Inter-cage dynamics in structure I, II, and H fluoromethane hydrates as studied by NMR and molecular dynamics simulations

Abstract

Prospective industrial applications of clathrate hydrates as materials for gas separation require further knowledge of cavity distortion, cavity selectivity, and defects induction by guest-host interactions. The results presented in this contribution show that under certain temperature conditions the guest combination of CH 3 F and a large polar molecule induces defects on the clathrate hydrate framework that allow intercage guest dynamics.13 C NMR chemical shifts of a CH 3 F/CH 4 /TBME sH hydrate and a temperature analysis of the 2 H NMR powder lineshapes of a CD 3 F/THF sII and CD 3 F/TBME sH hydrate, displayed evidence that the populations of CH 4 and CH 3 F in the D and D cages were in a state of rapid exchange. A hydrogen bonding analysis using molecular dynamics simulations on the TBME/CH 3 F and TBME/CH 4 sH hydrates showed that the presence of CH 3 F enhances the hydrogen bonding probability of the TBME molecule with the water molecules of the cavity. Similar results were obtained for THF/CH 3 F and THF/CH 4 sII hydrates. The enhanced hydrogen bond formation leads to the formation of defects in the water hydrogen bonding lattice and this can enhance the migration of CH 3 F molecules between adjacent small cages. © 2014 AIP Publishing LLC.

Disorder of Hydrofluorocarbon Molecules Entrapped in the Water Cages of Structure I Clathrate Hydrate

“…Reported enhanced water dynamics in CH 3 Fh ydrates (sI, sII, and sH) are likely to be attributable to special host-guest interactions. [41] In other gas hydrate systems, hydrogen bondingc an occur between guest and host, for example, the 5 12 cage of CO 2 in sI CO 2 hydrates. It is reported that the 5 12 cage occupancyo fC O 2 hydrate is about 70 %o rh igher,w hich is greater than that expected for am olecule of this size (the maximum van der Waals diameter (and volume) of CO 2 is 5.…”

“…One other effect of increased host–guest interactions, which are likely to result from hydrogen bonding, is the greater mobility of water molecules, as demonstrated by decreased activation energies for water reorientation. Reported enhanced water dynamics in CH 3 F hydrates (sI, sII, and sH) are likely to be attributable to special host–guest interactions …”

Quantitative analysis of CO2 hydrate formation in porous media by proton NMR

“…Henning et al studied the formation process of CO 2 hydrates from the deuterated ice at the pressure of 6.2 MPa by neutron diffraction. Trueba et al obtained a series of NMR spectra of various hydrates at temperature from 183 to 253 K. Similarly, a lot of experiments on the quantitative analysis of gas hydrates were conducted under extreme high pressure or extreme low temperature, deviated from the usual hydrate formation conditions of several MPa and degrees centigrade . In other words, the results of these researches may be different from actual conditions.…”

A molecular dynamics study of guest–host hydrogen bonding in alcohol clathrate hydrates

“…Previous MD simulations reported that the relaxation time is affected by the occupancy of clathrate hydrates. 59,64 More detailed analysis focusing on the occupancy and types of guest molecules should be performed in future considering varying degrees of occupancy of the small cages.…”