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The analysis of 0-0 bands of allowed $\Pi \ast \leftarrow n$ systems of isotopic variants of three different azabenzenes reveals that these molecules, each planar in the ground state, exhibit some large positive and negative inertial defects upon excitation. To account for the large inertial defects in the excited states, rotational mixing of the analyzed states with perturbing electronic states is proposed. Because of this mixing, the derived moments of inertia are effective values and are of limited value for structure determination. This work was supported in part by the National Science Foundation.