Revisiting the radical trapping activity of N-H and O-H in N-phenylhydroxylamine: a DFT study

Hydroxylamines have been identified as promising antioxidants that can effectively scavenge free radicals primarily through a hydrogen transfer mechanism. Specifically, for N-phenylhydroxylamines, it is believed that both N-H and O-H bonds serve as two hydrogen-donating centers responsible for this task. The M06-2X/6-311++G(d,p) and CBS-QB3 methods were used to re-evaluate the bond dissociation enthalpies of N-H and O-H and the results were found to be in agreement with each other. The revisited BDE(N-H) values in the gas phase, DMSO and water media are 74.8, 77.1, and 77.4 kcal mol^-1, respectively, while the BDE(O-H) values are about 5.0, 7.5, and 6.0 kcal mol^-1 lower in comparison. Additionally, the effect of substitution with halogen, electron-donating, and electron-withdrawing groups at the para site of the aromatic ring of ArNHOH on the BDEs of both N-H and O-H bonds was evaluated. In addition to examining the role of O-H and N-H bonds in the trapping of radicals, the current study incorporated a kinetic aspect to gain insight into the comprehension of the implicated mechanisms. Moreover, an evaluation of the N-phenylhydroxylamine's antioxidant capability was carried out through the execution of a DPPH assay.