Figure 3: Carbon monoxide distributions.

The distribution of carbon monoxide molecules in 5¹²6⁴ and 5¹² cages at 25 K (a) from Rietveld refinement; (b) from MD simulations with the 10% top possibilities of carbon monoxide molecular trajectories (oxygen in red and carbon in green) (Supplementary Fig. 3); and (c) from MEM analysis with isosurface level of 1.5 fm Å⁻³ for 5¹²6⁴ cage and 3 fm Å⁻³ for 5¹² cage. The Pauling’s half-hydrogen model with an average occupancy of 0.5 for each deuterium atom in water molecules was used for the Rietveld refinement and MEM analysis, and the ordered model was used for MD calculation. The green balls represent possible positions of carbon atoms, and the red balls represent oxygen atoms. In a 5¹²6⁴ cage, the 12-fold coordinated carbon and oxygen positions are each related by a three-fold symmetry axis, and are normal to the hexagonal faces along the body diagonal of the cubic unit cell. The carbon atoms point towards the four hexagons and oxygen atoms point towards the four co-vertexes of the three pentagons. In a 5¹² cage, the sixfold coordinated carbon and oxygen positions are each related by a three-fold body-diagonal axis, and the distribution of the carbon–oxygen bonds forms a solid angle of 102.6°. The carbon positions form a donut-shape ring in between the oxygen positions.