Substituent effects on the physical properties and pKa of phenol

Substituent effects on the physical properties and pK(a) of phenol. were studied using density functional theory [B3LYP/6-311G(d,p)] calculations. Substituents alter the physical properties of phenol such as the hydroxyl-group C-O and O-H bond lengths, the C-O-H bond angle, and the energy barrier to rotation about the C-O bond, and also influence the hydroxyl-group pKa. Except for the rotational barrier, Hammett or constants showed strong correlation with these property changes. Several quantum chemical parameters, including the natural charge on the phenolic hydrogen Q(n)(H) and the natural charge on the phenoxide oxygen Q(n)(O-), the HF/6-311G(dp) HOMO energy E-homo, and the proton-transfer energy DeltaE(prot), outperformed the empirical Hammett constants in modeling changes in the pK(a). All of these latter parameters yielded correlation coefficients \r \ > 0.94 for the pK(a). (C) 2001 John Wiley & Sons, Inc.