Molecular recognition in molecular tweezers systems: quantum-chemical calculation of NMR chemical shifts

Abstract

Quantum-chemical calculations for molecular tweezers systems are presented, where the focus is not only on the recognition process in the host–guest systems, but on the self aggregation of the tweezers host as well. Such intermolecular interactions influence the corresponding NMR spectra strongly by up to 6 ppm for proton chemical shifts, since ring-current effects are particularly important. The quantum-chemical results allow one to reliably assign the spectra and to gain information both on the structure and on the importance of intra- and intermolecular interactions. In addition, we study the accuracy of a variety of density functionals for describing the present host–guest systems, where we observe a considerable underestimation of ring-current effects on ¹H NMR chemical shifts at the density functional theory (DFT) level using smaller basis sets such as 6-31G**, so that larger bases like TZP are required. This stands in contrast to the behavior of the Hartree–Fock scheme, where small basis sets, such as 6-31G**, provide reliable ¹H NMR shieldings for molecular tweezers systems.