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A disordered racemic crystal of trispivaloylmethane

When two enantiomers have similar geometries, their racemic mixtures may crystallize with structural disorder, so that the same crystalographical sites are occupied by both enantiomeres. Often such enantiomers produce a continuous range of mixed (scalemic) crystals ranging from one pure enantiomer to the other. In such cases, the space group is expected to be the same for crystals of any enantiomeric composition [H.-B. Bürgi, J. D. Dunitz, Helv. Chim. Acta 76 (1993) 1115– 1166 ; H. D. Flack, G. Bernardinelli, Cryst. Eng. 6 (2003) 213-223.]. Trispivaloylmethane is a 1, 3, 3'-triketone with a C3 symmetry. It was obtained by reaction of 2, 2, 6, 6-tetramethylheptane-3, 5-dione with acetanhydride and barium metal. The compound crystalizes from dimethylsulfoxide as elongated colourless prisms. The molecules of the compound show conformational chirality. They pack in an achiral space group R3m. The molecules are placed in 3(a) special position of the space group with the methine C– H bond along the threefold axis. The M and P molecules are placed stohastically in the crystal structure what generates the aditional set of mirror planes. The random placement of the enantiomeric molecules can be explained by the lack of any significant interactions between neighbouring molecules. The shortest contact between molecules is a C– H...O interaction 3, 487 Å ; ; long. Crystal data: C16H28O3, trigonal, R3m (No. 160), a = 15, 717(5) Å ; ; , c = 5, 7927(17) Å ; ; , V = 1239, 2(6) Å ; ; 3, Z = 3, T = 293(2) K, R = 0, 074, wR = 0, 231, 3434 reflections observed.

triketone ; racemate ; enantiomeric disorder

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