Atmospheric radical cation chain reaction

The development of mass spectrometric ionization methods at atmospheric pressures (API), such as the atmospheric pressure chemical ionization (APCI)99 and the electrospray ionization mass spectrometry (ESI-MS)100 has made it possible to study liquid-phase solutions by mass spectrometry. Electrospray ionization mass spectrometry coupled to a micro-reactor was used to investigate radical cation chain reaction is solution101. The tris (p-bromophenyl)aminium hexachloro antimonate mediated [2 + 2] cycloaddition of trans-anethole to give l,2-bis(4-methoxyphenyl)-3,4-dimethylcyclobutane was investigated and the transient intermediates 9 + and 10 + were detected and characterized directly in the reacting solution. However, steady state conditions are necessary for the detection of reactive intermediates and therefore it is crucial that the reaction must not be complete at the moment of electrospray ionization to be able to detect the intermediates. 

A better clue comes from knowledge that several vital cell constituents, particularly mercapto-compounds [such as dihydrolipoic acid (2.25), see Section 11.7.3] are easily oxidized by atmospheric oxygen if traces of iron or copper are present. These oxidations lead to the formation of hydrogen peroxide and superoxide radicals ( O2 ) which, in the presence of the metal cations, produce a fulminating chain reaction so that a very small amount of metal can catalyse widespread destruction. In some model reactions of this kind, traces of cobalt have been found to act as a chainbreaker which greatly moderates the destruction (see Fig. 11.10). ... 

A distinct characteristic of photoinitiated cationic polymerisation is its lack of sensitivity towards atmospheric oxygen which does not scavenge cationic species, as well as its living character due to the fact that the propagating polymer cations are not reacting among themselves. In contrast to radical-initiated polymerisation, the chain reaction will thus continue to proceed in the dark after the UV-exposure, a feature which was used to assemble nontransparent materials by UV-curable laminating adhesives [56]. 

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