ESR Spin-Trapping Study of the Radicals Produced in NOx/Olefin Reactions. A Mechanism for the Production of the Apparently Long-Lived Radicals in Gas-Phase Cigarette Smoke.

Article abstract of DOI:10.1021/ja00330a006

Gas-phase cigarette smoke contains high concentrations of both oxygen- and carbon-centered free radicals.We have detected these radicals using several variations of the electron spin resonance (ESR) spin-trapping technique, including the use of spin traps in the solid state, to show that the radicals are trapped directly from the gas phase.These gas-phase radicals can still be trapped from gas-phase smoke that is more than 5 min old, a result that is clearly inconsistent with the highly reactive nature of oxygen- and carbon-centered radicals.To rationalize this apparent paradox, we hypothesize that free radicals are continuously produced and destroyed in cigarette smoke and exist in a steady state.We suggest that one mechanism by which radicals can be formed involves the slow oxidation of the relatively unreactive nitric oxide (which acts as a "radical reservoir") to the much more reactive nitrogen dioxide.Nitrogen dioxide can then react with a number of the species that are present in smoke to produce the radicals that we detect.As a model, we have studied the reactions of NO/air mixtures with unsaturated hydrocarbons.Isoprene is one of the most abundant species in smoke and is known to be very reactive toward NO2; therefore we have studied the nature of the radicals that can be spin trapped from gaseous mixtures of NO, isoprene, and air.We find that the NO/air/isoprene model system gives essentially the same types of radicals (oxygen and carbon centered) as does cigarette smoke.We have also studied the gas-phase reactions of NO2 with several small olefins and 1,3-butadiene and find evidence for peroxyl radical intermediates.In solution, NO2 reacts with isoprene much faster than it does with the spin-trap phenyl-tert-butylnitrone (PBN).We find that NO2 oxidizes PBN to benzoyl tert-butyl nitroxide and propose a mechanism for this reaction.