Dielectric and13C NMR studies of the carbon monoxide clathrate hydrate

Encapsulation kinetics and dynamics of carbon monoxide in clathrate hydrate

“…Figure 1 shows neutron diffraction patterns of CO hydrates obtained in two different synthesis experiments. Consistent with the earlier finding 15 16 , CO hydrate crystallized in sI at 173 bar and 243 K. Over the initial period of two and a half weeks, the CO gas continued to react with D 2 O ice to form sI hydrate. Interestingly, in both experiments, sII hydrate started to form and coexisted with sI hydrate after staying at the same conditions for additional amounts of time.…”

“…Note that, as a result of the disordered nature of guest molecules in the cages2226, the described CO distribution is only one of the most probable structural configurations from our Rietveld analyses, which, however, does not provide insight into the dynamics of guest molecules in the cages. On the other hand, a previous nuclear magnetic resonance (NMR) study on sI clathrate shows1516 that the CO molecules in the large cages undergo anisotropic reorientation with substantial mobility even at 77 K. The proposed model involves rapid motion of CO molecules among sites over each of the 14 faces, with the CO axis orientated towards the cage centre. Clearly, other techniques, such as NMR, Raman, infrared and inelastic scattering spectroscopies, are needed to provide more comprehensive understanding of the disordered state and dynamics of CO molecules in sII clathrate cages.…”

“…Together, these eight positions form a cubic geometry with an edge length of ~3 Å, which has the same orientation as the unit cell. In the 5 12 6 4 cages, CO molecules are distributed at the eight vertices of the cube, which is different from the oblate shape of the CO distribution at the 14 sites towards each face of 5 12 6 2 cage in sI clathrate1516. Such distributions suggest that the intra-cage CO-H 2 O interaction and inter-cage CO-CO interaction can provide a significant source of stability for the sII framework.…”

“…In addition, based on the theoretical calculation, Miller13 predicted that sII CO clathrate is energetically more favourable than its sI counterpart. However, contrary to this prediction, all experimental studies show that CO forms sI clathrate hydrate under moderate pressure and temperature conditions151617181920. Thermodynamic calculations using the classical Lennard–Jones–Devonshire cell model also concluded that CO sI hydrate would be more stable, giving a dissociation pressure that is 20% lower than that for sII clathrate12.…”

CO clathrate hydrate: Near to mid-IR spectroscopic signatures

“…The difference lies probably in the dipolar character or molecular polarizability of the CO molecule during clathration. Using dielectric loss measurements and 13 C NMR spectroscopies, they ascribe a CO reorientation in cages with a 0.14 kJ/mol barrier ($16.8 K), later detailed in Desando et al (1990), as a mean barrier between small-and large-cage ones, with 0.18 kJ/mol and 0.09 kJ/mol, respectively. The existence of such a barrier is confirmed in our spectra, and in Cwiklik and Devlin (2010) mixed EO-CO study at higher temperatures.…”