Theoretical and experimental study of the acetohydroxamic acid protonation: The solvent effect

The mechanism of the protonation of acetohydroxamic acid is investigated co mparing experimental results and ab initio calculations. Experimentally, th e UV spectral curves were recorded at different temperatures, at constant d ioxane/water concentration. and at very high concentrations of strong miner al acids. The process is followed by monitoring the changes in the UV curve s with increasing acid concentration. The molecular structures and the solv ation energies were calculated with the RHF B3LYP. and MP2 methods. The sol vent is treated as a continuum of uniform dielectric constant. The isolated molecule of acetohydroxamic acid exhibits two protonation sites, the carbo nyl oxygen and the nitrogen atom. In dioxane/water mixture, the RMF calcula tions predict the existence of a third cation of low stability, where the p roton is bonded to the OH oxygen. With the MP2 ab initio calculations, the for energies of the formation processes in solution of the two most stable cations, CH3COH-NHOH+ (O3H +) and CH3CO-NH2OH+ have been evaluated to be - 160.2 kcal mol(-1) and - 157.6 kcal moll. The carbonyl site is the most act ive center in solution and in the gas phase. The carbonyl site is also the most active center in the UV measurements. Experimentally, the ionization c onstant was found to be pK(O3H+) = 2.21 at 298.15 K, after the elimination of the medium effects using the Cox-Yates equation for hight activity level s. Experiments and ab initio calculations indicate that KO3H+ decreases wit h the temperature.