Absolute pKa determination for N-phenylbenzohydroxamic

Abstract

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/6K31CG(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].