THE DIVERSE NATURE OF DODECAHEDRAL CAGES IN CLATHRATE HYDRATES AS REVEALED BY XE-129 AND C-13 NMR-SPECTROSCOPY - CO2 AS A SMALL-CAGE GUEST

In this contribution it is shown that Xe-129 NMR chemical shift parame ters accurately reflect the relative size and geometry of the small do decahedral cages in different hydrate structures. The relative sizes a nd 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 larg er and asymmetric D cage in structure II hydrate. On the basis of an a pproximate CO2 distribution over the cages in structure I hydrate a lo wer 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 hydr ate, it was predicted that CO2 should also be a reasonable small-cage guest for this structure. Experimental results are presented that supp ort this prediction and show that CO2 indeed can serve as a helpgas fo r structure H hydrate.