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Fig. 4 Crystal structures of alS AgTlB (A = K,Rb,Cs), b NauKgTlisM (M = Mg,Zn,Cd,Hg), and c Mg2Zni 1, all of which contain centered icosahedral of the main group elements...

The prochirality concept is useful if it is applied to factored structures within a molecule rather than to the whole, because chiral compounds may also contain centers of prostereoisomerism that would become chiral if their homomorphic ligands were made distinct. The methylene carbons of cholesterol or C(3) of chiral trihydroxyglutaric acid (20b) are appropriate examples. 

These seemingly conflicting results are due to the heterogenous nature of the surface of alumina which contains centers of different acid strengths. Since catalytic activity is known to depend on acid strength... 

Optical isomerism is possible whenever the substituents X contain centers of asymmetry polymers obtained from pure enantiomeric monomers are optically active. However, the specific rotation of the polymers is in general clearly different from that of the monomers. Optical isomerism is also possible when asymmetrically substituted carbon atoms are placed in the main chain (see Example 3-25). 

The steady-state luminescence of water-organic complexes is strong and conceals the weaker characteristic luminescence of uranium containing centers, which can be detected by the difference in decay times only. The reason is that the decay time of water-organic complexes is characterized by two time intervals less then 30 ns and more then 10 ms. Since the uranium centers have decay times in the microseconds range, it is possible to detect them by time-resolved spectroscopy. In the time-delayed laser-induced spectroscopy, the luminescence spectra are recorded at a fixed moment after a laser pulse. These spectra maybe different from the integrated steady-state ones since after a certain time short luminescence will be practically absent. 

It is interesting to note that in magmatic apatites the luminescence of uranium containing centers have not been discovered before or after oxidizing heating. Thus it is reasonable to suppose that uranium is present mainly in the 11" + form. The U with an ionic radius of 0.97 A may be located in the apatite structure instead of Ca with the ionic radius of 0.99 A. The most likely way for achieving the excess charge compensation is the Na" for Ca " structural substitutions. 

Conjugate carbonyl compounds contain centers of different hardness (hard Co and soft C=C), consequently hard hydrides favour conversion of enones to allylic alcohols [25]. Such a concept could be operative also in our case since C=C bond is preferably reduced on going from Mgo to SrO. 

Recently interest has been shown in studies of the biochemical processes, which include the action of enzymes with vanadium-containing centers (like those of serine or tyrosine) that catalyze or inhibiting the redox and phosphorylation reactions, and the alkoxides were investigated as the structural models for the latter [1319, 424], A bimetallic phenoxide of vanadium (II, HI) was found to be an efficient reducive agent for molecular nitrogen in the methanol solutions [1034],... 

The procedures for the preparation of tellurium-containing heterocycles and their reactivity display some characteristic peculiarities in comparison with sulfur and selenium analogs (90M12). These are caused by the specificity of the electronic structure and chemical behavior of the tellurium-containing centers and are characterized by the following salient features. 

An equivalent procedure can be used for chains containing centers with S> 1. Since the matrix dimension of the eigenvalue problem increases significantly with higher S, smaller chain segments are used to extrapolate to N —> oo, increasing the uncertainty of this approach. Therefore, for chains with S > 5/2 centers, it is better to use an expression derived for classical spins (i.e., spin vectors that are not quantized with respect to spatial directions) ... 

NHC Ligands with N-Substituents Containing Centers of Chirality. 119... 

NHC ligands with N-substituents containing centers of chirality ... 

Among the NHC ligands with N-substituents containing centers of chirality, polydentate ligands that combine the NHC unit with an anionic functional group have been developed recently. They thus combine two complementary ligating anchor units, which avoids the rotation of the chiral substituents around the C - N axis. Arnold and coworkers reported the synthesis of the... 

A cathode, which is formed in the simplest approach by a conductive substrate (in contact with the membrane and permeable to protons) containing active centers for proton and electron recombination to H2. In a more challenging approach, the cathode contains centers able to chemisorb C02 and convert it catalytically (in the presence of electrons and protons) to fuels or valuable chemicals. 

As for chloroplast membranes, various compounds in mitochondrial membranes accept and donate electrons. These electrons originate from biochemical cycles in the cytosol as well as in the mitochondrial matrix (see Fig. 1-9) —most come from the tricarboxylic acid (Krebs) cycle, which leads to the oxidation of pyruvate and the reduction of NAD+ within mitochondria. Certain principal components for mitochondrial electron transfer and their midpoint redox potentials are indicated in Figure 6-8, in which the spontaneous electron flow to higher redox potentials is toward the bottom of the figure. As for photosynthetic electron flow, only a few types of compounds are involved in electron transfer in mitochondria—namely, pyridine nucleotides, flavoproteins, quinones, cytochromes, and the water-oxygen couple (plus some iron-plus-sulfur-containing centers or clusters). 

Similar experiments with nickel (60) showed that hydrogen-containing centers must be involved there also. The contribution of excitation was... 

Iron sulfur proteins These carry one electron and contain centers that chelate iron with organic and inorganic sulfur. Some centers contain a... 

IV) the isolation of desulforedoxin, a two-iron protein containing centers that represent a variation on the basic rubredoxin structure52). 

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