AB-INITIO CALCULATIONS OF THE POTENTIAL SURFACE FOR THE THERMAL-DECOMPOSITION OF THE PHENOXYL RADICAL

The thermal decomposition of the phenoxyl radical (1) to form CO plus C5H5., a key reaction in the high-temperature oxidation of benzene, ha s been studied using ab initio quantum mechanical electronic structure methods. The complete active space (GAS) SCF method was used for geom etry optimization of 10 stationary points on the ground-state potentia l energy reaction surface and computing their harmonic vibrational fre quencies. Subsequent calculations using the multireference second-orde r perturbation theory based on a CASSCF reference function (CASPT2) wi th 6-31G(d,p) basis set established the energetics along the two alter native reaction paths proposed by Benson and co-workers. The energetic s were further corrected for zero point vibrational energy at the CASS CF level of theory. The present study predicts the decomposition of 1 to occur preferably through an electrocyclic cyclization mechanism inv olving the formation of the 6-oxobicyclo[3.1.0]hex-3-en-2-yl radical ( 2) as intermediate, rather than by a ring-opening process leading to t he (3z)-6-oxo-1,3,5-hexatrien-1-yl radical (4) intermediate. In contra st to early reported experimental kinetic data, the preexponential fac tor of the thermal Arrhenius expression of the rate constant for the u nimolecular decomposition of 1 is predicted to have a normal value (A > 10(13.5) s(-1)).