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 mol­ecule] 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 [Fd{\overline 3}m] [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, β′ = 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 [Fd{\overline 3}m] model. As suggested from band splitting observed in the Raman spectra, the [5¹²]-type cages (one crystallographically unique in [Fd{\overline 3}m], four different in A2/n) entrap the hydro­carbon species (CH₄, C₂H₆, C₃H₈, i-C₄H₁₀). The [5¹²6⁴]-type cage was found to be unique in both structure types. It contains the larger hydro­carbon mol­ecules C₂H₆, C₃H₈ and i-C₄H₁₀.