Ab Initio Calculations of the Potential Surface for the Thermal Decomposition of the Phenoxyl Radical

Abstract

The thermal decomposition of the phenoxyl radical (1) to form CO plus C5H5' , a key reaction in the hightemperature oxidation of benzene, has been studied using ab initio quantum mechanical electronic structure methods. The complete active space (CAS) SCF method was used for geometry optimization of 10 stationary points on the ground-state potential energy reaction surface and computing their harmonic vibrational frequencies. Subsequent calculations using the multireference second-order perturbation theory based on a CASSCF reference function (CASPT2) with 6-3 lG(d,p) basis set established the energetics along the two altemative reaction paths proposed by Benson and co-workers. The energetics were further corrected for zero point vibrational energy at the CASSCF level of theory. The present study predicts the decomposition of 1 to occur preferably through an electrocyclic cyclization mechanism involving the formation of the 6-oxobicyclo[3.1 .O]hex-3-en-2-y1 radical (2) as intermediate, rather than by a ring-opening process leading to the (3~)-6-0~0-1,3,5-hexatrien-1-yl radical (4) intermediate. In contrast to early reported experimental kinetic data, the preexponential factor of the thermal Arrhenius expression of the rate constant for the unimolecular decomposition of 1 is predicted to have a normal value (A > 10'3.5 s-I).

Influence of the Temperature and 2,5-Dimethylfuran Concentration on Its Sooting Tendency

“…Subsequent reactions, which include as intermediates phenol (C 6 H 5 OH), phenoxy radicals (C 6 H 5 O), and a bicycle, result in the formation of the cyclopentadienyl radical (C 5 H 5 ). The overall reaction (R1), as a possible pathway to give the cyclopentadienyl radical, has also been confirmed in the works of Olivella et al (1995) and D'Anna (2005):…”

A quantitative kinetic analysis of CO elimination from phenoxy radicals

“…The mechanism for the CO elimination from phenoxy radical is already well established [4,8,9,32]. Ring contraction of the phenoxy radical (1) leads to the bicyclic intermediate (2), which upon C C bond fission converts to the 2,4-cyclopentadienyl formyl radical (3), which in turn dissociates to cyclopentadienyl (4) plus CO. An alternative pathway, which starts with C C bond fission in α position to the C O moiety in phenoxy, is higher in energy and not competitive [8].…”

“…Thus, their recommended rate expression is k(T ) = 7.5E11 exp[−43.9 kcal mol −1 /(RT )] s −1 . The first theoretical studies on the thermal decomposition of phenoxy radicals are those by Olivella et al [8] in 1995 using the CASSCF and CASPT2 methods, and the G2(M) investigation by Liu et al [9] in 1996. Both studies support the mechanism for reaction (1) suggested by Colussi et al [4], which involves formation of a bicyclic intermediate that ring-opens to the 2,4-cyclopentadienyl carbonyl radical and subsequently eliminates CO to form cyclopentadienyl radical (cf.…”

Unimolecular isomerization/decomposition of cyclopentadienyl and related bimolecular reverse process:ab initio MO/statistical theory study

“…7 We have previously investigated the mechanism for the formation of C 3 H 4 (propyne) from the CH 3 + C 2 H 2 reaction, the most likely precursor process of C 3 H 3 , 8 which can be produced readily by the abstraction reactions, X + C 3 H 4 → HX + C 3 H 3 , where X = H, OH, CH 3 , etc. The mechanism for the decomposition of the phenoxy radical producing CO + c-C 5 H 5 has been studied theoretically in detail, 9,10 whereas that for the decomposition of c-C 5 H 5 or its reverse process, C 3 H 3 + C 2 H 2 , is not well understood. This is the objective of the present investigation.…”