Quartz chirality

In inorganic salts such as NaC103 discussed above, or quartz, chiral structures are built up from achiral species. An interesting field has been opened by the analysis by Takezoe and colleagues of the so-called banana shaped molecules, which build up chiral domains together with phase separation, although, as is generally believed, they consist of achiral molecules. The analysis of such chiral phases by CD or ACD spectroscopy opens up a new field for an analysis of suprastructural chirality. 

In certain crystals, e.g. in quartz, there is chirality in the crystal structure. Molecular chirality is possible in compounds which have no chiral carbon atoms and yet possess non-superimposable mirror image structures. Restricted rotation about the C=C = C bonds in an allene abC = C = Cba causes chirality and the existence of two optically active forms (i)... 

The gas chromatographic separation of some sulphoxide enantiomers was observed on quartz fused silica capillaries coated with the chiral silicon phase chirasil-val280. 

Experiments with these catalysts, from an old study by Schwab and Rudolph,3 showed that enantioselectivity occurs at the metal-quartz interface during enantioselective dehydrations of racemic 2-butanol. Klabunovskii and colleagues expanded on this idea and prepared a chiral Ni-quartz catalyst that... 

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 the first half of the nineteenth century, it was known that certain minerals, the prime example being quartz, formed chiral crystals. Often, it was seen that rocks could be composed of a physical mixture of small but macroscopic right-handed and left-handed crystals. This kind of mixture, composed of macroscopic chiral domains (crystals) occurring in both enantiomeric forms, was termed a conglomerate. 

In recent years, stereochemistry, dealing with the three-dimensional behavior of chiral molecules, has become a significant area of research in modern organic chemistry. The development of stereochemistry can, however, be traced as far back as the nineteenth century. In 1801, the French mineralogist Haiiy noticed that quartz crystals exhibited hemihedral phenomena, which implied that certain facets of the crystals were disposed as nonsuperimposable species showing a typical relationship between an object and its mirror image. In 1809, the French physicist Malus, who also studied quartz crystals, observed that they could induce the polarization of light. 

This chromalographic method is convenient and useful in a number of cases. Crystals of quartz which are chiral exhibit different adbsoptive power for several antipodes and so also starch has been employed. Gas-liquid chromatography (GIC) is also employed to resolve racemates. For example to separate racemic a amino acids they are converted into esters with optically active alcohols like (-)2 butanol or trifluroracetyl devivatives, which are then separated by GLC. 

The observed distribution of chirality among quartz crystals within the Earth is extremely dose to 50 50, as would be expeded from a randomly sdeded sample of... 

Rg. 11.2 Single crystals of morphologically enantiomeric quartz. Note the mirror symmetry of the facets on the respective crystals. Unlike amino acids, the component silicon dioxide molecules have no chirality. The spontaneous resolution of quartz into crystals of opposite morphological handedness is an example of local symmetry breaking in the environment. 

Spontaneous resolution seems to be a general phenomenon, because it is also observed to occur from solutions of achiral sodium chlorate, which crystallizes into morphologically chiral crystals. In the same way as quartz, however, any given spontaneous crystallization produces nearly equal numbers of left- and right-handed sodium chlorate crystals. The explanation for this behavior is that the primary crystallization nuclei that form throughout a cooling concentrated solution must occur with a random distribution of the two morphological chiralities. 

As found for quartz, however, such processes occurring across the Earth will necessarily sum to zero net chirality. That is, because the choice of nucleating chirality is random, the chirality of the crystals deposited from disturbed solutions is also random, and over a large number of such events, the outcomes must sum to equality. 

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. 

This section discusses some simple chemical reactions which convert a chiral nonracemic compound containing no electron absorption band in an easily accessible spectral range into a derivative with absorption in either the visible or quartz ultraviolet region. This is a useful operation if a reliable correlation exists between absolute configuration (conformation) and chiroptical properties. A collection of useful chromophoric systems is found in reference 167. 

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. 

Quartz, which is chiral, has a 6482 topology (Wells uses the designation 6482-b, reserving 6482-a for the more symmetrical NbO net discussed below). In the structures of [M Au(CN)2 2], M is Zn or Co, M provides the 4-connected centers and Au(CN)2 acts as an approximately linear bridging group to generate quartz-like nets [31]. Six such nets, all of the same hand, then interpenetrate [31]. 

The first reported attempts of what was then called "absolute or total asymmetric synthesis" with chiral solid catalysts used nature (naturally ) both as a model and as a challenge. Hypotheses of the origin of chirality on earth and early ideas on the nature of enzymes strongly influenced this period [15]. Two directions were tried First, chiral solids such as quartz and natural fibres were used as supports for metallic catalysts and second, existing heterogeneous catalysts were modified by the addition of naturally occuring chiral molecules. Both approaches were successful and even if the optical yields were, with few exceptions, very low or not even determined quantitatively the basic feasibility of heterogeneous enantioselective catalysis was established. 

The first successful experiments were reported by Schwab [16] Cu, Ni and Pt on quartz HI were used to dehydrogenate racemic 2-butanol 23. At low conversions, a measurable optical rotation of the reaction solution indicated that one enantiomer of 23 had reacted preferentially (eeright-handed quartz gave the opposite optical rotation it was deduced that the chiral arrangement of the crystal was indeed responsible for this kinetic resolution (for a review see [8]). Later, natural fibres like silk fibroin H5 (Akabori [21]), polysaccharides H8 (Balandin [23]) and cellulose H12 (Harada [29]) were employed as chiral carriers or as protective polymer for several metals. With the exception of Pd/silk fibroin HS, where ee s up to 66% were reported, the optical yields observed for catalysts from natural or synthetic (H8, Hll. H13) chiral supports were very low and it was later found that the results observed with HS were not reproducible [4],... 

MIP films, applied to a QCM transducer, have been employed for chiral recognition of the R- and 5-propranolol enantiomers [107]. MIP films were prepared for that purpose by surface grafted photo-radical polymerization. First, a monolayer of 11-mercaptoundecanoic acid was self-assembled on a gold electrode of the quartz resonator. Then, a 2,2 -azobis(2-amidinopropane) hydrochloride initiator (AAPH), was attached to this monolayer. Subsequently, this surface-modified resonator was immersed in an ACN solution containing the MAA functional monomer, enantiomer template and trimethylolpropane trimethacrylate (TRIM) cross-linker. Next, the solution was irradiated with UV light for photopolymerization. The resulting MIP-coated resonator was used for enantioselective determination of the propranolol enantiomers under the batch [107] conditions and the FIA [107] conditions with an aqueous-ACN mixed solvent solution as the carrier. The MIP-QCM chemosensor was enantioselective to 5-propranolol at concentrations exceeding 0.38 mM [107]. 

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