Ring-chain tautomerism of thiosemicarbazones of salicylaldehyde and pyridinecarbaldehyde in acidic media

On the basis of NMR spectroscopic data, it has been found that 4-substituted thosemicarbazones of salicylaldehyde and α-, β-, and γ- pyridinecarbaldehydes in solutions of trifluoroacetic acid are tautomeric mixtures of protonated linear and cyclic 1, 3, 4-thiadiazolidine-2-imine forms, the linear tautomer having an azinethiol structure with localization of the proton on the N(2) nitrogen atom.