Structural NMR and ab Initio Study of Salicylhydroxamic and p-Hydroxybenzohydroxamic Acids: Evidence for an Extended Aggregation
journal contributionposted on 12.10.2007, 00:00 by Begoña García, Fernando Secco, Saturnino Ibeas, Asunción Muñoz, Francisco. J. Hoyuelos, José M. Leal, María L. Senent, Marcella Venturini
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 °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 π−π 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.