LAUSR

Stable tautomeric forms in a series of phosphonyl thioamides have been studied using DFT methods. The molecules studied in this contribution present a phosphonyl group in β-position of the C–S bond connected to the amine group. The three most stable tautomeric forms with double bonds on either C=N, C=S, or adjacent C=C have been described, and their relative energies together with the transition barriers have been evaluated. In such molecules a six-member ring can be formed by a hydrogen bond between the oxygen of the phosphonyl group and the H–N bond of the thioamide. The tautomeric form involving a C=N double bond is found less stable than the two other forms by more than 10 kcal/mol. The transition barriers between the various tautomers are calculated to be as large as 40 kcal/mol for the isolated molecules, but less than 30 kcal/mol in the presence of one water molecule. Similar results are obtained with various substituents on the phosphorus or on the nitrogen atom. Electronic vertical spectra have been calculated using the TD-DFT approach for the three stable tautomeric forms in a series of six substituted phosphonyl thioamides, and it is found that the signature of each tautomer is sufficiently specific to allow for their clear identification in a mixture using UV–Vis spectroscopy.