Quartz enantiomorphism

Another hypothesis on homochirality involves interaction of biomolecules with minerals, either at rock surfaces or at the sea bottom thus, adsorption processes of biomolecules at chiral mineral surfaces have been studied. Klabunovskii and Thiemann (2000) used a large selection of analytical data, provided by other authors, to study whether natural, optically active quartz could have played a role in the emergence of optical activity on the primeval Earth. Some researchers consider it possible that enantioselective adsorption by one of the quartz species (L or D) could have led to the homochirality of biomolecules. Asymmetric adsorption at enantiomor-phic quartz crystals has been detected L-quartz preferentially adsorbs L-alanine. Asymmetrical hydrogenation using d- or L-quartz as active catalysts is also possible. However, if the information in a large number of publications is averaged out, as Klabunovskii and Thiemann could show, there is no clear preference in nature for one of the two enantiomorphic quartz structures. It is possible that rhomobohedral... 

In 1822, the British astronomer Sir John Herschel observed that there was a correlation between hemihedralism and optical rotation. He found that all quartz crystals having the odd faces inclined in one direction rotated the plane of polarized light in one direction, while the enantiomorphous crystals rotate the polarized light in the opposite direction. 

Quartz is a naturally occurring chiral inorganic crystal. It exhibits either a dextrorotatory (d) or levorotatory (f) enantiomorph. Quartz has been considered as one of... 

Crystallization and reactivity in two-dimensional (2D) and 3D crystals provide a simple route for mirror-symmetry breaking. Of particular importance are the processes of the self assembly of non-chiral molecules or a racemate that undergo fast racemization prior to crystallization, into a single crystal or small number of enantiomorphous crystals of the same handedness. Such spontaneous asymmetric transformation processes are particularly efficient in systems where the nucleation of the crystals is a slow event in comparison to the sequential step of crystal growth (Havinga, 1954 Penzien and Schmidt, 1969 Kirstein et al, 2000 Ribo et al 2001 Lauceri et al, 2002 De Feyter et al, 2001). The chiral crystals of quartz, which are composed from non-chiral Si02 molecules is an exemplary system that displays such phenomenon. 

In place of the above-mentioned chiral organic compounds, chiral inorganic substrates have been examined as chiral initiators. Quartz (Si02) exhibits both dextrorotatory (d) and levorotatory (Z) enantiomorphs that exist in nature. Quartz is considered as one of the origins of chirality of organic compounds186. 

An assembly of molecules may be achiral for one of two reasons. Either all the molecules present are achiral, or the two kinds of enantiomorphs are present in equal amounts. Chemical reactions between achiral molecules lead to achiral products. Either all product molecules will be achiral or the two kinds of chiral molecules will be produced in equal amounts. Chiral crystals may sometimes be obtained from achiral solutions. When this happens, the two enantiomorphs will be obtained in (roughly) equal numbers, as was observed by Pasteur. Quartz crystals are an inorganic example of chirality (Figure 2-36b). Roughly equal numbers of left-handed and right-handed crystals are obtained from the achiral silica melt. 

Pure Si02 occurs in only two forms, quartz and cristobalite. The silicon atom is always tetrahedrally bound to four oxygen atoms, but the bonds have considerable ionic character. In cristobalite the Si atoms are placed as are the C atoms in diamond with the O atoms midway between each pair. In quartz there are helices, so that enantiomorphic crystals occur, and these may be easily recognized and separated mechanically. 

Enantiomorphic Mirror-image habit and optical characteristics right- and left-handed crystals Quartz... 

Another imperfection in crystals is called "twinning". This usualty happens when enantiomorphs are present, or possible. A good example is quartz, i.e.-... 

Chapter 1 considers the possible relationships of earthly clays and other minerals to the origin of chirality in organic molecules. Attempts to establish experimental evidence of asymmetric adsorption on clays were unsuccessfiil, but die search for chirality did find naturally occurring enantiomorphic crystals like quartz. Asymmetric adsorption of organic molecules on quartz crystals such as separation of racemic mixtures, like Co or Cr complexes, alcohols and other compounds, allowed for the conclusion that quartz crystals can serve as possible sources of chirality but not of homochirality. This latter conclusion results fi om the finding that all studied locations of quartz crystals contain equal amounts of d- and /-forms. The preparations of synthetic adsorbents such as imprinting silica gels are also considered. More than 130 references are analyzed. 

Vistelius A.B. (1950), The frequency of enantiomorphic quartz types, Zapiski Vsesoyusnogo Mineralog. Obshch. (Collections of All-Union MineralogicalSoc. USSR, russ.),V.19,. 191-195. 

Klabunovskii E.I.(2001) Can enantiomorphic crystals like quartz play role in the origin of homochirahty on Earth, 1, 127-131. 

These results have a threefold significance first, as a practical method of amplifying small chiral effects in organic reactions, second, as a possible way to discover and assign the chirality of substances, such as natural minerals, that can be used as chiral catalysts (or carriers), and third, as a way to search for the possibility of the origin of chirality on Earth, where enantiomorphic minerals like quartz can exist (see Klabunovskii... 

Enantiomorph i- nan-te-9- morf [GK enan-tios opposite (fi . enanti facing, ff. en in -F anti against) + ISR - morph] (1885) n. Enantiomer also, either of a pair of crystals (as of quartz) that are structural mirror images. 

This new inorganic optically active compound has several novel features. Werner s prototype, his "hexol", contained cobalt, a first-row element, and was a cation. F.G. Mann s rhodate, while an anion, contained a second row element. (+) -[Pt(S5)3]2- contains a third-row element. Further, this thioplatinate(IV) shares with the helicene hydrocarbon series the distinction of manifesting molecular optical activity while containing atoms of only two elements. Of course, enantiomorphism is well known in cryst ine binary assemblies (as in ciimabar, HgS, or quartz, Si02) and even in elements (like tellurium). 

Due to the possibility of two enantiomorphous forms of a-quartz several types of twin crystal may appear - Brazilian, Dauphine and Japanese twins. 

Table 12 summarizes on the basis of international point-group symbols the structural distribution with respect to piezoelectric classes, centrosymmetric classes, and enantiomorphism. For example, LiNb03, which belongs to the class 3 m, is piezoelectric, but not enantiomorphic. whereas a-quartz, in class 32, is both piezoelectric and enantiomorphic. The latter characteristic is illustrated in Figure 55. 

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