Molecular recognition in aqueous media. New binding studies provide further insights into the cation-.pi. interaction and related phenomena

We describe a large number of binding studies in aqueous media designed to provide new insights into noncovalent binding interactions, especially the cation-π interaction. The studies include 7 different hosts, over 70 guests, and over 150 new binding constants. In addition to the now standard NMR methods, circular dichroism has proven to be an especially useful tool for determining aqueous binding constants. We have found that, in addition to the alkyliminium and tetraalkylammonium guests we have studied previously, sulfonium and guanidinium guests also show substantial cation-π effects. Bromination of the host greatly enhances its binding ability in a general fashion, primarily as a result of hydrophobic interactions. Addition of methoxy groups did not enhance binding, apparently as a result of a collapse of the host into a conformation that is not suitable for binding. Replacement of two benzene rings of the host by furans or thiophenes also did not enhance binding. Ab initio calculations provide a rationalization for this effect and suggest a clearer model for the cation-π interaction.