The role of isomerism and medium effects on stability of anions of formo- and thioformohydroxamic acid

The anionic forms of formo- and thioformohydroxamic acids have been analyzed for their relative stabilities and barriers for interconversion between the tautomeric forms employing ab initio and DFT methods. The deprotonation affinities and pKa values are evaluated to differentiate the various deprotonation processes. The effect of medium on deprotonation behavior is analysed using Tomasi's PCM model and examining free energy changes of deprotonation from isolated molecules, molecule-water aggregates and subsequently from anion-water aggregates in the gas phase and aqueous phase respectively. Variation of geometrical parameters, charges, electron delocalizations, intramolecular H-bonding and relative stabilities upon water aggregation of the anions all point towards NH deprotonation as the most favored process.