Cis–Trans Isomerization of Chemically Activated 1-Methylallyl Radical and Fate of the Resulting 2-Buten-1-peroxy Radical

Abstract

The cis-trans isomerization of chemically activated 1-methylallyl is investigated using RRKM/Master Equation methods for a range of pressures and temperatures. This system is a prototype for a large range of allylic radicals formed from highly exothermic (∼35 kcal/mol) OH + alkene reactions. Energies, vibrational frequencies, anharmonic constants, and the torsional potential of the methyl group are computed with density functional theory for both isomers and the transition state connecting them. Chemically activated radicals are found to undergo rapid cis-trans isomerization leading to stabilization of significant amounts of both isomers. In addition, the thermal rate constant for trans → cis isomerization of 1-methylallyl is computed to be high enough to dominate reaction with O(2) in 10 atm of air at 700 K, so models of the chemistry of the (more abundant and more commonly studied) trans-alkenes may need to be modified to include the cis isomers of the corresponding allylic radicals. Addition of molecular oxygen to 1-methylallyl radical can form 2-butene-1-peroxy radical (CH(3)CH═CHCH(2)OO(•)), and quantum chemistry is used to thoroughly explore the possible unimolecular reactions of the cis and trans isomers of this radical. The cis isomer of the 2-butene-1-peroxy radical has the lowest barrier (via 1,6 H-shift) to further reaction, but this barrier appears to be too high to compete with loss of O(2).

Kinetics of the Methyl–Vinyl Radical + O2 Reactions Associated with Propene Oxidation

“…The calculated cis / trans population ratio, however, depended on the employed density functional theory method. The results of Dibble et al 25 may explain why the bimolecular rate coefficients of reactions 2a and 3a measured in this work do not depend on photolysis wavelength employed (193 or 248 nm), although the initial excitation of CH 3 CHCH radicals was significantly above the cis ↔ trans isomerization barrier.…”

Quantum Monte Carlo investigation of the H-shift and O2-loss channels of cis-2-butene-1-peroxy radical

“…[8]. Results of RO-B3LYP calculations that were obtained in trial wave function preparation are also included.…”

“…Following the methodology of Ref. [8], zero-point energy (ZPE) corrections for the FN DMC results were determined at B3LYP/6-311G(d, p) level and the ZPE values were scaled by a factor of 0.99.…”

“…From calculations using the composite CBS-QB3 method [8], the barrier height for 1,6-H-shift is ~3 kcal/mol higher than the endothermic energy for barrierless O 2 -loss. That such a small energy difference can be reliably achieved was demonstrated in a recent study of the OH bond dissociation energy in phenol where the difference between highly correlated theoretical levels was of order 4 kcal/mol [9].…”

“…A recent study of 2-butene-1-peroxy radical reaction channels [8] suggested that the 2-butene-1-peroxy radical, like other allylic peroxy radicals, appears to lose O 2 rather than undergo unimolecular reactions. The 1, 6-H-shift is the only reaction with the potential to compete to any extent with O 2 -loss and contribute to autoignition (Reactions 1, 2).…”