The hydrocarbon-bearing clathrasil chibaite and its host-guest structure at low temperature.

ABSTRACT: The natural sII-type clathrasil chibaite [chemical formula SiO₂·(M¹²,M¹⁶), where M^x denotes a guest mol-ecule] was investigated using single-crystal X-ray diffraction and Raman spectroscopy in the temperature range from 273 to 83?K. The O atoms of the structure at room temperature, which globally conforms to space group [V = 7348.9?(17)?ų, a = 19.4420?(15)?Å], have anomalous anisotropic displacement parameters indicating a static or dynamic disorder. With decreasing temperature, the crystal structure shows a continuous symmetry-lowering transformation accompanied by twinning. The intensities of weak superstructure reflections increase as temperature decreases. A monoclinic twinned superstructure was derived at 100?K [A2/n, V = 7251.0?(17)?ų, a' = 23.7054?(2), b' = 13.6861?(11), c' = 23.7051?(2)?Å, ?' = 109.47°]. The transformation matrix from the cubic to the monoclinic system is a_i ' = (½ 1 ½ / ½ 0 -½ / ½ -1 ½). The A2/n host framework has Si-O bond lengths and Si-O-Si angles that are much closer to known values for stable silicate-framework structures compared with the averaged model. As suggested from band splitting observed in the Raman spectra, the [5¹²]-type cages (one crystallographically unique in , four different in A2/n) entrap the hydro-carbon species (CH₄, C₂H₆, C₃H₈, i-C₄H₁₀). The [5¹²6⁴]-type cage was found to be unique in both structure types. It contains the larger hydro-carbon mol-ecules C₂H₆, C₃H₈ and i-C₄H₁₀.

SUBMITTER: Scheidl KS 

PROVIDER: S-EPMC6126654 | biostudies-literature | 2018 Sep

REPOSITORIES: biostudies-literature