Knowledge Bank

The perpendicular band of the $E1u$ vibration $\nu 14$ of benzene-$d6$ was recorded in the $800-830cm-1$ region with the aid of a large Fourier transform spectrometer at the Universit'e pierre et Marie Curie with resolution near the Doppler limit. $PPk$ branches up to $K=50$ and branches to $K=39$ have been assigned in the $v14$ band. The branches for K from 16 to 30 are strongly affected by an x,y-Coriolis resonance, with an avoided crossing observed between $K\prime =23$ and 24. The origin of the state responsible for this perturbation lies above $820cm-1$, close to the reported position of the infrared- and Raman-inactive $B2u$ fundamental $\nu 10$. However, a direct first-order interaction between $B2u$ and $E1u$ states is not allowed by symmetry, and alternative interpretations of the observed perturbation, including possible reassignment of the inactive $\nu 10$ and $\nu 19$ vibrations, will be discussed. Spectroscopic constants obtained from the analysis will be reported.