Comparison of Quantum Chemical Parameters and Hammett Constants in Correlating pK_a Values of Substituted Anilines

Historically, Hammett constants have been extremely effective in describing the influence of substituents on chemical reactivity and other physical and chemical properties, whereas variables derived from quantum chemical calculations have generally been less effective. Taking the experimental pK_as of substituted anilines as a representative physicochemical property, five ab initio quantum chemical indices are compared for effectiveness as one-parameter regression descriptors for pK_a. All of the tested descriptors performed well for a set of 19 mono-, 13 di-, and 4 trisubstituted anilines, and two performed somewhat better than the traditional Hammett σ constants. Among the calculated quantities, the best representation of the aniline pK_as is produced by the minimum average local ionization energy on the molecular surface.