Elementary reactions involved in pollutant-forming mechanisms

Abstract

The reactions of the hydroxyl radical (OH) with molecular chlorine (Reaction 1), methane (Reaction 2), and propane (Reaction 3) have been studied experimentally using a pulsed laser photolysis/pulsed-laser-induced fluorescence technique over wide ranges of temperatures (297-826, 298-1009, and 296-908 K, respectively) and at pressures between 6.68 and 24.15 kPascals. The rate coefficients for these reactions exhibit no dependence on pressure and exhibit positive temperature dependences that can be represented with modified three-parameter Arrhenius expressions within their corresponding temperature ranges: k1 = 3.59 x 10-16T1.35exp(-745K/T)cm3molecule-1sec-1, k2 = 3.82 x 10-19T2.38 exp(-1136K/T)cm3molecule-1sec-1, and k3 = 6.64 x 10-16T1.46 exp(-271K/T)cm3molecule-1sec-1. For the OH + Cl2 reaction, the potential energy surface has been studied using quantum chemical methods which suggests OH + Cl2 à HOCl + Cl as the main channel of this reaction. Density Functional Theory (DFT) along with Quadratic Configuration Interaction (QCISD(T)//DFT) calculations, with single, double, and triple electronic excitations, for the energetics of formation, stability, and reactivity of ortho-semiquinone, para-semiquinone, and the chloro-phenoxyl radicals have been performed using the 6-31G(d,p) basis set. Formation of these radicals from potential molecular precursors catechol, hydroquinone, and the chloro-phenols is readily achieved under combustion conditions through unimolecular scission of the phenoxyl-hydrogen bond or abstraction of the phenoxyl hydrogen by a hydrogen atom or hydroxyl radical. The resulting radicals are resonance stabilized and resist decomposition and oxidation. The calculations strongly suggest that combustion-generated semiquinone and chloro-phenoxyl radicals are sufficiently stable and resistant to oxidation to be considered persistent in the atmospheric environment. Semiquinone radicals (ortho- and para-hydroxy substituted phenoxyl radicals and various derivatives) are suspected to be biologically active and may lead to DNA damage, pulmonary disease, cardiovascular disease, and liver dysfunction. These radicals thought to be highly stable with low reactivity due to resonance stabilization including both carbon-centered and oxygen-centered radical resonance structures and been reported in cigarette tar. Chloro-phenoxyl radicals, on the other hand, are implicated in polychlorinated-dibenzodioxin and -dibenzofuran formation mechanisms, EPA pollutants, in the low temperature sections of hazardous waste combustion.