Peroxy radical isomerization in the oxidation of isoprene

We report experimental evidence for the formation of C-5-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) + HO2 is k(isom)(295)/k(ISO*2+HO2)(295) = (1.2 perpendicular to 0.6) x 10(8) mol cm(-3), or k(isom)(295) similar or equal to 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+HO2)(T) = 2.0 x 10(21) exp( 9000/T) mol cm(-3). The overall uncertainty in the isomerization rate (relative to k(ISO*2+HO2)) is estimated to be 50%. Peroxy radicals from the oxidation of the fully deuterated isoprene analog isomerize at a rate similar to 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.