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

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journal contribution
posted on 10.08.2021, 14:12 authored by Koushik Mondal, Souvick Biswas, Aparajeo Chattopadhyay, Piyali Chatterjee, Tapas Chakraborty
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.