nitrogen-dioxide and phenoxy-radical

The gas-phase oxidation mechanism of phenol initiated by OH radical was investigated using DFT and ab initio calculations. The initiation of the reaction is dominated by OH addition to ortho-position, forming P2, which subsequently combines with O2 at the ipso-position to form P2-1-OO adduct. A concerted HO2 elimination process from P2-1-OO was found to be much faster than the common ring closure to bicyclic intermediates. The HO2 elimination process from P2-1-OO forms 2-hydroxy-3,5-cyclohexadienone (HCH) as the main product and is also responsible for the experimental fact that the rate constants for reaction between P2 and O2 are about 2 orders of magnitude higher than those between other aromatic-OH adducts and O2. It was speculated that HCH would isomerize to catechol, which is thermodynamically more stable than HCH and was the experimentally observed main product, possibly through heterogeneous processes. Reaction of P2 with NO2 proceeded by addition to form P2-n-NO2 (n = 1, 3, 5), followed by HONO elimination from P2-1/3-NO2 to form catechol. The barriers for HONO elimination and catechol formation are below the separate reactants P2 and NO2, being consistent with the experimental observation of catechol in the absence of O2, while H2O elimination from P2-1/3-NO2 to form 2-nitrophenol (2NP) is hindered by high barriers. The most likely pathway for 2NP is the reaction of phenoxy radical and NO2.

Topics: Atmosphere; Catechols; Gases; Hydroxyl Radical; Kinetics; Models, Chemical; Nitrogen Dioxide; Nitrophenols; Oxidation-Reduction; Phenol; Phenols; Quantum Theory; Thermodynamics

The uptake coefficients of NO2 on aqueous solutions containing guaiacol, syringol and catechol were determined over the pH range from 1 to 13 using the wetted wall flowtube technique. The measured uptake coefficients were used to determine the rate coefficients for the reaction of the physically dissolved NO2 with the neutral and deprotonated forms of phenolic compounds listed above. These organic compounds are ubiquitous not only in biomass burning plumes but also in soils, where they form part of the building blocks of humic acids. The NO2 uptake kinetics on solutions containing guaiacol, syringol or catechol were observed to be strongly pH dependent with uptake coefficients increasing from below 10(-7), under acidic conditions, to more than 10(-5) at pH values above 10. This behaviour illustrates the difference of reactivity between the neutral phenolic species and the phenoxide ions. The corresponding second order rate coefficients were typically observed to increase from 10(5) M(-1) s(-1) for the neutral compounds to a minimum of 10(8) M(-1) s(-1) for the phenoxide ions.

Topics: Biomass; Catechols; Environmental Pollution; Fires; Guaiacol; Humic Substances; Hydrogen-Ion Concentration; Kinetics; Models, Chemical; Nitrogen Dioxide; Phenols; Pyrogallol; Soil Pollutants; Solutions; Water