Gas-Phase Oxidation of NO₂ to HNO₃ by Phenol: Atmospheric Implications

The thermal reaction between nitrogen dioxide and phenol in the gas phase under anaerobic conditions has been investigated by diluting the reactants in dry nitrogen in a glass reaction vessel. Infrared spectroscopic analysis reveals that nitric acid, nitric oxide, and o-nitrophenol are the major products of the reaction. The kinetic analysis reveals the reaction stoichiometry as 3NO₂ + PhOH → HNO₃ + NO + o-nitrophenol, and the corresponding reaction enthalpy is Δ_rH⁰ = −44.82 kcal/mol. Reaction monitoring by NO₂ concentration variation shows that HNO₃ formation is linearly correlated with the effective concentration of the nitrogen dioxide dimer (N₂O₄) formed, and the overall reaction follows a second-order kinetic behavior with respect to N₂O₄ and phenol, and the estimated rate constant value is (3.53 ± 0.56) × 10^–18 cm³ molecule^–1 s^–1 at 298 K. In the presence of excess NO₂, the reaction shows a pseudo-first-order kinetic behavior with a rate constant of (6.67 ± 0.12) × 10^–3 s^–1. The electronic structure calculation predicts that the N₂O₄–phenol complex can have multiple conformational minima, and in the lowest-energy conformer, the orientation of the two NO₂ molecules about the phenolic −OH group is similar to that of the charge-separated asymmetric ONONO₂ dimer of NO₂. A radical mechanism has been ruled out, as HONO has not been identified as a product. To the best of our knowledge, the formation of o-nitrophenol in the gas-phase reaction between phenol and NO₂ is reported here for the first time. The atmospheric implication of the reaction has been discussed.