Carbon-13 and Fluorine-19 NMR Spectroscopy of the Supramolecular Solid p-tert-Butylcalix[4]arene ·α,α,α-trifluorotoluene

Abstract

The supramolecular 1:1 host–guest inclusion compound, p-tert-butylcalix[4]arene·α,α,α-trifluorotoluene, 1, is characterized by 19F and 13C solid-state NMR spectroscopy. Whereas the 13C NMR spectra are easily interpreted in the context of earlier work on similar host–guest compounds, the 19F NMR spectra of solid 1 are, initially, more difficult to understand. The 19F{1H} NMR spectrum obtained under cross-polarization and magic-angle spinning conditions shows a single isotropic resonance with a significant spinning sideband manifold. The static 19F{1H} CP NMR spectrum consists of a powder pattern dominated by the contributions of the anisotropic chemical shift and the homonuclear dipolar interactions. The 19F MREV-8 experiment, which minimizes the 19F–19F dipolar contribution, helps to identify the chemical shift contribution as an axial lineshape. The full static 19F{1H} CP NMR spectrum is analysed using subspectral analysis and subsequently simulated as a function of the 19F–19F internuclear distance (DFF=2.25±0.01 Å) of the rapidly rotating CF3 group without including contributions from additional libration motions and the anisotropy in the scalar tensor. The shielding span is found to be 56 ppm. The width of the centerband in the 19F{1H} sample-spinning CP NMR spectrum is very sensitive to the angle between the rotor and the magnetic field. Compound 1 is thus an attractive standard for setting the magic angle for NMR probes containing a fluorine channel with a proton-decoupling facility.