Ambident Reactivity in the Phenyl + NO Radical Recombination

Area of Science:

• Chemical Kinetics
• Photochemistry
• Quantum Chemistry

Background:

• Oxynitrenes are reactive intermediates implicated in the photochemical isomerization of nitroso compounds.
• The formation mechanism of oxynitrenes via radical recombination, particularly through oxygen atom pathways, remains poorly understood due to experimental challenges under continuous irradiation.

Purpose of the Study:

• To investigate the thermal recombination reaction between phenyl radicals and nitric oxide (NO).
• To elucidate the mechanism of oxynitrene formation under controlled thermal conditions.
• To identify the role of oxygen atom recombination in the reaction pathway.

Main Methods:

• Generation of a proximal radical pair from nitrosobenzene photolysis in a solid argon matrix at 254 nm.
• Infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy for in-situ analysis.
• Annealing the matrix to 30 K to induce thermal reaction.
• NEVPT2 computational methods to model reaction pathways and electronic states.

Main Results:

• Thermal reaction of phenyl radicals with NO at 30 K yielded both nitrosobenzene and phenoxynitrene in comparable amounts.
• Phenoxynitrene formation was computationally shown to occur on the singlet surface, intersecting the triplet ground state.
• A crossing point with significant spin-orbit coupling (55 cm⁻¹) was identified, enabling two-state reactivity.

Conclusions:

• The study demonstrates that phenyl radical recombination with NO can proceed via oxygen atom attack, leading to phenoxynitrene formation under thermal conditions.
• Two-state reactivity, facilitated by spin-orbit coupling at the conical intersection, is proposed as the mechanism for this transformation.
• This work provides crucial insights into the elusive mechanism of oxynitrene formation from nitroso compounds.