Knowledge Bank

The phosphorescence spectra of benzene and 8 of the 12 possible deutero-isomers have been recorded using $C5D6$ as a solvent at $4.2\circ K$. The vibronic structure of all isomers can be completely analyzed in terms of vibrational modes which correlate with either the $e2g,b2g$, or $a1g$ modes of $C6H6$. Using the observed vibronic intensities of each fundamental for $C6H6$, the Herzberg-Teller mixing scheme, and a normal coordinate analysis, the relative phosphorescence intensities of each of the fundamental vibronic transitions were calculated for each isomer. The predicted intensities compare very favorably with the observed intensities. Also explained are several anomalies in the spectra of the isomers possessing $C2v$ symmetry ($C6H5D,1,2-C6H4D2$, and $1,3-C6H4D2$) where either $a1$ or $b1$ modes predominate depending upon the particular isomer.