests is found by comparing contour analyses of the $0-0$ and $6a10$ bands; it is shown that the effect of excitation of one quantum of $\nu 6a\prime \prime$ is to approximately equalize the two in-plane moments of inertia, by Increasing the moment about the symmetry axis and decreasing the other moment, each by about 2.3%. On the other hand, two solutions are found for the effect of electronic excitation. In both, the directions of changes of moments are the same as for excitation of $\nu 6a$. One solution is for an increase of the moment about the symmetry axis by 2% and a decrease of the other moment by 1.5%. The second solution uses the same upper-state moments but switches the one associated with the symmetry axis so that the changes are by +3.3% and $-$2.9%, respectively. Even these larger changes are appreciably smaller than those found recently for the analogous electronic change of the s-tetrazine $molecule.1$ Implications of these and other observations will be discussed.