Direct Space Methods for Powder X-ray Diffraction for Guest−Host Materials: Applications to Cage Occupancies and Guest Distributions in Clathrate Hydrates

Abstract

Structural determination of crystalline powders, especially those of complex materials, is not a trivial task. For non-stoichiometric guest-host materials, the difficulty lies in how to determine dynamical disorder and partial cage occupancies of the guest molecules without other supporting information or constraints. Here, we show how direct space methods combined with Rietveld analysis can be applied to a class of host-guest materials, in this case the clathrate hydrates. We report crystal structures in the three important hydrate crystal classes, sI, sII, and sH, for the guests CO 2 ,C 2 H 6 ,C 3 H 8 , and methylcyclohexane + CH 4 . The results obtained for powder samples are found to be in good agreement with the experimental data from single crystal X-ray diffraction and 13 C solid-state NMR spectroscopy. This method is also used to determine the guest disorder and cage occupancies of neohexane and tert-butyl methyl ether binary hydrates with CH 4 in the structure H clathrate hydrates. The results are found to be in good agreement with the results from the 13 C solid-state NMR and molecular dynamics simulations. It is demonstrated that the ab initio crystal structure determination methodology reported here is able to determine absolute cage occupancies and the dynamical disorder of guest molecules in clathrate hydrates from powdered crystalline samples.

Methanol incorporation in clathrate hydrates and the implications for oil and gas pipeline flow assurance and icy planetary bodies

“…The powder patterns for the codeposited samples were refined by the Le Bail method using the profile matching method within FULLPROF (27). The structural analysis of the THF hydrates followed method used in the previously reported work by Takeya et al (28). The direct space method using the FOX (free objects for crystallography) program was used to determine initial THF and methanol positions in the cages (29,30).…”

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

“…4 ( 33.3 3 )). [8,23] Another case is that of Cl 2 hydrate, in which there is ah alogen-bonding interaction, ClÀCl···OH 2 .T he diameter of Cl 2 (5.5 ) is also too large for the sI 5 12 cage, yet ac age occupancy of 32.5 %i sr eported. In each case, guest molecules are isotropically (or randomly)d istributed within the 5 12 cage in the same waya s those of HFC hydrates analyzed in this study.T his gives an indicationt hat these guest molecules are mobile in the 5 12 cage, and bonding between the guests and host water molecules occurs isotropically (or randomly) within the symmetrical 5 12 cage.…”

“…The initial crystallographic model for host structuresc omposed of water molecules was introduced from the previously reported models of XRD data of CH 4 hydrate [22] for the directs pace method. [23] The crystal structures, including cage occupancies and distributions of guest molecules within the sI cages,w ere analyzed by using rigid-body constraints for each guest molecule. Because the measured diffraction patterns revealed that the hydrates amples were mixtureso fg as hydrate and as malla mount of unreactedh exagonal ice (Ih) of space group P6 3 /mmc,b ut not the solidified guest molecules, multiphase refinement was performed.…”

Clathrate Hydrates

“…Takeya et al have determined structures of clathrate hydrates containing the guests CO 2 , C 2 H 6 , C 3 H 8 , and methyl cyclohexane + CH 4 using X-ray methods with Rietveld analysis [38]. Shin et al [39] have studied clathrate hydrate hosts with methanol guests via X-ray diffraction [39].…”