The protonation equilibria of N-phenylbenzohydroxamic, benzohydroxamic, salicylhydroxamic, and N-p-tolylcinnamohydroxamic acids have been studied at 25 degrees C in concentrated sulfuric, hydrochloric, and perchloric acid media; the UV-vis spectral measurements were analyzed using the Hammett equation and the Bunnett-Olsen and excess acidity methods. The medium effects observed in the UV spectral curves were corrected with the Cox-Yates and vector analysis methods. The H(A) acidity function based on benzamides provided the best results. The range of variation of the solvation coefficient m is similar to that of amides, this indicating similar solvation requirements for amides and hydroxamic acids. For the same substrate, the observed variations of pK(BH)(+) with the mineral acid used was justified by formation of solvent-separated ion pairs; for the same mineral acid, the observed changes in pK(BH)(+) can be explained by the solvation of BH(+). The change of the pK(BH)(+) values was in reasonably good agreement with the sequence of the catalytic efficiency of the mineral acids used, HCl > H(2)SO(4) > HClO(4).

The binary mixtures of methanol with (C(1)-C(4)) alkylbenzoates and of (C(1), C(3), C(5), C(7), C(9), C(11)) alkan-1-ols with methylbenzoate were used as solvents to look into the preferential solvation and intermolecular interactions of the solvatochromic indicators 2-nitroanisole, 4-nitroaniline, 4-nitrophenol, and Reichardt's dye by UV-vis measurements. The experimental data at 298.15 K have served to deduce the corresponding Reichardt and Kamlet-Taft parameters of the mixed solvents. The solvation effects exerted on the solvatochromic probes by the solvents used, either pure or binary mixed, were analyzed by means of the preferential solvation model. Likewise, the (1)H NMR, (13)C NMR, and IR spectroscopic parameters measured for the mixed solvents corroborate the structural effects. The sets of experimental data gathered shed abundant light on the underlying solute-solvent and solvent-solvent interactions. The alkanol/methylbenzoate mixtures display stronger solvation ability than the pure solvents.

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Indomethacine (1) and acemethacine (2) contain the substituents -OCH 3 , -CH 2 COOH, -CH 2 COOCH 2 COOH, C-indole and O-benzoyl, which are susceptible to protonation in highly acidic media. To determine the protonation sites and the substituent effects reliably, the dissociation constants of the set of structurally related compounds 1-benzoyl-3-methylindole (3), 1-benzoyl-5-methoxyindole (4), 1-benzoylindole (5), 5-methoxy-2-methylindole (6), (2-methylindol-3-yl)acetic acid (7) and (5-methoxy-2-methylindol-3-yl)acetic acid (8) were investigated in concentrated perchloric acid. The UV/Visible spectral curves were studied by the Hammett, Bunnett-Olsen and excess acidity methods, and the observed medium effects of 1 to 5 were analysed by the vector analysis and excess acidity methods. Acemethacine (2) dis-

The acid-base behavior and self-aggregation of salicylhydroxamic (SHA) and p-hydroxybenzohydroxamic acids (PHBHA)have been investigated by UV and 1HNMR spectroscopy, respectively. The acid-base parameters, measured in H2O at 25 degrees C and I=0.1 M, were pK1=7.56, pK2=9.85 for SHA and pK1=8.4, pK2=9.4 for PHBHA. The 1H NMR signals for salicylhydroxamic and p-hydroxybenzohydroxamic acids measured in acetone indicate that both acids self-aggregate according to a mechanism where two monomers produce planar E-E dimers stabilized by horizontal H-bonds. Further dimer aggregation yields sandwich-like tetramer structures stabilized by vertical H-bonds and pi-pi interactions. The p-hydroxybenzohydroxamic tetramers, less stable than those of salicylhydroxamic, contain two water molecules in their structures. The gas-phase structures of salicylhydroxamic acid and its anions were investigated by ab initio calculations using the density functional theory at the B3LYP/AUG-cc-pVDZ level. The SHA most stable gas-phase conformer is the A-Z amide, a structure with all three phenolate (OP), carboxylate (OC), and hydroxamate (OH) oxygen atoms in the cis position. The B-Z amide, with the OP oxygen trans to OC, lies 5.4 kcal above the A-Z amide. The most stable monoanion is the N-deprotonated A-Z amide.