Cooperative Interaction of Hydronium Ion with an Ethereally Fenced Hexaarylbenzene-Based Receptor: An NMR and Theoretical Study

Using 1H and 13C NMR and DFT calculations, the structure and interactions of the symmetric ethereally fenced hexaarylbenzene receptor 1 with hydronium ions were studied. Both 1 and its equimolecular complex 1·H3O+ exhibit C 3v symmetry. According to DFT, two similar optimal structures of the complex exist, the more stable one being 15.4 kJ/mol lower in energy. The equilibrium between 1 and 1·H3O+ complexes is characterized by the stabilization constant K = 1.97 × 106 (i.e., the binding constant η = 6.3) according to both proton and carbon NMR spectra. The exchange dynamics between 1 and the complex measured by the delay-varied CPMG sequence had to be corrected for the internal exchange processes in both 1 (conformation change) and the complex (vacillation between the two minima). After this correction, the correlation time of exchange was found to be 4.76 × 10−5 s. Such relatively fast exchange can be explained only by it being mediated by the excess water molecules present in the system.