1.2- Dithiole-3-thiones spectra

Potential 2-hydroxy- and 2-mercapto-l,3,4-thiadiazoles have been examined both by infrared and by ultraviolet spectra in the solid state and in solution by Sheinker et They concluded that these compounds exist in the 2-oxo and 2-thione forms, respectively. To 2,5-dimercapto-l,3,4-thiadiazole the 2-mercapto-5-thione structure (155) was given. The structure of this compound has been the subject of some controversy. Stanovnik and TiSler have added some valuable arguments to the discussion. They measured the pKfl values of 155, its iV-methyl, iV -phenyl, and iV -phenyl-/S-methyl derivatives (156), and of the conjugate acids of these and the S-methyl derivative (pKn ) (Table III). In all compounds 156 with R = H, the infrared spectrum showed an absorption band near 2300 cm characteristic of the SH group. They also had pK i values near —1.5, Avhich in connection with the infrared evidence was taken as characteristic of an SH group in this situation. Since the 2,5-dithiol structure is excluded by ultraviolet spectral evidence, the 2-mercapto-5-thione structure (155) seems rather well established. It has previously been shown bj Thorn to predominate in chloroform solution, whereas he concluded that the dithione form (157) should predominate in ethanol solution. However, the pK i value for 155, — 1.36, makes it rather probable that Thorn used the monoanion of 155 instead of the acid itself for spectral comparison, and in that case his conclusions have a very weak foundation. 

Reduction of l,3-dithiole-2-thione yields an anion-radical (150) whose ESR spectrum at low temperature has been measured the very low proton splitting (indicated in gauss) implies the spin population to be highly concentrated in the trithiocarbonate moiety. Consistent with this, no proton hyperfine splittings were resolved in the ESR spectrum of the anion-radical of benzo-l,3-dithiole-2-thione. ... 

The NMR spectrum of l,3-dithiole-2-thione in 1 1 FS03H-Sbp5 in SOj indicates that protonation occurs on the thiocarbonyl sulfur. This C spectrum consists of two peaks at 145.2 and 140.0 ppm, attributed to C-2 and C-4 respectively and these shifts, very similar to those of 213, show the formation of a 2-mercapto-l,3-dithiolium cation. 

A modified CNDO method has been used for the calculation of the electronic spectra of l,2-dithiole-3-thiones and l,2-dithiol-3-ones259,260 good agreement with observed data was found. The method has further been used to interpret the modification of the electronic spectrum of l,2-dithiole-3-thione by protonation in sulfuric acid between 1 and 20 A,261 forming the 3-mercapto-l,2-dithiolylium ion 174. On the basis of acidity functions it has... 

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