Fast (E)-(Z) Isomerization Mechanisms of Substituted Allyloxy Radicals in Isoprene Oxidation.

Unusually rapid (E) ⇌ (Z) isomerization mechanisms are proposed and theoretically quantified for substituted allyloxy radicals, R'RC═CH-CH2O(•), with R and R' alkyl or oxygenated substituents, termed below β,γ-enoxy radicals. These conversions are shown to occur by a sequence of (i) ring closure to nearly isoergic oxiranyl-C(•)RR' radicals, (ii) internal rotation of the oxiranyl-moiety over 180°, and (iii) oxiranyl-ring reopening to yield the (E) ⇌ (Z)-isomerized oxy radicals. The barriers for all three steps were computed at the CCSD(T)/aug-cc-pVTZ//QCISD/6-311(d,p) level of theory to be only ≈5 ± 2 kcal mol(-1), and the rate constants at 298 K for the overall reactions were evaluated using transition-state theory to be in the range of 10(8)-10(9) s(-1). Specifically, and of relevance to the isoprene oxidation mechanism, it is predicted that the (E)-δ-hydroxy-isoprenyloxy radicals resulting from isoprene oxidation at high NO levels should isomerize to their (Z)-analogues at a rate of about 1.5 × 10(9) s(-1), much faster than the competing 1,5-H shift that was proposed earlier as the major fate of these (E)-oxy radicals ( Dibble, T. S. J. Phys. Chem. A 2002, 106, 6643-6650 ). It is concluded that under high-NO conditions the (E)- and (Z)-δ-hydroxy-isoprenylperoxy precursors should yield identical and therefore indistinguishable C5-hydroxycarbonyls as main products.