Base centered

The committee recommends that the Bureau fund a modest number of university-based centers focused on in-situ processing of dilute resources. This initiative would complement the one proposed for DOE. 

A,BorC = base-centered in the be-, ac or ab plane, respectively F = face-centered (all faces)... 

When the metal complexes constitute the peripheral units (Fig. lb) and/or belong to the branches (Fig. 1 c) of a dendrimer, a number of equivalent metal-based centers are present since dendrimers are usually highly symmetric species by their own nature. The metal-based centers may or may not interact, depending on distance and nature of the connector units. Multielectron redox processes can therefore be observed, whose specific patterns are related to the degree of interaction among the various units. 

Four Ru(tpy) + units with or without carborane-type substituents have been linked to a pentaerythritol core. The resulting compounds exhibit only one cyclic voltammetric wave assigned to the simultaneous oxidation of the four metal-based centers [33]. 

Recently, novel bifunctionalized zinc catalysts have been developed (compounds (N) and (P), Scheme 55). They have both Lewis-acid and Lewis-base centers in their complexes, and show remarkable catalytic activity in direct aldol reactions.233-236 A Zn11 chiral diamine complex effectively catalyzes Mannich-type reactions of acylhydrazones in aqueous media to afford the corresponding adducts in high yields and selectivities (Scheme 56).237 This is the first example of catalytic asymmetric Mannich-type reactions in aqueous media, and it is remarkable that this chiral Zn11 complex is stable in aqueous media. 

GTAP (Global Trade, Assistance, and Production) (2002). The GTAP5 Data Base, Center for Global Trade Analysis. West Lafayette Purdue University. 

The picture of the nitrogen atoms in diazadiboretidines acting as Lewis base centers is also supported by the formation of a 1 1 coordination compound with TiCl4 [Eq. (58)] (91). The B-NMR signal of 22.7 ppm indicates a highfield shift, which cannot be due to d-electrons from tetravalent d -titanium. X-Ray structural analysis shows that bridging chlorine atoms provide the observed electronic saturation of the boron atoms. 

The TMSCN addition on aldehydes has been reported to be catalyzed by the ionic liquid [omim][PFJ [202]. The influence of the counter anion in activating the TMSCN cannot be neglected, since the TMSCN addition on aldehydes can be also catalyzed by a Lewis base. The imidazolinium-dithiocarboxylate 72 has been recently shown to catalyze the reaction also in good yields up to 99% (Scheme 80) [203]. One could assume, that the zwitterion incorporates a Lewis acid and Lewis base center. The reaction did not proceed in the absence of the catalyst. 

While a fully dehydrated surface presents only Lewis acid and Lewis base centers of variable strength, the real oxide surfaces are variably covered by hydroxyl groups [44,83,84]. 

Orthorhombic crystals are similar to both tetragonal and cubic crystals because their coordinate axes are still orthogonal, but now all the lattice parameters are unequal. There are four types of orthorhombic space lattices simple orthorhombic, face-centered orthorhombic, body-centered orthorhombic, and a type we have not yet encountered, base-centered orthorhombic. The first three types are similar to those we have seen for the cubic and tetragonal systems. The base-centered orthorhombic space lattice has a lattice point (atom) at each comer, as well as a lattice point only on the top and bottom faces (called basal faces). All four orthorhombic space lattices are shown in Figure 1.20. 

The crystal descriptions become increasingly more complex as we move to the monoclinic system. Here all lattice parameters are different, and only two of the interaxial angles are orthogonal. The third angle is not 90°. There are two types of monoclinic space lattices simple monoclinic and base-centered monoclinic. The triclinic crystal, of which there is only one type, has three different lattice parameters, and none of its interaxial angles are orthogonal, though they are all equal. 

A crystal lattice is an array of points arranged according to the symmetry of the crystal system. Connecting the points produces the lattice that can be divided into identical parallelepipeds. This parallelepiped is the unit cell. The space lattice can be reproduced by repeating the unit cells in three dimensions. The seven basic primitive space lattices (P) correspond to the seven systems. There are variations of the primitive cells produced by lattice points in the center of cells (body-centered cells, I) or in the center of faces (face-centered cells, F). Base-centered orthorhombic and monoclinic lattices are designated by C. Primitive cells contain one lattice point (8 x 1/8). Body-centered cells... 

The Seven Systems of Crystals are shown in Figure 2.2. The relationship between the trigonal and rhombohedral systems is shown in Figure B.la. The possibilities of body-centered and base-centered cells give the 14 Bravais Lattices, also shown in Figure 2.2. A face-centered cubic (fee) cell can be represented as a 60° rhombohedron, as shown in Figure B.lb. The fee cell is used because it shows the high symmetry of the cube. 

Simple monoclinic Base-centered monoclinic Simple orthorhombic Body-centered orthorhombic Base-centered orthorhombic Face-centered orthorhombic... 

Why is there no face-centered tetragonal space lattice Why is there no base-centered tetragonal ... 

Within a given crystal system, a supplementary subdivision is necessary to be made, in order to produce the 14 Bravais lattices. In this regard, it is necessary to make a distinction between the following types of Bravais lattices, that is, primitive (P) or simple (S), base-centered (BC), face-centered (FC), and body-centered (BoC) lattices [1-3]. 

Simple orthorhombic (SO), body-centered orthorhombic (BoCO), face-centered orthorhombic (FCO), and base-centered orthorhombic (BCO) Simple monoclinic (SM) and base-centered monoclinic (BCM)... 

---