Bond dissociation energy of the phenol OH bond from ab initio calculations

The phenol OH bond dissociation energy (BDE) is currently disputed, despite its importance in combustion chemistry and in the reactions of antioxidants such as vitamin E. We have studied this BDE using the computational methods G3, G3B3 and CBS-APNO, with bond-isodesmic work reactions. These calculations yield a BDE of 89.0 ± 1.0 kcal mol−1, which supports other recent calculations [B.J. Costa Cabral, S. Canuto, Chem. Phys. Lett. 406 (2005) 300]. From our BDE we determine the enthalpy of formation of the phenoxy radical to be 13.9 ± 1.0 kcal mol−1. Comparison of the phenol CH BDE with the vinyl alcohol HCHCHOH BDE reveals the bond in phenol to be around 3 kcal mol−1 stronger than that in vinyl alcohol. Replacement of a H atom with the OH group on benzene or ethylene strengthens the CH bonds on adjacent carbons by 3 to 4 kcal mol−1.