Knowledge Bank

The sharp 3280 $\backslash AA$ absorption system of benzothiadiazole-$d4$ (BTD-$d4$) vapour has been studied and found to be similar to that reported by Hollas $etal1$ for BTD-$d0$. The most striking new feature is the appearance of a strong $776cm-1$ fundamental which probably corresponds to a BTD-$d0$ mode which is weakened by Fermi resonance. Polarized spectra of BTD as a guest in a variety of host crystals confirm that the 3280 $\backslash AA$ system is electronically ($B2\leftarrow A1$) and that $b2$ vibrations are active in intensity stealing. The stronger transition near 3060 $\backslash AA$ records an ($A1\leftarrow A1$) transition. A complicated pattern of sharp bands is seen in the vapour spectrum of BTD-$d0$ and ---$d4$ in the region where the 3280 and 3060 $\backslash AA$ transitions overlap, but does not appear in 4-amino BTD where the two transitions are more widely separated.