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 formation of radicals from the gas-phase pyrolysis of hydroquinone, catechol, and phenol over a temperature range of 400-750 degrees C was studied using the technique of low-temperature matrix isolation electron paramagnetic resonance (LTMI EPR). Cooling the reactor effluent from pyrolysis in a nitrogen carrier gas to 77 K produces a cryogenic matrix that exhibits poorly resolved EPR spectra. However, using carbon dioxide as a carrier gas formed a matrix that, upon annealing by slowly raising the matrix temperature followed by rapid recooling to 77 K, yielded more resolved, identifiable spectra. Annealed spectra of all three samples resulted in the generation of EPR spectra above 700 degrees C with 6 lines, hyperfine splitting constant approximately 6.0 G, and peak to peak width approximately 3 G that was readily assignable, based on comparison to the literature and theoretical calculations, as that of cyclopentadienyl radical. Pyrolysis at temperatures below 700 degrees C generated a carbon dioxide matrix isolation spectrum with a high g-value (>2.0040) that is attributed to oxygen-containing radicals such as semiquinone or phenoxyl. Conclusive identification of anticipated semiquinone, phenoxyl, and hydroxycyclopentadienyl radicals was complicated by the ability of these radicals to exist in carbon-centered and oxygen-centered resonance structures that can give different EPR spectra.

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The fractional pyrolysis of Bright tobacco was studied in an inert nitrogen environment at atmospheric pressure over a temperature range of 200-700 °C, using a continuous-flow reactor system. The effect of pyrolysis conditions on the generation of free radicals and their distribution among total particulate matter (TPM), char residue, and the gas phase was characterized using electron paramagnetic resonance (EPR) and EPR spin-trapping techniques. It was found that the fractional pyrolysis of tobacco led to the generation of free radicals with g-factors of 2.0035-2.0040. They were formed during pyrolysis at temperatures of >380 °C and were present in both char and TPM. The concentration of these radicals in TPM varied from 0 spins/g at 240 °C to 11 × 10 16 spins/g of TPM at 620 °C. The concentration of these radicals in the char residue increased as the pyrolysis temperature increased; a maximum concentration of 1.2 × 10 19 spins/g of char at 480 °C was obtained, and then the value declined. The g-values of 2.0035-2.0040 are consistent with (i) surface-associated, carbon-centered radicals, where the unpaired electron is vicinal to an oxygen-containing functional group, or (ii) partially delocalized, polymeric, phenoxyl-type radicals. The analysis of the gas phase of tobacco smoke revealed the formation of alkoxyl radicals. The N-tert-butyl-R-phenylnitrone (PBN) adduct of the alkoxyl radicals is characterized by hyperfine coupling constants of a N ) 13.9 G and a H ) 2.0 G and a g-factor of 2.0064. The concentration of these radicals in the gas phase was insignificant ((5 ( 1) × 10 14 spins/g of tobacco) and was not dependent on the pyrolysis temperature. After exposure to air, the concentration of TPM radicals increased, because of the production of "new" radicals, in addition to the "old" radicals, with the increasing time of storage, reaching a maximum value and then declining. The g-factor of these new radicals is 2.0053 ( 0.0003, and their TPM concentration achieved a maximum for a pyrolysis temperature of 280 °C. At this temperature, the concentration of these new radicals is 8.1 × 10 15 spins/g of tobacco, which is 25 times greater than the concentration of radicals measured immediately after collection. The EPR spectra of the new radicals are consistent with oxygen-centered, semiquinone-type radicals. These results suggest that hydroquinone/ catechol-type species, which are free or chemically bound to TPM and do not produce EPR signals, are converted to semiquinone-type radicals by atmospheric oxidation.