Electronic descriptors for analytical use of the benzidine-based compounds and the mechanism of oxidative coupling of anilines

A number of hybrid exchange-correlation functionals, namely, B3LYP, BMK, M06-2X, PBE0, CAM-B3LYP, BH&HLYP, and HSE06, have been employed to calculate the electronic spectra of a series of the N,N'-derivatives of benzidine, their radical cations, and dications; the results are compared with available experimental data. In most cases studied, the time-dependent density functional theory results based on the CAM-B3LYP and BMK functionals indicate a better agreement with the experimental absorption bands in the UV-Vis spectra. The other five functionals provide statistically comparable but lower accuracy. On the basis of the time-dependent density functional theory calculations with both the CAM-B3LYP and BMK functionals and the 6-311 + G(d,p) basis set including effect of polar medium simulation, the electronic spectrum of the benzidine dication has been completely assigned. Because the latter dication is formed during oxidative coupling of two aniline molecules, as the reductive reagents for analytical determination of strong oxidants, the mechanism of this reaction has been clarified on the ground of density functional theory calculations including effect of dispersion for all possible reaction paths and intermediates. By the analysis of spectral properties and ionization energies of several aniline derivatives, a few simple electronic descriptors that help to justify a reagent molecule as a potentially favorable for colorimetric use have been proposed. Copyright (C) 2014 John Wiley & Sons, Ltd.