Association of free radicals with oxygen

The majority of FPTRMS investigations have been of the reaction of a free radical with an excess of a stable molecule, making the radical decay pseudo-first order. Of this class of reaction, the most frequently studied has been association with 02 to form a peroxy radical. These reactions are of importance in combustion and in the atmosphere. Bayes and coworkers have reported a series of FPTRMS investigations of radical/02 association reactions, following the decay of ions formed by photoionization of the radicals. 

Lenhardt et al. [30] conducted the first direct study of the reaction of butyl radicals with 02, reporting room-temperature rate coefficients for n-butyl, s-butyl, t-butyl, and 3-hydroxy s-butyl, where the radicals were prepared by broadband flash photolysis of the iodides. The bimolecular rate coefficients were independent of pressure over the range 1 to 4 torr, showing that these association reactions are in the high-pressure limit. The rate coefficients increased in the order n-butyl s-butyl f-butyl 5-hydroxy s-butyl. On the other hand, the CH3 + 02 association has been shown to be well into the unimolecular falloff at pressures from 0.5 to 6 torr at room temperature [62]. Falloff behavior is not unexpected for the smaller CH3 radical, in contrast with C4H9 radicals. Methyl was generated by 193-nm photolysis of nitromethane. 

FLASH PHOTOLYSIS WITH TIME-RESOLVED MASS SPECTROMETRY 

The effect of chlorine substitution is to reduce the C-O bond energy of the peroxy radicals from 32kcal/mol for CH302 to 20kcal/mol for CC1302. 

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