Study of static and dynamic structure of β-quinol-methanol clathrate by 13C high-resolution solid-state NMR and proton T1 measurements

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Abstract
13C high‐resolution solid‐state NMR spectra and proton spin‐lattice relaxation times T1 have been measured for β‐quinol clathrate with trapped methanol molecules over a wide temperature range of 4.2–363 K. Above the transition point (∼67 K), a single and axially‐symmetric 13C chemical shift tensor is observed for the trapped methanol, demonstrating that the molecules reorient themselves among three potential minima in the cavity. Its anisotropy allows us to obtain a temperature‐dependent angle ϑ between the CO bond and the crystallographic c axis. Analysis of the T1 results gives the motional parameters (Ea = 0.7±0.2 kcal/mol, τ0 = 3.7×10−12 s for C3‐ reorientation and Ea = 2.4±0.1 kcal/mol, τ0 = 4.5×10−14 s for inversion about the cavity center) and the position of the methanol molecule in the cavity. Each 13C absorption line for both methanol and quinol is found to split into three below the transition point. This splitting indicates that the methanol molecules are settled down in any of the six orientations and simultaneously the quinol lattice is distorted from the space group R3̄ below the transition point. From the methanol 13C chemical shift tensor, the direction and the librational frequency of the settled CO‐bond axis are determined. Ordering of the methanol dipoles, methyl tunneling motion, and residual C3‐reorientation below the transition point are also discussed.