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.