Active quartz crystals

Mason and Tranter performed calculations and alleged that a 1.4% excess of /- over r/-quartz on the surface of Earth is the inevitable consequence of a parity-violating energy difference (PVED). They calculated that the /-quartz is stabilized by lO kT per Si02 unit This allegation is based on a report that there is 50.17% /-quartz and 49.3% r/-quartz in 16,807 samples collected worldwide that is an excess of 1.4% of /-quartz. But a more recent survey of 27,053 samples yields 50.17 % r/-quartz and 49.83 % /-quartz, which is a. 0.39 % excess of r/-quartz. Therefore it is necessary to accept the fact that the terrestrial distribution of optically active quartz crystals is quite random and, consequently, no basis exists for calcula-... 

Thus, optically active quartz crystals remain the only relatively effective stereoselective inorganic adsorbent and this fact must be used in discussions of the role of as5mimetric adsorption on quartz crystals in the origin (or amplification) of optical activity on the surface of the Earth (see Bernal and... 

This chapter summarizes data about the application of chiral metal catalysts supported on optically active quartz crystals in hydrogenation and other reactions. Despite the low enantioselective efficiency of these catalysts, recent result show that almost 100% enantioselectivity results when they are involved in autocatalytic processes. 

Schwab and Rudolph " prepared chiral catalysts by supporting the metals on the surfaces of ferreted fine powdered optically active quartz crystals, which proved to be active during as3nnmetric dehydrogenation and dehydration of racemic butan-2-ol. The dehydration-dehydrogenation reactions of butan-2-ol (Scheme 2.1.) were carried out in the vapor phase at... 

Recently it has been shown that optically active quartz crystals as asymmetric inductors become very effective in autocatalytic enantioselective reactions. Soai et al. have shown that in asymmetric autocatalysis, the action of small amounts of chiral reaction products (involved in the reaction cycle) may enhance the enantioselective excess by a factor of 94 after introduction of an intermediate into the reaction. Optically active synthetic quartz crystals were used in this reaction with ratios of 1 1.9 quartz to aldehyde and 1 2.2 quartz to diisopropyl-zinc. 

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... 

Six years later Sir John Herschel 13> observed that the algebraic sign of the optical rotation of quartz crystals could be correlated with their shape, and the dextro- and levorotatory crystals look like mutual mirror images. This correlation was extended from crystals to molecules by Pasteur 14>, who postulated that the spatial arrangement of atoms in the molecules is responsible for their optical activity, and the dextro-and levorotatory molecules are in a mirror image relation. 

The monotonic increase of immobilized material vith the number of deposition cycles in the LbL technique is vhat allo vs control over film thickness on the nanometric scale. Eilm growth in LbL has been very well characterized by several complementary experimental techniques such as UV-visible spectroscopy [66, 67], quartz crystal microbalance (QCM) [68-70], X-ray [63] and neutron reflectometry [3], Fourier transform infrared spectroscopy (ETIR) [71], ellipsometry [68-70], cyclic voltammetry (CV) [67, 72], electrochemical impedance spectroscopy (EIS) [73], -potential [74] and so on. The complement of these techniques can be appreciated, for example, in the integrated charge in cyclic voltammetry experiments or the redox capacitance in EIS for redox PEMs The charge or redox capacitance is not necessarily that expected for the complete oxidation/reduction of all the redox-active groups that can be estimated by other techniques because of the experimental timescale and charge-transport limitations. 

The Mirror Plane, cr Most flowers, cut gems, pairs of gloves and shoes, and simple molecules have a plane of symmetry. A single hand, a quartz crystal, an optically active molecule, and certain cats at certain times4 do not possess such a plane. The symmetry element is a mirror plane, and the symmetry operation is the reflection of the molecule in the mirror plane. Some examples of molecules with and without mirror planes are shown in Fig. 3.1... 

A Acoustically active area of the quartz crystal resonator... 

Oscillation of the crystal is due to the AC electric field applied across the crystal from metal electrodes. Typical electric fields are quite low, 10-20 V cm-1. The electrodes are deposited directly on the quartz crystal (Fig. 4.4) although, in principle, a noncontact activation of the crystal is also possible. 

The structures of tridymite and cristobalite are more open than that of quartz as shown by their densities quartz, 2.655 tridymite, 2.26, and cristobalite, 2.32 g/cm3. Quartz crystals are optically active, that is,... 

Alternate anti-vs procoagulant activity of human whole blood on a LbL assembly between chitosan and dextran sulfate has been achieved [149,150]. Furthermore, the technique permits the formation of biodegradable nanostructures with nanometer-order thickness on surfaces, which is an important requirement for biomedical applications. The alternating enzymatic hydrolysis of a LbL assembly formed from chitosan and dextran sulfate by chitosanase was demonstrated via measurements with a quartz crystal microbalance (QCM) [151]. The hydrolysis of the assembly was clearly dependent on the surface component. The hydrolysis of the assembly with the dextran sulfate surface was saturated within 10 min and was much faster than the hydrolysis of the assembly with the chitosan surface, although chitosanase can hydrolyse chitosan (Fig. 14). 

Busch and Bailar1 obtained optically active solutions of one of the enantiomers of the ethylenediaminetetraacetato-cobaltate(III) ion by selective adsorption on optically active quartz and by fractional crystallization of the strychnine salt. More recently Dwyer, Gyarfas, and Mellor2 reported the complete resolution using d and Z-tris(ethylenediamine) cobalt(III) chloride. Precipitation of the diastereoisomers was effected by addition of ethanol to the aqueous solution. The volume of ethanol used was critical and often merely the potassium salt separated. 

As discussed above in Chapter 3, ellipsometry and quartz crystal microbalance (QCM) approaches provide a useful insight into the adsorption of both the supporting interfacial assembly and the proteins themselves. Beyond monitoring the adsorption dynamics and the structural integrity of the biomolecule, the orientation of the active site is of prime importance. For example, if the active site itself binds to the self-assembled monolayer, transport of the substrate or co-enzyme may be blocked. 

As with all supramolecular structures, one of the most important issues is whether a direct relationship between the structure of a material and its function or properties can be established. In the following, some examples of polymer systems which show such a correlation will be discussed. The materials addressed will include block copolymers, polyalkylthiophenes and a multilayer system based on the self-assembly of polyelectrolytes. Detailed studies on the electrochemical properties of redox-active polymers, based on poly(vinyl pyridine) modified with pendent osmium polypyridyl moieties, have shown that electrochemical, neutron reflectivity and electrochemical quartz crystal microbalance measurements can yield detailed information about the structural aspects of thin layers of these materials. 

Oyama and Tatsuma measured the resonant frequency and resonant resistance of quartz crystal resonators coated with several redox active polymers [58-60], DNA [61] and tungsten oxide [62]. 

An ELCHEMA Model EQCN-700 electrochemical quartz crystal nanobalance was used in combination with an ELCHEMA Model PS-205B potentiostat in special experiments. As working crystal, an ELCHEMA QC-10-AuPB laboratory quartz crystal with 14 mm of diameter (active area of 0.196 cm2) was used. 

The fundamental resonant frequency (/0) shifts when a thin film is deposited on the surface of the quartz crystal. Under the assumption that the density and the shear modulus of the film are the same as those of quartz and that the film is uniform (constant density and thickness) and covering the acoustically active area of the whole... 

Generally, optical activity is observed when electrons are displaced along chiral paths by an applied electric field. This does not necessarily require the involvement of dissymmetric molecules. Quartz crystals, in which the first observation < of optical activity was achieved, have no chiral molecules or ions, and their optical activity arises simply from the helical placement of atoms in the crystal. Likewise, a crystal composed of achiral untwistable molecules may become optically active if there are strong helical interactions between neighboring molecules. 

Silver evaporations were done in the same preparation chamber using an inhouse-constructed evaporation assembly containing a resistively heated tungsten basket. Ag coverages were controlled by a quartz crystal thickness monitor. The deposition rates were typically 0.01 to 0.1 A/s depending on the desired coverage. Coverages were subsequently calibrated by XPS analysis of the Ag 3d and polymer core level (C Is and O Is) intensities as compared to known standards, and by neutron activation analysis (111. 

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