Hammett constants were developed, and initially employed, to describe the reactivity at an atom directly bonded to an aromatic, <i>meta</i>- or <i>para</i>- to a substituent.  It is not immediately obvious why a constant that was developed to describe the effects of substitution on the ionization of substituted benzoic acids should correlate arene-arene non-covalent binding energies. To varying degrees, non-covalent interactions of aromatics with other aromatics, with cations, and with anions have also been understood in terms of Hammett substituent constants. Of these three general types of aromatic non-covalent interactions, studies of arene-arene interactions were the first to employ Hammett constants as a means of understanding the binding energy, generally in the context of experimental physical organic investigations.  Inductive effects and resonance effects are highly correlated to Hammett constants and suggest that these effects should dominate in determining the binding energies of aromatic interactions. The aromatic quadrapole in describing arene-arene interactons was the focus of early computational studies. Later experimental and computational work suggested that other forces, such as induction, dispersion, and exchange forces might also be important.  Investigations which compare π-π and substituent-subtituent interactions should aid in understanding aromatic interactions.  Experimental, theoretical, and computational work related to the investigation of aromatic interactions is reviewed and summarized.

Why are Hammett constants so effective in predicting the non-covalent binding energies of aromatic interactions: A review

255th National Meeting (2018)

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