UV induced proton transfer in thioacetamide: first observation of thiol form of simple thioamide

Abstract

A photoreaction transforming the thione–amino form of thioacetamide into the thiol–imino tautomer has been observed for the monomers of the compound isolated in argon and nitrogen low temperature matrices. Good agreement between the experimental infrared spectrum recorded after UV irradiation and the spectrum theoretically calculated (at the DFT(B3LYP)/6-31++G(d,p) level) for thioacetamide in the thiol tautomeric form allows reliable identification of the photoproduct. Four conformers of thiol thioacetamide were produced in the photoreaction. The relative populations of these conformers in the matrix were found to depend significantly on the wavelengths of the UV light used for irradiation. Upon λ > 300 nm irradiation the photoproducts with NH bond in anti orientation with respect to the sulfur atom are predominantly generated, while upon λ > 270 nm irradiation all four possible forms of the thiol tautomer of thioacetamide emerge in comparable amounts. The infrared spectra of the photoproduct(s) resulting from irradiation with UV light of different wavelength were analysed by comparison with theoretically predicted spectra for the four conformers of the compound in its thiol–imino tautomeric form.