Generation of a Plastic Crystal Including Methane Rotator within Metal-Organic Cavity by Forcible Gas Adsorption

The structural determination of saturated adsorbed methane inside a metal-organic cavity by the forcible pressure
swing adsorption method (ca. 13 MPa) through a gas-adsorption equilibrium state gives a methane inclusion crystal
even at 298 K. The adsorbed methane molecules regularly locate in the pocket-like narrow corners of the necks
of the 1-D channel without disorder. The thermal motion of the pseudo-spherical methane molecules seems to be
effectively suppressed in its translation mode but allowed rotation. In cooling to 90 K, the crystal structure remained
essentially unchanged while the thermal motion decreased, indicating that a lower temperature reduces the rotation
of the adsorbed pseudo-spherical methane. The observed crystal structure could also be influenced by a reduction
of the vibrational magnitude, and a phase transition from a static disordered structure to an ordered state might
occur. The observed crystal state at a higher temperature should have a plastic crystal nature in terms of the
randomness of the orientation of incorporated guests. The single-crystal adsorbent is effective for crystallographic
observation of the thermal activated guest forced into regular alignment in the crystal lattice, which can be used
as a model of the supercritical fluid.