Crystal absolute polarity

From the relative intensities of the two diffracted waves one can deduce the absolute direction of the vector W —> Y with respect to the b axis. This type of reasoning was exploited by Nishikawa and Matsukawa (14) in 1928 and independently by Coster, Knol, and Prins (15) in 1930 to determine the absolute polarity of successive layers of zinc and sulfur in a polar crystal of zinc sulfide. In the zinc sulfide crystal, planes of zinc and sulfur alternate parallel to the face (111), as shown in Scheme 3 the distance between the close pairs of zinc and sulfur planes is one-quarter of the whole 111 spacing. 

In principle, an anisotropic reaction performed on a crystal of polar symmetry may fix the absolute direction of the polar axis. In the case of an asymmetric reaction carried out in a centrosymmetric (enantiopolar) crystal, one may establish the absolute configuration of the chiral product. The degree of reliability of the assignment will depend on knowledge of the various states of the reaction pathway. Here we briefly describe some heterogeneous reactions in polar and enantiopolar crystals that illustrate this approach. 

In this study, an important piece of information was gleaned regarding the mechanism of the reaction from the assignment of the absolute sign of the polar axis. Of course, in transformations of this type, if the reaction mechanisms are well established, one may proceed in a reverse manner and assign the absolute polarity of the crystal (and therefore the absolute configuration of the chiral molecules) by determining the preferential direction of the attack. 

Macroscopic physical phenomena in polar crystals, such as pyroelectricity, piezoelectricity, and optical activity, would appear to be useful for the assignment of absolute polarity of crystals, provided that one can explain such phenomena at the required level in terms of the atomic arrangement. It appears from the... 

DKPs are soluble in hot absolute alcohol. Young et reported that DKPs are readily crystallized from ether, acetone, ether—acetone, and methanolic solutions. DKPs that separate out on the concentration of the alcoholic solutions or extracts may be recrystallized from alcohol or ethyl acetate. Crystals of polar DKPs can be obtained by slow evaporation of aqueous solutions. ... 

In 2,3,7,8-tetramethoxythianthrene 3-III, the less stable (lower in both density and absolute value of lattice energy) monoclinic modification is obtained under kinetic conditions rapid crystallization from polar diisopropyl ether, whereas the more stable (higher density and lattice energy) orthorhombic modification is thermodynamically obtained from a non-polar hydrocarbon solvent. 

Single crystals In principle, this classical technique makes it possible to obtain nearly perfect orientation and absolute polarization assignments. However, both the measurement and the interpretation are often difficult. Very thin crystals are required for electronic absorption studies. Intermolecular interactions (Davydov splitting) often complicate the spectra. The solution is to find a single crystal of a suitable host molecule, transparent in the region of interest. These two conditions are often very difficult to meet simultaneously. 

F. Assignment of the Absolute Configuration of a Chiral Crystal Containing a Polar Axis... 

The morphological differences between crystals grown in the presence and absence of the additive would then indicate the direction of the substrate molecule W-Y with respect to the polar axis. Consequently, the absolute configuration of the crystal and of the chiral molecular constituents can be derived. The additive need not be chiral, and even if it is, the assignment of the absolute configuration... 

In the orthorhombic point group mm2 there is an ambiguity in the sense of the polar axis c. Conventional X-ray diffraction does not allow one to differentiate, with respect to a chosen coordinate system, between the mm2 structures of Schemes 15a and b (these two structures are, in fact, related by a rotation of 180° about the a or c axis) and therefore to fix the orientation and chirality of the enantiomers with respect to the crystal faces. Nevertheless, by determining which polar end of a given crystal (e.g., face hkl or hkl) is affected by an appropriate additive, it is possible to fix the absolute sense of the polar c axis and so the absolute structure with respect to this axis. Subsequently, the absolute configuration of a chiral resolved additive may be assigned depending on which faces of the enantiotopic pair [e.g., (hkl) and (hkl) or (hkl) and (hkl)] are affected. 

I. Determination of the Absolute Direction of a Polar Axis in a Chiral or Achiral Crystal... 

V. ABSOLUTE CONFIGURATION FROM CHEMICAL REACTIVITY IN POLAR AND ENANTIOPOLAR CRYSTALS... 

These effects have proved important in improving the methods available for resolution of enantiomers by crystallization (267). Furthermore, by studies of the morphological changes induced, one may determine the faces at which the impurities are dominantly attached (270,271). Then, in suitable systems, it is possible to determine the absolute configuration of a polar crystal if one knows that of the impurity (272), or to determine that of the impurity if one knows the structure of the centrosymmetric crystal with which it interacts (270). 

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