Structural NMR and ab initio study of salicylhydroxamic and p-hydroxybenzohydroxamic acids: evidence for an extended aggregation.
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.
García
Begoña
B
Universidad de Burgos, Departamento de Química, 09001 Burgos, Spain. begar@ubu.es
Secco
Fernando
F
Ibeas
Saturnino
S
Muñoz
Asunción
A
Hoyuelos
Francisco J
FJ
Leal
José M
JM
Senent
María L
ML
Venturini
Marcella
M
D002614:Chelating Agents / Q000737:chemistry*; D006877:Hydroxamic Acids / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D012457:Salicylamides / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
D002614:Chelating Agents; D006877:Hydroxamic Acids; D012457:Salicylamides; C005703:salicylhydroxamic acid