CARBON-DIOXIDE CLATHRATE HYDRATE EPITAXIAL-GROWTH - SPECTROSCOPIC EVIDENCE FOR FORMATION OF THE SIMPLE TYPE-II CO2 HYDRATE

It is known that small atomic and diatomic species such as argon, krypton, nitrogen, and oxygen preferentially form a structure-II clathrate hydrate through occupation of the structure-II small (and large) cages. There have apparently been no similar reports indicating that small triatomic molecules having a diameter of 5.4 angstrom or less also disobey the classical clathrate hydrate size-structure rule by forming a structure-II hydrate. However, an exception has now been recognized in FT-IR studies of CO2 clathrate hydrate metastable thin films prepared by using epitaxial cryogenic methods. For the type-I CO2 simple hydrate, grown epitaxially to the ethylene oxide clathrate hydrate under kinetic control, an occupation of both the small and the large cages is indicated by CO2 nu-3-band peak frequencies of 2347 and 2335 cm-1, respectively. More surprisingly, the small CO2 molecules, with a maximum van der Waals parameter of 4.7 angstrom, are also capable of "stabilizing" the type-II hydrate through interactions within the small cages. This is evident from a single nu-3 absorption band at 2345 cm-1 for the simple hydrate grown epitaxially to the structure-II clathrate hydrate of tetrahydrofuran at 150 K. Thermal effects on the spectra of these simple hydrates, attributed to the onset of rotational motion of the guest molecules within the large cages, are also examined. For example, a transition dipole coupling of the CO2 guest molecules of neighboring cages, apparent from a comparison of the bands of the isotopomers of the naturally abundant C-12 and C-13 isotopes, is disrupted by warning.