The ultra-violet spectra of some heterocyclic thioamides and hydrogen bonding

The ultra-violet spectra of thiazoline-2-thione, thiazolidine-2-thione, benzthiazoline-2-thione, 3(N)-methyl-benzthiazoline-2-thione and 2-methyl-thiobenzthiazole in a range of solvents have been determined, solvent effects noted and assignments given for the main absorption bands. The spectra of thiazoline-2-thione and thiazolidine-2-thione are interpreted by their relationship to the spectra of dithiocarbamates, the intense long-wavelength ultra-violet absorption band in the spectra being assigned to an intramolecular charge-transfer transition involving the thioamide grouping. The ultra-violet spectra of benzthiazoline-2-thione and 3(N)-methyl-benzthiazoline-2-thione are related to the spectra of benzthiazole and thioamides. The very intense long-wavelength band in the spectra of these compounds is also assigned to an intra-molecular charge-transfer transition involving the thioamide grouping. The spectrum of 2-methyl-thiobenzthiazole is related to that of benzthiazole and also contains an intense long-wavelength band which is assigned to an intra-molecular charge-transfer transition involving the NCS grouping.

The spectrum of benzthiazoline-2-thione in twenty-five solvents is reported and the solvent dependence of the charge-transfer band noted. Blue shifts are observed in hydroxylic solvents where hydrogen bonding from solvent to the thione sulphur atom occurs and red shifts in solvents where the benzthiazoline-2-thione functions as a hydrogen bond donor. In solvents where the compound acts as both a donor and acceptor zero shifts are observed.

These conclusions are supported by the studies, in a range of critical solvents, of the spectrum of the N-methyl derivative which can function as a hydrogen bond acceptor only, and hence shows blue shifts only.