title: Conformations, protonation sites, and metal complexation of benzohydroxamic Acid. A theoretical and experimental study.
abstract: A theoretical and experimental study on the structure and deprotonation of benzohydroxamic acid (BHA) has been performed. Calculations at the RHF/cc-pVDZ level, refined by the B3LYP/AUG-cc-pVDZ method, indicate that, in the gas phase, Z amide is the most stable structure of both neutral and deprotonated BHA. (1)H-(1)H nuclear Overhauser enhancement spectroscopy and (1)H-(1)H correlation spectroscopy spectra in acetone, interpreted with ab initio interatomic distances, reveal that BHA is split into the Z and E forms, the [E]/[Z] ratio being 75:25 at -80 degrees C. The formation of E-E, Z-Z, and E-Z dimers has been detected; in the presence of water, the dimers dissociate to the corresponding monomers. The rates of proton exchange within the Z and E forms and between E and Z were measured by dynamic (1)H NMR in the -60 to 40 degrees C temperature range; an increase in water content lowers the rate of exchange of the E isomer. The effect of D(2)O on the NMR signals indicates a fast hydrogen exchange between D(2)O and the E and Z amide forms. The sequence of the acid strength at low temperatures is (N)H(E)) approximately (O)H(E) < (O)H(Z) approximately (N)H(Z). The kinetics of complex formation between BHA and Ni(2+), investigated by the stopped-flow method, show that both neutral BHA and its anion can bind Ni(2+). Whereas the anion reacts at a "normal" speed, the rate of water replacement from Ni(H(2)O)(6)(2+) by neutral BHA is about 1 order of magnitude less than expected. This behavior was interpreted assuming that, in aqueous solution, BHA mainly adopts a closed (hydrogen-bonded) Z configuration, which should open (with an energy penalty) for the metal binding process to occur.
authors.affiliation: Departamento de Química, Universidad de Burgos, 09001 Burgos, Spain. begar@ubu.es
authors.forename: Begoña
authors.initials: B
authors.lastname: García
authors.forename: Saturnino
authors.initials: S
authors.lastname: Ibeas
authors.forename: José M
authors.initials: JM
authors.lastname: Leal
authors.forename: Fernando
authors.initials: F
authors.lastname: Secco
authors.forename: Marcella
authors.initials: M
authors.lastname: Venturini
authors.forename: Maria L
authors.initials: ML
authors.lastname: Senent
authors.forename: Alfonso
authors.initials: A
authors.lastname: Niño
authors.forename: Camelia
authors.initials: C
authors.lastname: Muñoz