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Theoretical and Experimental Study of the Acetohydroxamic Acid Protonation: The Solvent Effect
Author(s) -
García Begoña,
Ibeas Saturnino,
Leal José María,
Senent María Luisa,
Niño Alfonso,
MuñozCaro Camelia
Publication year - 2000
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20000717)6:14<2644::aid-chem2644>3.0.co;2-d
Subject(s) - protonation , chemistry , ab initio , solvation , acetohydroxamic acid , proton affinity , ab initio quantum chemistry methods , molecule , solvent effects , solvent , computational chemistry , inorganic chemistry , ion , organic chemistry , urease , enzyme
The mechanism of the protonation of acetohydroxamic acid is investigated comparing experimental results and ab initio calculations. Experimentally, the UV spectral curves were recorded at different temperatures, at constant dioxane/water concentration, and at very high concentrations of strong mineral acids. The process is followed by monitoring the changes in the UV curves with increasing acid concentration. The molecular structures and the solvation energies were calculated with the RHF, B3LYP, and MP2 methods. The solvent is treated as a continuum of uniform dielectric constant. The isolated molecule of acetohydroxamic acid exhibits two protonation sites, the carbonyl oxygen and the nitrogen atom. In dioxane/water mixture, the RHF calculations predict the existence of a third cation of low stability, where the proton is bonded to the OH oxygen. With the MP2 ab initio calculations, the free energies of the formation processes in solution of the two most stable cations, CH 3 COHNHOH + (O3H+) and CH 3 CONH 2 OH + have been evaluated to be −160.2 kcal mol −1 and −157.6 kcal mol −1 . The carbonyl site is the most active center in solution and in the gas phase. The carbonyl site is also the most active center in the UV measurements. Experimentally, the ionization constant was found to be p K O3H+ =−2.21 at 298.15 K, after the elimination of the medium effects using the Cox‐Yates equation for hight acidity levels. Experiments and ab initio calculations indicate that K O3H+ decreases with the temperature.