Chemical Shift Changes and Line Narrowing in 13C NMR Spectra of Hydrocarbon Clathrate Hydrates

The solid-state C-13 NMR spectra of various guest hydrocarbons (methane, ethane, propane, adamantane) in clathrate hydrates were measured to elucidate the local structural environments around hydrocarbon molecules isolated in guest host frameworks of clathrate hydrates. The results show that, depending on the cage environment, the trends in the C-13 chemical shift and line width change as a function of temperature. Shielding around the carbons of the guest normal alkanes in looser cage environments tends to decrease with increasing temperature, whereas shielding in tighter cage environments tends to increase continuously with increasing temperature. Furthermore, the C-13 NMR line widths suggest, because of the reorientation of the guest alkanes, that the local structures in structure II are more averaged than those in structure I. The differences between structures I and II tend to be very large in the lower temperature range examined in this study. The C-13 NMR spectra of adamantane guest molecules in structure H hydrate show that the local structures around adamantane guests trapped in structure H hydrate cages are averaged at the same level as in the a phase of solid adamantane.