Substituent effects on the electronic structure and pKa of benzoic acid



The effects of substituents on the pKas of a set of 16 substituted benzoic acids have been examined using density functional theory [B3LYP/6-311G(d, p)] calculations. A variety of quantum chemical parameters were examined as indicators for the variations observed in the experimental pKas, including the Löwdin, Mulliken, AIM, and natural population analysis charges (QL, QM, QA, and Qn) on atoms of the dissociating carboxylic acid group, the energy difference (ΔEprot) between the carboxylic acid and its conjugate base, and the Wiberg bond index. Several of these calculated quantities yielded excellent correlations with the experimental pKa: ΔHprot, r2= 0.958; QL(H), r2= 0.963; Qn(CO2H), r2= 0.969; QL(COmath image), r2= 0.970; and QL(CO2H), r2= 0.978. The best correlation observed with the pKa, as expected, was from the empirical Hammett constants designed for this purpose (r2= 0.999). © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002