Water assisted and solvation energies of intramolecular proton transfer process in thioformohydroxamic acid structures

A quantum chemistry calculation has been applied in order to investigate the intramolecular proton transfer process in thioformohydroxamic in gas phase and in water. The global isomeric structures, the transfer potential surfaces, the harmonic frequency and transition states geometries of intramolecular proton transfer were calculated at the B3LYP/6-31++G** // B3LYP/6-31G* levels of theory. The free energies, enthalpies of solvation and equilibrium constants are calculated at the same level of theory. The 1Z and 1E are thione forms whereas 2Z and 2E are thiol forms and S1 is the charge-separated structure. The stability order for thioformohydroxamic acid molecules are 2Z>1Z>2E>1E>>>S1. The presence of waters molecules stabilizes the keto-1Z form of the molecules of thioformohydroxamic acid by approximately 2-3 kcal/mol. The thermodynamic and the kinetic parameters revels that the hydrogen binding with water stabilized both 1Z.2H2O and 1Z.3H2O to greater extent than 1E.2H2O and 1E.3H2O. The reaction pathways for the interconversion between tautomers were through the transition structures TS1, TS2, TS3 and TS4. The TS3 was involved in the rate-determining step. The ring strain is clearly affecting the activation barrier. Some anionic structures are also examined to look whether the thiohydroxamic acids are O-acid, N-acid or S-acid.

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