The diverse nature of dodecahedral cages in clathrate hydrates as revealed by 129Xe and 13C NMR spectroscopy:: CO2 as a small-cage guest

In this contribution it is shown that Xe-129 NMR chemical shift parameters accurately reflect the relative size and geometry of the small dodecahedral cages in different hydrate structures. The relative sizes and geometric relationships were explored with CO2 as a probe molecule. Whereas CO2 is a relatively poor guest for the D cage in structure I hydrate, it appears to be a very efficient guest for the somewhat larger and asymmetric D cage in structure II hydrate. On the basis of an approximate CO2 distribution over the cages in structure I hydrate a lower limit on the hydration number of CO2 hydrate is determined to be 7.0. On the basis of Xe chemical shift, parameters for structure H hydrate, it was predicted that CO2 should also be a reasonable small-cage guest for this structure. Experimental results are presented that support this prediction and show that CO2 indeed can serve as a helpgas for structure H hydrate.