Peroxy radical isomerization in the oxidation of isoprene

Abstract

We report experimental evidence for the formation of C5-hydroperoxyaldehydes (HPALDs) from 1,6-H-shift isomerizations in peroxy radicals formed from the hydroxyl radical (OH) oxidation of 2-methyl-1,3-butadiene (isoprene). At 295 K, the isomerization rate of isoprene peroxy radicals (ISO∙_2) relative to the rate of reaction of (ISO∙_2) + HO_2 is (k^(295)_(isom))/(k^(295)_((ISO∙_2) + HO_2) = (1.2±0.6) × 10^8 mol cm^(-2), or k^(295)_(isom)≃0.002 s^(-1). The temperature dependence of this rate was determined through experiments conducted at 295, 310 and 318 K and is well described by (k_(isom))T))/(k_((ISO∙_2)+HO_2) (T) = 2.0 × 10^(21) exp(-9000/T)mol cm^(-3). The overall uncertainty in the isomerization rate (relative to k_((ISO∙_2)+HO_2) is estimated to be 50%. Peroxy radicals from the oxidation of the fully deuterated isoprene analog isomerize at a rate of ~15 times slower than non-deuterated isoprene. The fraction of isoprene peroxy radicals reacting by 1,6-H-shift isomerization is estimated to be 8–11% globally, with values up to 20% in tropical regions.

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