Absolute pKa determination for N-phenylbenzohydroxamic acid derivatives

Ab initio calculations on isomers of N-phenylbenzohydroxamic acid derivatives and their deprotonation process have been performed. Gas phase geometries were optimised using complete basis set method, CBS-QB3 as implemented in gaussian 98 suite. It was observed that in the gas phase, the Z-isomer of the molecule and the E-isomer of the anion are stable. Acid dissociation constants obtained using CBS-QB3 gas phase energies and HF/6−31+G(d)/CPCM hydration energies closely agree with the experimental values determined in 50% ethanol in water or 50% methanol in water media, provided that the most stable isomer for the molecule and the anion in water is taken as the Z-isomer. The following pKa values were obtained for the compounds listed in the order (experimental-ethanol; experimental-methanol; theoretical); N-phenylbenzohydroxamic acid-[9.81,9.81,10.13]; N-(o-chloro)phenylbenzohydroxamic acid-[9.64,9.61,10.34]; N-(o-bromo)phenylbenzohydroxamic acid-[9.72,9.66,9.94] and N-(o-methoxy)phenylbenzohydroxamic acid-[9.97, 9.97, 9.59].