A computational analysis of substituent effects on the O-H bond dissociation energy in phenols: Polar versus radical effects

O-H bond dissociation energies (BDEs) of phenol, p-aminophenol, and p-nitrophenol have been computed using ab initio and density functional theory (DFT) methods. The MP2 and MP4 methods consistently overestimate the absolute BDEs but provide reasonable relative BDEs. Spin projected MP2 and MP4 energies are not able to reproduce the substituent effects on the BDE. The BLYP and B3LYP DFT methods provide more reliable and economical approaches for prediction of phenol BDEs. B3LYP/6-31G** computed Delta BDEs for 10 substituted phenols have been compared with values determined by different experimental approaches. The computed values are in most cases within the uncertainty of the measurements. It is shown that the substituent effects on the BDEs can be interpreted in terms of polar and radical stabilization. The polar stabilization is found to be related to the ability of the substituent to delocalize the lone pair on the phenol oxygen. The radical stabilization is dependent on the degree of spin delocalization. A method for estimating relative polar and radical stabilization energies based on computed electrostatic potentials and spin densities is presented.