Computational models involved in calculating the energies of substituted benzene dimers stacking in a sandwich configuration reveal enhanced interactions of all substituted dimers relative to un-substituted benzene. In addition, many current models demonstrate that increasing the number of substituents on the aromatic ring can result in heightened stabilization. While these observations appear to be inconsistent with the Hunter-Sanders model, there still is a lack of consensus on whether electrostatic or dispersion effects control p-p interactions. Thus, experimental models are needed that will supplement the computational studies and further elucidate the nature of p-p interactions. Here, we use liquid chromatography to assess substituent effects on the separation of small chiral molecules. Molecules of varying substitution and electronic character were synthesized, and their separation on Pirkle-type columns was investigated. The results suggest that the presence of the offset p-p interaction orients the molecules on the column, promoting good separation. Interestingly, the strength of the p-p interaction of the analyte and the column material is dependent on the location, number, and the electronic nature of the substituent on the aromatic rings. These results suggest that electrostatics may have a significant contribution towards aromatic interactions in solution, and may encourage better understanding of the chiral recognition mechanism and substituent effects that control aromatic interactions.

Assessment of substituent effects associated with p-p interactions using chiral chromatography

246th National Meeting (2013)

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