| Abstract
| - The acid−base behavior and self-aggregation of salicylhydroxamic (SHA) and p-hydroxybenzohydroxamicacids (PHBHA)have been investigated by UV and 1HNMR spectroscopy, respectively. The acid−baseparameters, measured in H2O at 25 °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-hydroxybenzohydroxamicacids measured in acetone indicate that both acids self-aggregate according to a mechanism where twomonomers produce planar E−E dimers stabilized by horizontal H-bonds. Further dimer aggregation yieldssandwich-like tetramer structures stabilized by vertical H-bonds and π−π interactions. The p-hydroxybenzohydroxamic tetramers, less stable than those of salicylhydroxamic, contain two watermolecules in their structures. The gas-phase structures of salicylhydroxamic acid and its anions wereinvestigated by ab initio calculations using the density functional theory at the B3LYP/AUG-cc-pVDZlevel. 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 theOP oxygen trans to OC, lies 5.4 kcal above the A-Z amide. The most stable monoanion is theN-deprotonated A-Z amide.
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