The structure of deuterated methane–hydrate

Abstract

We present the results of a high-resolution neutron diffraction experiment with a fully deuterated methane hydrate type I at temperatures of 2, 100, and 150 K. Precise crystallographic parameters of the ice-like D2O lattice and the thermal parameters of the encaged methane molecules have been obtained. The parameters of the host lattice differ only slightly from values found for hydrates with asymmetric guests included, which leads to the conclusion that the host lattice of structure I is only a little adaptive. At low temperatures (2 K) the methane molecules in both types of cages present in structure I occupy positions in the center of the cages. At higher temperatures the thermal parameters in both types of cages reflect the surrounding cage geometries or more precisely the translational potentials of the cages. The orientational scattering length density of the CD4 molecules has been analyzed in terms of a multipole expansion with symmetry adapted functions [Press and Hüller, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. A29, 252 (1972); Press, ibid. A29, 257 (1972)]. In both types of cages we found only small modulations of a spherically symmetric scattering density accounting for almost free rotations of the methane molecules. The large and asymmetric cage leads to a somewhat more pronounced modulation of the orientational density than in the small dodecahedral cage. The orientational probability distribution function (PDF) remains nearly unchanged from 2 to 150 K. At 200 K we observed the time-resolved decomposition of the hydrate structure I into hexagonal ice Ih.

Solubility isotope effects in aqueous solutions of methane

“…The free rotation of the methane molecules in the pure liquid and aqueous phases is corroborated by recent inelastic neutron scattering studies which showed that CH 4 molecules rotate almost freely in the cages of methane hydrates. 24 Our ACF results can also be compared with the values reported by Laaksonen and Stilbs. 20 It must be noted that in their study the conditions are slightly different: water and methane are modeled by the earlier TIP4P and united-atom OPLS potentials, respectively, and the aqueous solutions of methane are more concentrated ͑they contain four methane particles per simulation box͒.…”

Methane hydrate formation in confined nanospace can surpass nature

“…Under these conditions, new XRPD peaks suddenly appeared (in o10 min), corresponding to the sI structure of the methane hydrate [27][28][29] . The formation process was completed in 30 min, and peak intensities did not change when continuing the process for another 30 min.…”

“…Under these conditions, natural hydrates crystallize in a cubic structure known as type sI. This structure is constituted by cages of two different types: (i) six large cages having 12 pentagonal and 2 hexagonal faces (denoted by 5 12 6 2 ) formed by 24 water molecules and (ii) two small cages having 12 pentagonal faces (denoted by 5 12 ) formed by 20 water molecules 2 . These cages have an average cavity radius of 0.433 and 0.395 nm, respectively, which is enough to host one methane molecule per cage, resulting in a nominal stoichiometry 1CH 4 Á 5.75 H 2 O.…”

C–C Stretching Raman Spectra and Stabilities of Hydrocarbon Molecules in Natural Gas Hydrates: A Quantum Chemical Study

“…The molecular structures of various water cages ( Figure 2) are obtained from the crystalline structures of sI, 57 sII, 58 sH, 59 and sK. 7 To obtain a likely most stable structure of the empty water cage, we adjusted the arrangement of the hydrogen atoms to minimize the number of the nearest neighbor pairs that both have a dangling H-bond as in Refs.…”