Ambident Reactivity in the Phenyl + NO Radical Recombination

The recombination reaction of nitric oxide (NO) with radicals via its oxygen atom, rather than the more common nitrogen site, offers an intriguing pathway to the formation of oxynitrenes. The photochemical isomerization of nitroso compounds into their corresponding oxynitrenes may involve such a step. However, because this reaction typically proceeds under continuous irradiation, the formation mechanism has so far remained obscure. In this work, we investigate the recombination of the phenyl radical with NO strictly under thermal conditions, using infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy. A proximal radical pair is generated in a solid argon matrix by photolysis of nitrosobenzene at 254 nm. Upon annealing the matrix to 30 K, the radicals undergo a thermal reaction that yields both nitrosobenzene and phenoxynitrene, notably in comparable amounts. NEVPT2 computations show that phenoxynitrene forms on the 1-¹A' singlet surface, intersecting the 1-³A'' triplet ground state 1.0 kcal mol^-1 below the separated reactants. At this crossing point, a substantial spin-orbit coupling (55 cm^-1) enables two-state reactivity, thereby illuminating the mechanism behind this elusive transformation.