Chiral crystal classes

Broadly speaking, chiral space groups may be divided into two classes those that contain polar axes, for example, the commonly observed space groups P2, and C2 and those that do not, such as P2,2,2,. Crystal structures belonging to the latter class contain polar directions, but these do not coincide with the crystal axes. We shall focus on chiral crystals containing polar axes, although the method can in principle be applied to all chiral crystals. 

Finally, reference must be made to the important and interesting chiral crystal structures. There are two classes of symmetry elements those, such as inversion centers and mirror planes, that can interrelate. enantiomeric chiral molecules, and those, like rotation axes, that cannot. If the space group of the crystal is one that has only symmetry elements of the latter type, then the structure is a chiral one and all the constituent molecules are homochiral the dissymmetry of the molecules may be difficult to detect but, in principle, it is present. In general, if one enantiomer of a chiral compound is crystallized, it must form a chiral structure. A racemic mixture may crystallize as a racemic compound, or it may spontaneously resolve to give separate crystals of each enantiomer. The chemical consequences of an achiral substance crystallizing in a homochiral molecular assembly are perhaps the most intriguing of the stereochemical aspects of solid-state chemistry. 

Eleven acentric crystal classes are chiral, i.e., they exist in enantiomorphic forms, whereas ten are polar, i.e., they exhibit a dipole moment. Only five (1,2, 3, 4, and 6) have both chiral and polar symmetry. All acentric crystal classes except 432 possess the same symmetry requirements for materials to display piezoelectric and SHG properties. Both ferroelectricity and pyroelectricity are related to polarity a ferroelectric material crystallizes in one of ten polar crystal classes (1, 2, 3,4, 6, m, mm2, 3m, 4mm, and 6mm) and possesses a permanent dipole moment that can be reversed by an applied voltage, but the spontaneous polarization (as a function of temperature) of a pyroelectric material is not. Thus all ferroelectric materials are pyroelectric, but the converse is not true. 

Crystal system Crystal class Chiral (enantiomorphism) Optical activity (circular dichroism) Polar (pyroelectric) Piezoelectric, SHG... 

Certain aspects of the photoacoustic effect suggest that this technique might be generally applicable to all chiral solids regardless of crystal class, size or perfection, or strength of absorption. Although subsequent theoretical developments and experimental results have caused us to limit considerably the predicted scope of this method, nevertheless, it is possible now to say clearly that the experiment does work and offers prospects for unique results. In this paper we review briefly the nature of the theory and practice of condensed phase photoacoustic spectroscopy and its extension to the measurement of natural circular dichroism, and present initial results for single crystals and powders. 

Liquid crystalline behavior occurs in the exocuticle of certain classes of beetles. The bright iridescent colors that are reflected from the surface of Scarabaeid beetles originates from a petrified chiral nematic stmctural arrangement of chitin crystaUites in the exocuticle (38). It is suggested that this chiral nematic texture forms spontaneously in a mobile, Hquid crystal phase that is present during the initial stages of the exocuticle growth cycle. 

The nine crystal structures were arranged into three classes A, B and C (Table 12). In class A 1 1 complexes of chiral alcohols and DBTA can be found. Class B is the group of 2 1 complexes of achiral alcohols. Finally, in class C one can find the 2 1 complex of racemic 2-methylcyclohexanol (50, Scheme 13) and racemic DBTA. Common feature of all crystalline complexes is the lack of water or other solvate molecule even all of them was prepared from DBTA monohydrate. This obsrevation fits well to the thermoanalytical measurements. 

Belsky, V. K., Zorkii, P. M. (1977) Distribution of organic homomolecular crystals by chiral types and structural classes, Acta. Crystallogr. Sect. A, 33A, 1004-1006. 

D-Pantothenic acid is also traditionally produced by chemical processes which involve efficient but troublesome and costly crystallization of diastereomeric salts of pantoate and chiral amines. After lactonization of the isolated D-pantoate, d-pantolactone is reacted with / -alanine to give D-pantothenate. Because the monovalent salts of pantothenic acid are highly hygroscopic, conversion into the calcium salt is essential for convenient formulation. The third class of synthetic processes for optically active compounds makes use of biotechnology. For natural com-... 

If a molecule or a crystal is chiral, it is necessarily optically active. The converse is, however, not true. There are non-enantiomorphous symmetry classes of crystals that may exhibit optical activity. 

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