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

摘要

Unusually rapid (E) (sic) (Z) isomerization mechanisms are proposed and theoretically quantified for substituted allyloxy radicals, R'R=C-CH-CH2O(center dot), with R and R' alkyl or oxygenated substituents, termed below beta,gamma-enoxy radicals. These conversions are shown to occur by a sequence of (i) ring closure to nearly isoergic oxiranyl-(CRR)-R-center dot' radicals, (ii) internal rotation of the oxiranyl-moiety over 180, and (iii) oxiranyl-ring reopening to yield the (E) (sic) (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 approximate to 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)-delta-hydroxy-isoprenyloxy radicals resulting from isoprene oxidation at high NO levels should isomerize to their (Z)-analogues at a rate of about 1.5 X 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)-delta-hydroxy-isoprenylperoxy precursors should yield identical and therefore indistinguishable C-5-hydroxycarbonyls as main products.