Through-Space Stabilization of an Imidazolium Cation by Aromatic Rings

Imidazole-based compounds are widely found in natural products, synthetic molecules, and biomolecules. Noncovalent interactions between the imidazole ring and other functional groups play an important role in determining the function of diverse molecules. However, there is a limited understanding of the underlying noncovalent interactions between imidazoles and aromatic systems. In this work, we report physical-organic chemistry studies on 2-(2,6-diarylphenyl)-1H-imidazoles and their protonated forms to investigate the noncovalent interactions between the central imidazole ring and two flanking aromatic rings possessing substituents at the para/meta position. Hammett analysis revealed that pK_a values and proton affinities correlate well with Hammett σ values of para-substituents at the flanking rings. Additional quantitative Kohn-Sham molecular orbital and energy decomposition analyses reveal that through-space π-π interactions and NH-π interactions contribute to the intramolecular stabilization of the imidazolium cation. The results are important because they clearly demonstrate that the imidazolium cation forms energetically favorable noncovalent interactions with aromatic rings via the through-space effect, a knowledge that can be used in rational drug and catalyst design.