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This mechanism has been confirmed by the mass spec-trometric observation of the OH-alkene adducts themselves (e.g., Morris et al., 1971; Hoyermann and Sievert, 1983). Only at low pressures for the smaller alkenes is decomposition back to reactants significant.

However, this does illustrate the importance of understanding the fundamental mechanisms in order to extrapolate to atmospheric conditions reliably. A number of experimental techniques used for studying gasphase kinetics and mechanisms require low pressures and, under these conditions, decomposition of the OH-alkene adduct can predominate. As long as the fundamental mechanisms are understood and the kinetics determined as a function of pressure, extrapolation to atmospheric conditions is possible. Clearly, confirmation using studies at atmospheric pressure is also important.

Hydrogen atom abstraction can occur to a small extent, particularly with larger and more highly branched compounds. However, the contribution of this path is, overall, relatively small. For example, for the reaction with 3-methyl-f-butene, where there is a weaker allylic C-H bond, ~5-f0% of the reaction proceeds by abstraction at 1 atm in air (Atkinson et al., 1998).

For all but the two smallest alkenes, ethene and propene, the rate constants are at their high-pressure limits at 1 atm, and even for these two compounds, the effective rate constant is within ~ 10% of /cx.

In the case of unsymmetrical alkenes, the OH radical can add to either end of the double bond. There is evidence that, as expected, it preferentially adds to form the secondary radical. For example, for the propene reaction (Cvetanovic, 1976), ~65% of the adducts formed correspond to (34a) and 35% to (34b):

OH + CH3CH=CH2 -» CH3CH—CH2OH, (34a) -> CH3CH(OH)—CH2.

Because the /J- hydroxyalkyl radicals formed are substituted alkyl radicals, they react with 02 to form alkylperoxy radicals, e.g.,

These /3-hydroxyalkyperoxy radicals undergo the same reaction discussed earlier for R02 radicals, i.e., reaction primarily with NO,

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