H. Tavakol and S. Zakery,  Investigation of Hydroxamic Acid Tautomers by DFT Method  13th Intern. Conf. on the Applic. of Density Functional Theory in Chemistry and Physics, DFT09, Lyon, France, 31 August-4 September 2009

Abstract

Hydroxamic acids [1] are very important natural compounds. They are among the most well studied compounds due to their significance in biological activity since the first reported by Lossen in 1869 [2].

 Hydroxamic acids are a class of compounds, which display interesting chemical and biological properties. They are also capable for the inhibition of a wide variety of enzymes [3] and there is no doubt that their inhibitory effect is in correlation with chelation of the metals by the hydroxamate function. Because of their importance, hydroxamic acids have been the subject of some computational investigations including semi empirical and ab initio [4] at different levels of theory that some of those have focused on their acid-base properties [5] and other included their protonated species. 

Although the growing interest in the study of  hydroxamic acid is observed, but structures of hydroxamic acids are still the subject of many controversies. Because potentials for tautomerism, their transition state energy and properties have been recognized as a particular characteristic of hydroxamic acids and these aspects have not been investigated to their full extent.

In this research, all of important aspects of some simple hydroxamic acids, their tautomers and transition states in gas phase and solvent were explored using DFT calculations. Three tautomers (a,b,c) were defined for each molecule. As well as, two transition states between these tautomers can exist. In all cases, the order of stability of tautomers is tautomer a (keto form, N-hydroxy amide), tautomer b (iminol form, α-hydroxy oxime) and tautomer c (α-nitroso alcohol), respectively. In addition, T.S₁ (between tautomers a and b) is more stable than T.S₂ (between tautomers b and c). All tautomers and transition states are optimized at the B3LYP/6-311++G** level. Our results confirm the observed experimental data about most stability of tautomer a. Also because of great quantity of all barriers, converting of these tautomers to each other is impossible at room temperature.