H. Tavakol, T. Hadad, The Effect of Water on the Equilibrium and Rate Constant of Tautomerism in Hydroxamic Acids, 14th Iranian Physical Chemistry Conference, Kish Iceland, Kish, February 2011
Three tautomers were identified for each HAs, namely the keto form (N-hydroxy amide, a), iminol form (α-hydroxy oxime, b) and nitroso form (α-nitroso alcohol, c). In all cases, the order of stability of tautomers is found to be as tautomer a > tautomer b > tautomer c. Furthermore, two transition states are found between tautomers. TS1 (between a and b) is more stable than TS2 (between b and c). The energetic data confirm that because of high energy barriers between tautomers, their interconversions are very slow at room temperature.
Solvation data show no important difference between gas phase and solvent media. As an example, the maximum rate constant in acetone is 1.2x10-15 Sec-1, very near to that in the gas phase (1.3x10-15 Sec-1) and all energetic results in solvent (ΔGsolv) have a little difference with those in the gas phase.
To examine water-assisted tautomerism, geometry optimization of the tautomers and transition state species of molecule 1 and location of its reaction path have been carried out in the presence of 1-3 water molecules. Activation barriers in the presence of water molecules are in general lower than those in the gas phase. This difference increases with the increase of water molecules. Our results show that the presence of water molecules has more intensive effects on kinetic results (rate constants) versus thermodynamic results (equilibrium constants).