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-0 and 0-H bond lengths, the C-0-H bond angle, and the energy barrier to rotation about the C-0 bond, and also influence the hydroxyl-group pK •. Except for the rotational barrier, Hammett a 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 Q,,(0), the HF /6-311G(d,p) HOMO energy E110m0 , and the proton-transfer energy f'..Eprot, outperformed the empirical Hammett constants in modeling changes in the pKa. All of these latter parameters yielded correlation coefficients lrl > 0.94 for the pKa. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 85: 569-579, 2001 Key w ords: Hammett constants; partial atomic charges; pKa