Optical activity crystal classes

Thus there are two classes of optically active crystals. One, like quartz and sodium chlorate, only shows the effect in the crystalline state. Here the ability to rotate the plane of polarized light is related to the fact that the atoms in the crystal are arranged as right-handed or left-handed spirals or other asymmetric shapes. The second class of optically active crystals contains molecules or ions, such as certain tartrates, that are themselves asymmetric the effect persists even when the crystal melts or is dissolved. More details on this subject are given in Chapter 14. 

Table 2. Crystal Systems, Laue Classes, Non-Centrosymmetric Crystal Classes (Point Groups) and the Occurrence of Enantiomorphism and Optical Activity 31...

Crystal System Laue Class" Non-Centrosymmetric Crystal Classesa,b Enantiomorphism Optical Activity"... 

Extensive research has already been carried out on incorporation of fluorine into molecules which can lead to profound and unexpected results on biological activities and/or physical properties [ 1 - 5]. In particular, optically active fluorine-containing molecules have been recognized as a relatively important class of materials because of their interesting characteristics and potential applicability to optical devices such as ferroelectric or antiferroelectric liquid crystals [6-11]. Recent investigations in this field have opened up the possibility for the... 

The N-sulfonyloxaziridines are an important class of selective, aprotic oxidizing reagents.12 Enantiomerically pure N-sulfonyloxaziridines have been used in the asymmetric oxidation of sulfides to sulfoxides (30-91% ee),13 selenides to selenoxides (8-9% ee),14 disulfides to thiosulfinates (2-13% ee),5 and in the asymmetric epoxidation of alkenes (19-65% ee).15-16 Oxidation of optically active sulfonimines (R S02N=CHAr) affords mixtures of N-sulfonyloxaziridine diastereoisomers requiring separation by crystallization and/or chromatography.13... 

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-... 

Of the thirty-two crystal classes, twenty-two lack an inversion center and are therefore known as non-centrosymmetric, or acentric. Crystalline and polycrystalline bulk materials that belong to acentric crystal classes can exhibit a variety of technologically important physical properties, including optical activity, pyroelectricity, piezoelectricity, and second-harmonic generation (SHG, or frequency doubling). The relationships between acentric crystal classes and physical properties of bulk materials are summarized in Table 9.1.1. 

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

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. 

The relation between optical activity and enantiomorphism is not quite so simple for a crystal. Of the thirty-two crystal classes eleven are enantiomorphic ... 

A particular crystal having the symmetry of one of these classes is either left- or right-handed, and if suitable faces happen to develop when the crystal grows hand-sorting may be possible. Optical activity is also theoretically possible in four of the non-enantiomorphic classes ... 

In these classes directions of both left- and right-handed rotation of the plane of polarization must exist in the same crystal. An earlier claim that optical activity is exhibited by a crystal in class m (mesityl oxide oxalic methyl ester) has since been disproved, but it has been experimentally verified in both 4 (CdGa2S4) and 42m (AgGaSj). )... 

For intrinsically chiral species that are inert enough to be resolved conventionally, the measurement of natural optical activity in crystals has the same advantages as single crystal absorption measurements. In addition, however, it also affords the opportunity to determine rotational strengths of species which do not exhibit optical activity in solution. There are two classes of such materials 1) intrinsically achiral chromophores which crystallize in enantiomorphous space groups, and 2) intrinsically chiral but labile chromophores which spontaneously resolve on crystallization. 

Optical activity is observable in any direction for crystals belonging to the two cubic enantiomorphic classes 23 and 432, but, in general, optical activity can only be observed in certain symmetry limited directions. For example, optical activity in the other enantiomorphic classes is only readily observed in a direction fairly close to an optic axis. This is because the different refractive indices that apply to light polarised vertically and horizontally masks the effect in directions further from an optic axis. In the non-enantiomorphic groups, no optical activity is found along an inversion axis or perpendicular to a mirror plane. Thus no optical activity occurs along the optic axis... 

The sodium ammonium salt crystallized from racemic tartaric acid has been found to crystallize in the orthorhombic P2j2j2j space group and contains four molecules in the unit cell [28]. This particular crystal class is noncentrosymmetric, and as a result individual crystals will be optically active. In fact, efficient growth of this tartrate salt only takes place if all the (i ,i )-tartrate molecules crystallize in one ensemble of crystals, and if all the (, /S)-tartrate molecules crystallize in another ensemble. When formed below a temperature of 26°C, the preferred molecular packing does not permit the intermingling of the enantiomers to yield a true racemic crystal. The crystallization of sodium ammonium tartrate below 26°C results in a spontaneous resolution of the substance into physically separable enantiomers. Interestingly, a different polymorph forms above 26°C that requires a completely different packing pattern that allows for the formation of a racemic modification of sodium ammonium tartrate. 

The appearance of E1-E2 optical activity is restricted to those symmetry groups in which the components of a second rank odd-parity tensor are totally symmetric. As pointed out by Jerphagnon and Chemla, optical activity may be observed even in nonenantiomorphous systems due to the nonpseudoscalar parts of the optical activity tensor—only enantiomorphous crystal classes having a nonvanishing pseudoscalar part. 

Also classed with cholesteric liquid crystals are the so-called chiral nematics, whose molecules have a composition which is characteristic of nematic liquid crystal molecules but which possess an optical activity, e.g.. 

Generally, in liquid crystals, it is only phases that contain optically active materials that exhibit chiral liquid crystal modifications. Thus at least one substance in the liquid crystal system must be a stereoisomer that contains at least one asymmetrically substituted atom, and which is present in a greater concentration than its opposite enantiomer. It is the configurational isomers in the system that give rise to chiral properties. Included in configurational isomers are two distinct classes of stereoisomers enantiomers and diastereoisomers [2]. Enantiomers are two molecules that are related to one another as object and nonsuperimposable mirror image, as shown in the upper part of Fig. 1. Diastereoisomers usually contain more than one asymmetric atom, and pairs of diastereoisomers do not share a superim-posable mirror image, as shown in the lower part of Fig. 1 [1, 2]. 

Over the last three decades, chiral carbon-rich compounds have evolved into an interesting and useful class of materials due to the many unique properties that result from the installation of conjugated chromophores within a chiral framework. Chiral carbon-rich materials possess distinct chirooptical and electronic properties that may prove useful for a number of intriguing applications ranging from optically active liquid crystals to nonlinear optical materials. Chiral carbon-rich compounds also represent a potentially valuable scaffold for use in asymmetric reactions and catalysis. 

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