Intermolecular distance measurements in supramolecular solids: 13C–19F REDOR NMR spectroscopy of p-tert-butylcalix[4]arene–fluorobenzene

The distance between the host and guest molecular components of the supramolecular inclusion compound p-tert-butylcalix[4]arene–fluorobenzene is examined by solid-state rotational-echo double resonance (REDOR) NMR and single-crystal X-ray diffraction (XRD) techniques. Unlike many compounds studied with REDOR, no isotopic enrichment is required for either interacting nuclei. The 13C NMR signal of the host methyl group is modulated by heteronuclear dipolar interactions with the 19F-containing guest in REDOR NMR experiments. REDOR curves using internuclear distances derived directly from the XRD structural unit cell fail to match those determined experimentally. This discrepancy arises from two limitations: (1) consideration of only a single C–F internuclear distance, and (2) use of a static structural model, which neglects the effect of motional averaging on the heteronuclear dipolar interaction. Dipolar interactions between the guest fluorine and both the host methyl group of the immediate host cavity and the crystallographically-equivalent methyl carbon of the adjacent host cavity must be considered in this SI2-spin system. Additionally, rapid t-butyl methyl group reorientation reduces the effective C–F dipolar interaction. When a structural model incorporating both multiple spin interactions and molecular reorientation is used, the REDOR curve approaches the experimental curve within experimental error. A mathematical treatment of the angular and distance dependence of a generalized SIn system of n-spins under motional conditions is developed. These results extend the scope of distance-determining NMR techniques such as REDOR to systems complicated by disorder, molecular reorientation and multiple-spin interactions.