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

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 CH₃F and a large polar molecule induces defects on the clathrate hydrate framework that allow intercage guest dynamics. ¹³C NMR chemical shifts of a CH₃F/CH₄/TBME sH hydrate and a temperature analysis of the ²H NMR powder lineshapes of a CD₃F/THF sII and CD₃F/TBME sH hydrate, displayed evidence that the populations of CH₄ and CH₃F in the D and D^′ cages were in a state of rapid exchange. A hydrogen bonding analysis using molecular dynamics simulations on the TBME/CH₃F and TBME/CH₄ sH hydrates showed that the presence of CH₃F enhances the hydrogen bonding probability of the TBME molecule with the water molecules of the cavity. Similar results were obtained for THF/CH₃F and THF/CH₄ 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 CH₃F molecules between adjacent small cages.