NMR line shapes of tunneling methyl groups in enclathrated molecules

Abstract

1H NMR line shapes, obtained at low temperatures (T<5 K), are reported for CH3Br, CH3CD2Cl, and CH3CD2Br molecules enclathrated in the heavy water clathrate hydrates, and for CH3CD2I and CH3CCl3 molecules enclathrated together with D2S, in the heavy water double clathrate hydrates. Line shapes for CH3Br and CH3CCl3 are characteristic of very fast and slow tunneling, respectively. Those obtained for the ethyl halides indicated intermediate tunneling rates. To account for the latter line shapes it is necessary to include a distribution in tunneling frequencies in the Apaydin and Clough line shape calculation. Satisfactory simulated spectra are given by a log-normal distribution. The methyl group rotational barrier heights derived from the tunneling frequencies for a threefold sinusoidal barrier can be correlated with the guest molecule dimensions relative to the size of the clathrate hydrate cage.