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2-Octyl

0.35''

3-Octyl

0.34''

4-Octyl

0.32''

" Adapted from Lightfoot et al. (1992). 6 Aschmann et al. (1999) have measured 0.15 for the sum of (2-hexyl + 3-hexyl), 0.19 for(2-heptyl + 3-heptyl + 4-heptyl), and 0.24 for (2-octyl + 3-octyl + 4-octyl).

" Adapted from Lightfoot et al. (1992). 6 Aschmann et al. (1999) have measured 0.15 for the sum of (2-hexyl + 3-hexyl), 0.19 for(2-heptyl + 3-heptyl + 4-heptyl), and 0.24 for (2-octyl + 3-octyl + 4-octyl).

There is evidence that in some cases, the alkoxy radical formed in the ROz + NO reaction contains sufficient excess energy that it can decompose under atmospheric conditions. This is the case, for example, for some of the alkoxy radicals formed in the oxidation of alternate CFCs (see Chapter f3.D.2a). It has also been postulated for the alkoxy radical formed from the NO reaction with H0CH2CH202, formed in the OH + C2H4 reaction (Orlando et al., 1998). in the latter case, about 25% of the excited (H0CH2CH20)* decomposes to HCHO + CH2OH, with the remainder being stabilized. The stabilized radicals then decompose to HCHO + CH2OH or react with 02.

b. Reactions with H02 and R02

Alternate fates of R02 are reactions with H02 or with other R02 radicals:

The reaction of simple alkylperoxy radicals with HOz is believed to occur primarily by path (24a) to form the hydroperoxide, although for more complex R02 radicals there is some evidence for some contribution of the other paths. The rate constant (k24) at room temperature is 5.2 X 10"12 for R = CH3, increasing to 7.7 X 10"12 for C2H5 and -1.5 X fO"11 cm3 molecule ~ 1 s ~ 1 for the larger radicals (Lightfoot et al., 1992; Fenter et al., 1993; Atkinson, 1997a).

The self-reaction of ROz radicals, i.e., ROz + R02, or their reaction with alkylperoxy radicals of different structure, i.e., R02 + R'02 as shown in reaction (25), is complex, in principle, one expects an excited intermediate (ROOOOR)* to be formed, which could decompose by a number of different paths; i.e., the overall reaction could be one of the following paths:

The term "branching ratio" is used to describe the relative importance of each path. Thus the branching ratio for (25a) is the fraction of the total reaction proceeding by this path and is given by k25d/(k25d + ^25b + Clearly, the sum of the branching ratios for a reaction must add up to 1.0.

Table 6.6 gives some of the values of the recommended rate constants for the self-reaction, k25, and

TABLE 6.6 Recommended Rate Constants and Branching Ratios at Room Temperature for the Self-Reactions of Some ROz Radicals"

Branching ratios

TABLE 6.6 Recommended Rate Constants and Branching Ratios at Room Temperature for the Self-Reactions of Some ROz Radicals"

Branching ratios

ro2

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