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1.  Pseudomerohedrally twinned monoclinic structure of unfolded ‘free’ nona­ctin: comparative analysis of its large conformational change upon encapsulation of alkali metal ions 
The title compound, C40H64O12, crystallizes in a pseudo­merohedrally twinned primitive monoclinic cell with similar contributions of the two twin components. There are two symmetry-independent half-mol­ecules of nona­ctin in the asymmetric unit. Each mol­ecule has a pseudo-S 4 symmetry and resides on a crystallographic twofold axis; the axes pass through the mol­ecular center of mass and are perpendicular to the plane of the macrocycle. The literature description of the room-temperature structure of nona­ctin as an order–disorder structure in an ortho­rhom­bic unit cell is corrected. We report a low-temperature high-precision ordered structure of ‘free’ nona­ctin that allowed for the first time precise determination of its bond distances and angles. It possesses an unfolded and more planar geometry than its complexes with encapsulated Na+, K+, Cs+, Ca2+ or NH4 + cations that exhibit more isometric overall conformations.
doi:10.1107/S0108270109033083
PMCID: PMC2816929  PMID: 19805886

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