Experimental and theoretical calculation of pKa values of substituted-2,4,6-trinitrodiphenylamines

Abstract

The pKa values of eighteen 2,4,6-trinitrodiphenylamine derivatives were determined experimentally from their UV–vis spectra and their values were also calculated using quantum chemical methods. The accuracy of the various levels of theory and basis sets were assessed using the solvation model based on density and conductor-like screening model. Linear correlation coefficients of the calculated pKa values are in the range 0.44 – 0.89, 0.11 – 0.90 and 0.43 – 0.90, when compared with those obtained experimentally, for the 6-311++G(d,p), 6-31+G(d,p) and 6-31G(d) basis sets respectively. Increasing the size of the basis set by adding polarization and diffuse functions increase the average errors in the calculations, for ωB97XD functional in acetonitrile. Such variation was not observed in other functionals. Furthermore, the mean signed errors (MSE) for calculations with semiempirical methods are highly comparable but with very poor linear correlation coefficients. The M06-2X functional, in combination with the 6-31+G(d,p), 6-311++G(d,p) and 6-31G(d) basis sets, gave a much better performance than the other functionals used in the present study. The effect of empirical dispersion in the PM6-DH + method was not significant, as the results are similar to those obtained with AM1 and PM7. A strong correlation was observed for the pKa values obtained experimentally with Hammett σ constants, CN bond lengths and charges.