Anisotropy of the 19F Chemical Shift in Trapped CH3F Molecules

We have grown β-quinol clathrates with trapped CH3F and studied the anisotropy of the chemical shift of 19F. It is known that molecules trapped in these compounds become aligned at low temperatures so that effectively single-crystal studies of these molecules become possible. The NMR structure showed that the C–F bond was aligned parallel to the trigonal crystal axis of the clathrates. The resonance was observed at several frequencies with a corresponding constant magnetic field applied successively parallel and perpendicular to the crystal axis. It was found that the frequency of the line center was always highest with the C–F bond parallel to the field. This frequency difference between both directions gave a chemical shift anisotropy σ⊥—σ∥=66±8 ppm. This anisotropy has a different sign from that expected from recent theories for fluorine bonds and the results are discussed in relation to the chemical shifts for certain other fluorine compounds.