The C=S stretching vibration in the infrared spectra of some thiosemicarbazones

The infrared spectra of some aldehyde and ketone thiosemicarbazones have been compared over the region 1 200 to 700 cm −1 with the spectra of the corresponding selenosemicarbazones. A band in the former at 830 to 805 cm −1 is replaced by a band in the latter at 800 to 775 cm −1 , attributed to v(C==Se). No other systematic differences are observed in this spectral region. The formation of 1:1 Cu(I) complexes of the thiosemicarbazones results in the removal of bands in two regions, 1 110 to 1 075 cm −1 and 830 to 805 cm −1 , from the spectra of the uncomplexed thiosemicarbazones. It is concluded that there is a contribution from v(C==S) in both regions, but that absorption in the lower region represents the more nearly pure C==S stretching vibration. Examination of the spectra of N–methyl substituted thiosemicarbazones indicates that, even in this 830 to 805 cm −1 region, C==S stretching is coupled with other vibrations, perhaps N—C—N stretching.