Molecular dynamics simulation of 13C NMR powder lineshapes of CO in structure I clathrate hydrate

Guest molecules in nonspherical cages of inclusion compounds can possess non-uniform spatial distributions and motion. This can lead to anisotropic lineshapes in the solid-state NMR spectra of the guest species. In this work, we use our previously developed molecular dynamics-based methodology to calculate the lineshape anisotropy of guest CO species in cages of the structure I (sI) clathrate hydrate as an example of the above phenomenon. The linear CO molecules in the oblate large sI clathrate hydrate cages have a preferential alignment in the plane parallel to the two hexagonal faces of the cages. Molecular dynamics simulations are performed at three temperatures in the stability range of the CO sI clathrate to determine the angular distribution of the CO guests in the large and small cages. The experimental 13C NMR powder lineshapes of CO guests in the large cages become narrower as the temperature is increased from 77 K to 220 K (the limit of the stability of the clathrate). Good agreements between the calculated and experimental powder lineshapes are obtained. No assumptions regarding the nature of the guest motions in the cages are required. The dynamics of guest rotation are characterized by studying the orientational autocorrelation function (OACF) for the CO molecules in the large and small cages at different temperatures.