Theoretical investigation of the structure and acid-base properties of potential 2-thiolumazine tautomeric forms using the AM1 semiempirical method.

The relative stabilities of potential tautomers, in both gas and aqueous phases, have been calculated taking into account the entropy effects over the tautomeric equilibria, in order to determine the structure and acid-base properties of the most stable tautomers of 2-thiolumazine in different pH conditions. In each medium, the tautomer with the lowest energy must be the most representative form at the corresponding pH. Knowledge of the effect of the medium in the tautomerization energies allows us to evaluate the possible effect of the medium on the molecular stability. Clearly, the results show that in the gas phase the basicity of the potential donor atoms is N5<N8<O4<S2<N1<N3, and in the aqueous phase S2<(O4 approximately N5)<N8<N1<N3, with the higher basicity of N3 and N1 being common to the two phases. In the aqueous phase, the sulfur atom is usually found in the thiol form, whereas the oxygen atom is in the keto form only in the most stable species. Moreover the acid-base character of 2-thiolumazine in aqueous solution has been evaluated from the corresponding AM1 thermodynamic parameters. The results agree well with the experimental data. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00894-002-0094-9