Knowledge Bank

The spectra of $p-C6H4Cl2$ and $p-C6H4Cl2$ have been assigned over the range of $D-2500cm-1$. The spectra are dominated by totally symmetric vibrations and vibronically allowed $b3g$ vibrations. Overtones of several out of plane bending modes are observed. The S1 frequencies are markedly reduced from $S0$ values, indicating that the molecules are significantly less rigid in $S1$. Isotopic frequency shift ratios due to chlorine and deuterium/hydrogen substitution are in good agreement with those derived from ab initio calculations of vibrational frequencies and normal modes.