Ethyl (3S)-3-[(3aR,5R,6S,6aR)-6-hydroxy-2,2-dimethyltetrahydrofuro[4,5-d][1,3]dioxol-5-yl]-3-{(3S)-3-[(3aR,5R,6S,6aR)-6-hydroxy-2,2-dimethyltetrahydrofuro[4,5-d][1,3]dioxol-5-yl]-5-oxoisoxazolidin-2-yl}propanoate chloroform monosolvate

The Flack and Watkin 2AD plot based on 1941 acentric Friedel pairs for the title chloro­form solvate shows that the observed intensity differences for Friedel opposites are dominated by random and systematic errors, erasing information about resonant scattering.

The title compound, C₂₂H₃₃NO₁₂·CHCl₃, was obtained as a product of a double aza-Michael addition of hydroxyl­amine on a Chiron with a known absolute configuration. The enanti­opure compound crystallized as a chloro­form solvate, in space group P1, and diffraction data were collected at room temperature with Ag Kα radiation. The Flack parameter refined to x = −0.01 (16); however, the Flack and Watkin 2AD plot clearly shows that differences between Friedel opposites (the D component of the plot) do not carry any reliable information about resonant scattering of Cl atoms, and are rather dominated by random and systematic errors. The R_D factor calculated using 1941 acentric Friedel pairs is R_D = 0.995. On the other hand, the 2A component of the plot, related to average intensities of Friedel pairs, shows that data are of good quality (R_A = 0.069). This example illustrates that while using Ag Kα radiation (λ = 0.56083 Å), scatterers heavier than Cl should be present in a chiral crystal in order to determine confidently the absolute structure of the crystal.

Deciphering the hydrogen-bonding scheme in the crystal structure of triphenylmethanol: a tribute to George Ferguson and co-workers

The disordered crystal structure of tri­phenyl­methanol features tetra­hedral chiral clusters formed through weak hydrogen bonds, leading to the formation of three-dimensional supra­molecular motifs having left or right handedness.

The crystal structure of tri­phenyl­methanol, C₁₉H₁₆O, has been redetermined using data collected at 295 and 153 K, and is compared to the model published by Ferguson et al. over 25 years ago [Ferguson et al. (1992 ▸). Acta Cryst. C48, 1272–1275] and that published by Serrano-González et al., using neutron and X-ray diffraction data [Serrano-González et al. (1999 ▸). J. Phys. Chem. B, 103, 6215–6223]. As predicted by these authors, the hy­droxy groups are involved in weak inter­molecular hydrogen bonds in the crystal, forming tetra­hedral tetra­­mers based on the two independent mol­ecules in the asymmetric unit, one of which is placed on the threefold symmetry axis of the R space group. However, the reliable determination of the hy­droxy H-atom positions is difficult to achieve, for two reasons. Firstly, a positional disorder affects the full asymmetric unit, which is split over two sets of positions, with occupancy factors of ca 0.74 and 0.26. Secondly, all hy­droxy H atoms are further disordered, either by symmetry, or through a positional disorder in the case of parts placed in general positions. We show that the correct description of the hydrogen-bonding scheme is possible only if diffraction data are collected at low temperature. The pro­chiral character of the hydrogen-bonded tetra­meric supra­molecular clusters leads to enanti­omorphic three-dimensional graphs in each tetra­mer. The crystal is thus a racemic mixture of ^supS and ^supR motifs, consistent with the centro­symmetric nature of the R space group.