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 an d chemical properties, whereas variables derived from quantum chemical calc ulations have generally been less effective. Taking the experimental pK(a)s of substituted anilines as a representative physicochemical property, five ab initio quantum chemical indices are compared for effectiveness as one-p arameter regression descriptors for pK(a). All of the tested descriptors pe rformed well for a set of 19 mono-, 13 di-, and 4 trisubstituted anilines, and two performed somewhat better than the traditional Hammett sigma consta nts. Among the calculated quantities, the best representation of the anilin e pK(a)s is produced by the minimum average local ionization energy on the molecular surface.