Ab initio study of the mechanism for the thermal decomposition of the phenoxy radical

Various ab initio methods have been applied to the studies of the molecular structure and energetics for the low-lying electronic states of the phenoxy radical and of the mechanism of its thermal decomposition. The calculations confirm the decomposition mechanism I (Scheme 1) suggested earlier from experimental consideration. It involves the formation of the bicyclic intermediate E2 followed by alpha-CC bond cleavage to produce E3 and the elimination of the CO group from the latter leading to the main reaction products, C5H5 + CO. The rate-determining step is E2 --> E3 with transition stale T2. The activation energy, 52 kcal/mol, at our best G2M(rcc,MP2) level, is 8 kcal/mol higher than the value derived from experiment. A multistep RRKM calculation has been performed for the decomposition reaction. The theoretical rate constants agree well with the experimental results at lower temperatures (T < 1200 K) but deviate rom experiment above 1300 K. Plausible reasons fur the deviation have been discussed. On the basis of state-specific and state-averaged CASSCF calculations, the observed electronic transitions for the phenoxy radical have been assigned.