Comment on: Quasi-elastic neutron scattering investigation of the guest molecule dynamics in the bromomethane clathrate hydrate

This comment addresses several points presented in a recent paper in the Journal that affect the interpretation of data obtained in a study of quasi-elastic neutron scattering in methyl bromide clathrate hydrate. The clathrate belongs to a class known as cubic structure I and it has two sites in its hydrogen-bonded network suitable for guest molecules, one in the large cavity in the structure, the other in the small cavity. The data obtained for methyl bromide hydrate were assigned to the two presumed populations of methyl bromide molecules. However, previous composition measurements and NMR data have shown that the small cavity in fact does not contain methyl bromide molecules. This also becomes obvious once the dimensions of the methyl bromide and the host cavity are calculated, the molecule being too large to fit into the cavity by nearly 0.1 nm. Thus the interpretation of data as assigned in the paper cannot be correct. Finally, although the authors claim new insights into guest -host interactions from guest dynamics, this should be presented in light of previous relevant work where this topic has been thoroughly evaluated.