Substituent effects on the physical properties and pKa of phenol

Substituent effects on the physical properties and pKa 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 σ constants showed strong correlation with these property changes. Several quantum chemical parameters, including the natural charge on the phenolic hydrogen Qn(H) and the natural charge on the phenoxide oxygen Qn(O−), the HF/6‐311G(d,p) HOMO energy Ehomo, and the proton‐transfer energy ΔEprot, outperformed the empirical Hammett constants in modeling changes in the pKa. All of these latter parameters yielded correlation coefficients ∣r∣>0.94 for the pKa. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001