Crystal structure base-centered

For particles of the same size, the higher the coordination number of the crystal is, the greater the number of particles in a given volume. Therefore, as the coordination numbers in Figure 12.24 indicate, a crystal structure based on the face-centered cubic unit cell has more particles packed into a given volume than one based on the body-centered cubic unit cell, which has more than one based on the simple cubic unit cell. Let s see how to pack identical spheres to create these unit cells and the hexagonal unit cell as well ... 

The NMR spectrum of the syn isomer shows evidence of a diamagnetic ring current, based on both the relatively low-field position of the vinylic hydrogens and the upfield shift of the methylene hydrogens. The anti isomer shows much less pronounced shifts. The X-ray crystal structure of the syn isomer shows a moderate level of bond alternation, ranging ftom 1.36 to 1.45 A (Fig. 9.4A). In the anti isomer, bond alternation is more pronounced, vith the double bond in the center ring being essentially a localized double bond (Fig. 9.4B). 

It appears that Cluster C catalyzes the chemistry of CO oxidation and transfers electrons to Cluster B, which donates electrons to external acceptors such as ferredoxin. Since a crystal structure of this protein does not exist, the proposed structure of Cluster C is based on spectroscopic measurements. In some cases, the EPR spectrum of a metal center is diagnostic of the type of center. However, the EPR spectra of Cluster C are unusual. The paramagnetic states of Cluster C (Credi and Cred2) have g-values that are atypical of standard [4Fe-4S] clusters (Table III) and are similar to those in a variety of structurally unrelated systems including a t-oxo bridged ion dimer), a [Fe4S4] ... 

Yet another common crystal lattice based on the simple cubic arrangement is known as the face-centered cubic structure. When four atoms form a square, there is open space at the center of the square. A fifth atom can fit into this space by moving the other four atoms away from one another. Stacking together two of these five-atom sets creates a cube. When we do this, additional atoms can be placed in the centers of the four faces along the sides of the cube, as Figure 11-28 shows. 

Reduction of both nickel porphyrins and thiaporphyrins to Ni1 species has been studied by EPR and 2H NMR spectroscopy.179, 2 58 The Ni1 complex of 5,10,15,20-tetraphenyl-21-thiaporphyrin has been isolated and characterized. Reaction of this complex with sulfur dioxide produced a paramagnetic five-coordinated Ni1 S02 adduct, while reaction with nitrogenous base ligands (amines, pyridines, imidazoles) yielded five- and six-coordinate complexes. In addition, the crystal structure of Ni1 diphenyldi-p-tolyl-21-thiaporphyrin has been determined. The coordination geometry about the nickel center is essentially square planar with extremely short Ni—N and Ni—S bonds (Ni—N = 2.015(2) A, 2.014(12) A, and 1.910(14) A and Ni—S = 2.143(6) A).2359... 

The initial solution of the crystal structure of the Torpedo enzyme [28], followed by the mammalian AChE structure [29], revealed that the active center serine lies at the base of a rather narrow gorge that is lined heavily with aromatic residues (Fig. 11-6). The enzyme carries a net negative charge, and an electrostatic dipole is oriented on the enzyme to facilitate diffusional entry of cationic ligands. Crystal structures of several inhibitors in a complex with AChE also have been elucidated [25]. 

Fabry-Perot cavity, 14 849, 850 Fabry-Perot etalons, 11 151, 152 Face-centered cube lattice, 8 114t Face-centered cubic (FCC) crystal structure in Ni-base alloys, 13 512 of spinel ferrites, 11 60 Facial makup, 7 846-847 Facial preparations, 7 842t Facial tridendate ligand, 7 578 Facihtated transport, 15 826-827 carrier, 15 845-846... 

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