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

Abstract

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 ΔrH0 = −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.