The acid-base behavior and self-aggregation of salicylhydroxamic (SHA) and
p-hydroxybenzohydroxamicacids (PHBHA)have been investigated by UV and
1HNMR s
pectrosco
py, res
pectively. The acid-base
parameters, measured in H
2O 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 - 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(O
P), carboxylate (O
C), and hydroxamate (O
H) oxygen atoms in the
cis position. The B-
Z amide, with theO
P oxygen
trans to O
C, lies 5.4 kcal above the A-
Z amide. The most stable monoanion is theN-de
protonated A-
Z amide.
