Computational study of simple and water-assisted tautomerism of hydroxamic acids

All important aspects of some simple hydroxamic acids (HAs), their tautomers and transition states in the gas phase and solvent are explored using DFT-B3LYP/6-311++G** calculations. Three tautomers as presented in Fig. 1 are identified for each HAs (molecules 1-8), namely the keto form (N-hydroxy amide, tautomer a), iminol form (α-hydroxy oxime, tautomer b) and nitroso form (α-nitroso alcohol, tautomer 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 tautomers a and b) is more stable than TS2 (between tautomers b and c). Our results confirm the available experimental data approving higher stability of the tautomer a. Because of high energy barriers between tautomers, their interconversions are very slow at room temperature. Moreover, geometry optimization of the tautomers and transition state species of molecule 1 and location of its reaction path have been carried out at the same level of theory 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.