We demonstrate that stable and relatively unreactive "environmentally persistent free radicals (PFRs)" can be readily formed in the post-flame and cool-zone regions of combustion systems and other thermal processes. These resonance-stabilized radicals, including semiquinones, phenoxyls, and cyclopentadienyls, can be formed by the thermal decomposition of molecular precursors including catechols, hydroquinones and phenols. Association with the surfaces of fine particles imparts additional stabilization to these radicals such that they can persist almost indefinitely in the environment. A mechanism of chemisorption and electron transfer from the molecular adsorbate to a redox-active transition metal or other receptor is shown through experiment, and supported by molecular orbital calculations, to result in PFR formation. Both oxygen-centered and carbon-centered PFRs are possible that can significantly affect their environmental and biological reactivity.

The reactions of 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 kPa. The rate coefficients obtained for reactions 1-3 demonstrate no dependence on pressure and exhibit positive temperature dependences that can be represented with modified three-parameter Arrhenius expressions within their corresponding experimental temperature ranges: k 1 ) 3.59 × 10 -16 T 1.35 exp(-745 K/T) cm 3 molecule -1 s -1 , k 2 ) 3.82 × 10 -19 T 2.38 exp(-1136 K/T) cm 3 molecule -1 s -1 , and k 3 ) 6.64 × 10 -16 T 1.46 exp(-271 K/T) cm 3 molecule -1 s -1 . For the OH + Cl 2 reaction, the potential energy surface has been studied using quantum chemical methods, and a transition-state theory model has been developed on the basis of calculations and experimental data. Model predictions suggest OH + Cl 2 f HOCl + Cl as the main channel of this reaction. The model results in the expression k 1 ) 1.35 × 10 -16 T 1.50 exp(-723 K/T) cm 3 molecule -1 s -1 for the temperature dependence of the reaction 1 rate coefficient extrapolation outside the experimental range to low temperatures down to 200 K and to high temperatures up to 3000 K. A temperature dependence of the rate coefficient of the HOCl + Cl f OH + Cl 2 reaction has been derived on the basis of the experimental data, modeling, and thermochemical information.

Formation of radicals from the pyrolysis of catechol (CT) and hydroquinone (HQ) over a temperature range of 350-900 °C was studied using low-temperature matrix isolation electron paramagnetic resonance (LTMI EPR) spectroscopy. Comparative analysis of the pyrolysis mechanisms of these isomeric compounds was performed, and the role of semiquinone-type carrier radicals was studied. Pathways of unimolecular decomposition of intermediate radicals and molecular products were identified from the examination of the potential energy surface of catechol calculated at B3LYP hybrid density functional theory and composite CBS-QB3 levels. The results were compared with the experimental observations and mechanistic pathways previously developed for the pyrolysis of hydroquinone.