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Non-covalent interactions of aromatics are important in a wide range of chemical and biological applications. The past two decades have seen numerous reports of arene-arene binding being understood in terms Hammett substituent constants, and similar analyses have recently been extended to cation-arene and anion-arene binding. It is not immediately clear why electrostatic Hammett parameters should work so well in predicting the binding for all three interactions, given that different intermolecular forces dominate each interaction. This review explores such anomalies, and summarizes how Hammett substituent constants have been employed to understand the non-covalent binding in arene-arene, cation-arene and anion-arene interactions.

A computational study aimed at accurately predicting the strength of the anion-π binding of substituted benzenes is presented. The anion-π binding energies (E) of 37 substituted benzenes and the parent benzene, with chloride or bromide were investigated at the MP2(full)/6-311++G** level of theory. In addition, energy decomposition analysis was performed on 27 selected chloride-arene complexes via symmetry adapted perturbation theory (SAPT), using the SAPT2+ approach. Initial efforts aimed to correlate the anion-π E values with the sum of the Hammett constants σ (Σσ) or σ (Σσ), as done by others. This proved a decent approach for predicting the binding strength of aromatics with electron-withdrawing substituents. For the Cl-substituted benzene E values, the correlation with the Σσ and Σσ values of aromatics with electron-withdrawing groups had r values of 0.89 and 0.87 respectively. For the Br-substituted benzene E values, the correlation with the Σσ and Σσ values of aromatics with electron-withdrawing groups had r values of 0.90 and 0.87. However, adding aromatics with electron-donating substituents to the investigation caused the correlation to deteriorate. For the Cl-substituted benzene complexes the correlation between E values and the Hammett constants had r = 0.81 for Σσ and r = 0.84 for Σσ. For the Br-substituted benzene complexes, the respective r values were 0.71 for Σσ and 0.79 for Σσ. The deterioration in correlation upon consideration of substituted benzenes with electron-donating substituents is due to the anion-π binding energies becoming more attractive regardless of what type of substituent is added to the aromatic. A similar trend has been reported for parallel face-to-face substituted benzene-benzene binding. This is certainly counter to what electrostatic arguments would predict for trends in anion-π binding energies, and this discrepancy is further highlighted by the SAPT2+ calculated electrostatic component energies (E). The E values for the Cl-substituted benzene anion-π complexes are all more binding than the E value for the Cl-benzene complex, with the exception of chloride-1,3,5-trimethylbenzene. Again, this is a similar trend to what has been reported for parallel face-to-face substituted benzene-benzene binding. A discussion on this surprising result is presented. In addition, an improved approach to predicting the relative anion-π binding strength of substituted benzene is developed using the results of the SAPT2+ calculations.