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

The distance between the host and guest molecular components of the supramo lecular inclusion compound p-tert-butylcalix[4]arene-fluorobenzene is exami ned by solid-state rotational-echo double resonance (REDOR) NMR and single- crystal X-ray diffraction (XRD) techniques. Unlike many compounds studied w ith REDOR, no isotopic enrichment is required for either interacting nuclei . The C-13 NMR signal of the host methyl group is modulated by heteronuclea r dipolar interactions with the F-19-containing guest in REDOR NMR experime nts. REDOR curves using internuclear distances derived directly from the XR D 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 int eraction. Dipolar interactions between the guest fluorine and both the host methyl group of the immediate host cavity and the crystallographically-equ ivalent methyl carbon of the adjacent host cavity must be considered in thi s SI2-spin system. Additionally, rapid t-butyl methyl group reorientation r educes the effective C-F dipolar interaction. When a structural model incor porating both multiple spin interactions and molecular reorientation is use d, the REDOR curve approaches the experimental curve within experimental er ror. A mathematical treatment of the angular and distance dependence of a g eneralized SIn system of n-spins under motional conditions is developed. Th ese results extend the scope of distance-determining NMR techniques such as REDOR to systems complicated by disorder, molecular reorientation and mult iple-spin interactions.