Twinned crystal structure determination

Liu S, Lu YJ, Kappes MM, Ibers JA (1991) The Structure of the Cw Molecule-X-ray Crystal-Structure Determination of a Twin at 110K, Science 254 408-410... 

In the series of the binary halides of selenium and tellurium, the crystal structure determinations of tellurium tetrafluoride (100) and of tellurium tetrachloride on twinned crystals (65, 66) were the key to understanding the various and partly contradictory spectroscopic and other macroscopic properties (e.g., 66,161,168,169,219,220, 412), as well as the synthetic potential of the compounds. In contrast to the monomeric molecular i//-tbp gas phase structures with C2v symmetry (417), the solid state structures of both are polynuclear. As the prototype of the chlorides and bromides of selenium and tellurium, crystalline tellurium(IV) chloride has a cubane-like tetrameric structure with approximate Td symmetry (Fig. 1). Within the distorted TeCla+a octa-hedra the bonds to the triply bridging chlorine ligands are much longer than to the terminal chlorines. The bonding system can be described either covalently as Te4Cli6 molecules, or, in an ionic approximation, as [(TeCl Cn4] with a certain degree of stereochemical activity of the lone pairs toward the center of the voluminous cubane center (65, 66). 

The single crystal structure determination by Threadgold [63THR] of the natural product may be not perfect due to extensive twinning of the material. Hellyerite forms predominantly twin crystals to enhance the low crystal symmetry. Therefore the crystal structure needs to be refined to resolve its subtleties. 

Overall, therefore, the introduction of CCD area detectors for laboratory X-ray diffraction has provided an opportunity to determine structures from samples that were previously not feasible or impractical (crystals too small or twinned). In addition, data quality and therefore structure quality have improved and data collection times have decreased. To organometallic chemists, who often rely upon crystal structure determination to characterize new and unusual ligand bonding where unambiguous spectroscopic characterization is difficult or impossible, this change is a great step forward. 

P2i/c Z = 4 Dx = 1.392 R = 0.22 (all crystals were twinned) for 3,298 intensities. The structure determination was undertaken in order to establish the configuration. The cyclohexane ring is slightly distorted, owing to st/n-axial 0-2-0-4 interaction. The acetate groups are planar within 6°. 

Of the morphological phenomena mentioned in the last few paragraphs, that of twinning is likely to be of most frequent value in identification problems, but all the phenomena are significant from the point of view of crystal structure and the relation between internal structure and growth characteristics. The subject of crystal morphology in relation to internal structure will not, however, be pursued further at present it will be taken up again in Chapters VII and VIII. For the present, we shall continue our consideration of the problem of the identification of microscopic crystals we pass oij to discuss crystal optics, the relation between optical properties and crystal shape and symmetry, and the determination of refractive indices and other optical characteristics under the microscope. 

There are four independent molecules in the crystal of 205 at 138 K two of them show rotational disorder about their central bonds. Another phase formed on cooling consists of twinned crystals, which cannot be used for a structure determination. Approximate sysmmetry and bond lengths of about 1.60 and 1.53 A (corrected for the... 

Nevertheless, it is clear that any anisotropic - growth that results from the preferential binding of organic species to certain crystal faces relies on the crystal structure of the seed nanoparticles. Whether the seeds are single crystalline or whether they possess any twin planes or other defects, will determine the type and orientation of the crystal faces that are exposed to the growth medium in the first place. This is all the more apparent when we consider that in most syntheses a range of particle shapes are observed and yet the same shaped particle can be the major product of very different syntheses. 

---