Substituent effects on the electronic structure and pKa of benzoic acid

The effects of substituents on the pK(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 pK(a)s, including the Lowdin, 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 (DeltaE(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 pK(a): DeltaH(ptop), r(2) = 0.958; Q(L)(H), r(2) = 0.963; Q(n)(CO2H), r(2) = 0.969; Q(L)(CO2), r(2) = 0.970; and Q(L)(CO2H), r(2) = 0.978. The best correlation observed with the pK(a), as expected, vas from the empirical Hammett constants designed for this purpose (r(2) = 0.999). (C) 2002 Wiley Periodicals, Inc.