Redistribution of electron density in pyridinium and pyrazinium guests induced by complexation with cucurbit[6]uril

Abstract

Since the discovery of the cucurbit[6]uril (CB6) macrocycle, the binding modes of its supramolecular complexes with organic cations have been elucidated using NMR spectroscopy. The observed changes in chemical shifts of guests induced by this complexation follow generally accepted rules. Here we report some exceptions to this usual behavior for complexes of CB6 with pyridinium and pyrazinium guests, as determined by using ¹H, ¹³C, and ¹⁵N NMR spectroscopy. We use quantum chemical calculations to rationalize our observations by the concept of redistribution of the electron density in the aromatic part of the guest caused by complexation with the macrocycle. Furthermore, we used a simple model employing point charges that mimic the CB6host to analyze the electron deformation density induced by complexation. It indicates that the changes in the nuclear magnetic shielding observed for the aromatic part of the guest result from the redistribution of electrons induced mainly by the electrostatic repulsion of the oxygen atoms of the CB6 portal.