Hydroperoxy radicals, determination

Reaction 2-6 is sufficiently fast to be important in the atmosphere. For a carbon monoxide concentration of 5 ppm, the average lifetime of a hydroxyl radical is about 0.01 s (see Reaction 2-6 other reactions may decrease the lifetime even further). Reaction 2-7 is a three-body recombination and is known to be fast at atmospheric pressures. The rate constant for Reaction 2-8 is not well established, although several experimental studies support its occurrence. On the basis of the most recently reported value for the rate constant of Reaction 2-8, which is an indirect determination, the average lifetime of a hydroperoxy radical is about 2 s for a nitric oxide concentration of 0.05 ppm. Reaction 2-8 is the pivotal reaction for this cycle, and it deserves more direct experimental study. 

Direct determinations of rate constants are needed for almost all the reactions of hydroperoxy radical and ROj. 

It appears, then, that alkylperoxy radical isomerization is capable of producing hydroperoxyalkyl radicals during the oxidation of all alkanes and that alkene-hydroperoxy radical addition will serve a similar function during the oxidation of those alkanes which contain a high proportion of primary C—H bonds. It remains to determine the proportion of hydroperoxy alkyl radicals arriving by each route as equilibrium is approached. 

They proposed that the initial stage involves addition of HO2 to form the hydroperoxy radical which represents the rate determining step. They obtained an activation energy of TOkJmoF for reaction (51) which they ascribed to the addition step of HO2 to ethene. 

Laser magnetic resonance, which has already been used to detect the free hydroxyl, methynyl (CH), hydroperoxy, formyl (HCO), and amino radicals in low-pressure gases and could be used to determine rate constants for the reactions of the smaller free radicals. 

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