A Theoretical Investigation of the Geometries and Binding Energies of Molecular Tweezer and Clip Host-Guest Systems.

A quantum chemical study of host-guest systems with dimethylene-bridged clips and tetramethylene-bridged tweezers as host molecules and six different aliphatic and aromatic substrates as guests is presented. The geometries and binding energies of the complexes are investigated using the recently developed density functional theory with empirical corrections for dispersion interactions (DFT-D) in combination with the BLYP functional and basis sets of TZVP quality. It is found that the DFT-D method provides accurate geometries for the host-guest complexes that compare very favorably to experimental X-ray data. Without the dispersion correction, all host-guest complexes are unbound at the pure DFT level. Calculations of the clip complexes show that the DFT-D binding energies of the guests agree well with those from a more sophisticated SCS-MP2/aug-cc-pVTZ treatment. By a partitioning of the host into molecular fragments it is shown that the binding energy is clearly dominated by the aromatic units of the clip. An energy decomposition analysis of the interaction energies of some tweezer complexes revealed the decisive role of the electrostatic and dispersion contributions for relative stabilities. The calculations on the tweezer complexes show that the benzene spaced tweezer is a better receptor for aliphatic substrates than its naphthalene analogue that has a better topology for the binding of aromatic substrates. The tweezer with a OAc substituent in the central spacer unit is found to favor complex formation with both aliphatic and aromatic substrates. The theoretical results are qualitatively in very good agreement with previous experimental findings although direct comparison with experimental binding energies which include solvent effects is not possible. The good results obtained with the DFT-D-BLYP method suggest this approach as a standard tool in supramolecular chemistry and as the method of choice for theoretical structure determinations of large complexes where both electrostatic and dispersive interactions are crucial.