Indexed on: 09 Jun '09Published on: 09 Jun '09Published in: Journal of Chemical Theory and Computation
Competing pathways arising from the reaction of hydroxycyclohexadienyl radical (1) with O2, a key reaction in the oxidative degradation of benzene under tropospheric conditions, have been investigated by means of density functional theory (UB3LYP) and quantum-mechanical (UCCSD(T) and RCCSD(T)) electronic structure calculations. The energetic, structural, and vibrational results furnished by these calculations were subsequently used to perform conventional transition-state computations to predict the rate coefficients and evaluate the product yields. The trans stereoisomer of the peroxyl radical (4) produced by the O2 addition to position 2 of benzene ring in radical 1 is energetically more stable than the cis one, although the rate coefficients at 298 K for the formation of both isomers are predicted to be similar. The cyclization of the cis isomer of 4 to a bicyclic allyl radical (5) involves calculated barrier heights (ΔU(⧧), ΔE(⧧), ΔH(⧧), and ΔG(⧧)) significantly lower than those of the cyclization of the trans isomer of 4. This implies that the formation of the cis isomer of 4 can lead to irreversible loss of radical 1 and that the observed chemical equilibrium 1 + O2 ↔ 4 essentially involves the trans isomer of 4. Although the reaction enthalpies computed for the O2 addition to position 4 of benzene ring in radical 1, affording the cis and trans stereoisomers of a peroxyl radical (6), are similar to those for the addition to position 2, the latter addition mode is clearly preferred because it involves lower barrier heights. The barrier heights computed for the cyclization of either the cis or the trans isomers of 6 to a bicyclic radical bearing a peroxy bridge (7) are about twice those computed for the cyclization of either the cis or the trans isomers of 4. Thus, under tropospheric conditions, it is unlikely that the O2 addition to position 4 of the benzene ring in radical 1 can contribute to the formation of benzene oxidation products.