Interaction of two functional groups through the benzene ring: theory and experiment

Energies of 132 benzene para bis-derivatives calculated within the framework of the density functional theory at the level B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p) were used for correlations of two types. Correlation with the experimental enthalpies of formation clearly revealed that the published experimental data are generally not dependable and may be loaded with errors of more than 10 kJ mol(-1). On the other hand, the calculated relative energies are biased so that the interaction of the two substituents is systematically overestimated. This shortcoming was insignificant for our correlations of the second type, in which the interaction of substituents expressed in terms of isodesmic reactions was analyzed depending on the effects of inductive and resonance. The results depended strongly on the character of substituents. When one substituent is an electron donor and the other is an acceptor, the inductive-resonance model works and the classical resonance picture is adequate. With two acceptor substituents, this model is still acceptable with lower precision (as crossed conjugation), but with two donors it fails completely and may be acceptable only for a much restricted subclass of strong donors. Many correlations described in the literature must be viewed with great caution when they are based only on a relatively small number of data, in which substituents of different types are not represented in a comparable number.