Substituent effects on the electronic structure and p K a of benzoic acid

The effects of substituents on the p K a s 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 p K a s, including the Löwdin, Mulliken, AIM, and natural population analysis charges (Q L , Q M , Q A , and Q n ) on atoms of the dissociating carboxylic acid group, the energy difference (ΔE prot ) between the carboxylic acid and its conjugate base, and the Wiberg bond index. Several of these calculated quantities yielded excellent correlations with the experimental p K a : ΔH prot , r 2 = 0.958; Q L (H), r 2 = 0.963; Q n (CO 2 H), r 2 = 0.969; Q L (CO ), r 2 = 0.970; and Q L (CO 2 H), r 2 = 0.978. The best correlation observed with the p K a , as expected, was from the empirical Hammett constants designed for this purpose (r 2 = 0.999).