Shape arrangements

We have seen previously shape-selective catalysis by ZSM-5 in the conversion of methanol to gasoline (Chapter 15).-7 Other commercial processes include the formation of ethylbenzene from benzene and ethylene and the synthesis of p-xylene. The efficient performance of ZSM-5 catalyst has been attributed to its high acidity and to the peculiar shape, arrangement, and dimensions of the channels. Most of the active sites are within the channel so a branched chain molecule may not be able to diffuse in, and therefore does not react, while a linear one may do so. Of course, once a reactant is in the channel a cavity large enough to house the activated complex must exist or product cannot form. Finally, the product must be able to diffuse out. and in some instances product size and shape exclude this possibility. For example, in the methylu-uon of toluene to form xylene ... 

An L-shaped arrangement of bromine atoms has been identified by spectroscopy and theoretical studies on the 2 1 71-complex (8) generated by electrophilic bromination of tetraneopentylethylene (6). The reaction stops at the stage (8) which, for the first time, allowed its detection and determination of its thermodynamic parameters by UV spectroscopy (Scheme 1). Theoretical calculations predict an alkene-Br2 Br2 rather than the Br2-alkene-Br2 structure.22... 

In an atom, the hybridization of s and p orbitals to form sp orbitals provides electron probability areas where bonds can form to make a molecule more stable than if the bonding had occurred in the individual s and p orbitals. The sp orbitals have one large lobe and one small lobe and are aligned along x, y, and z coordinates so that four sp orbitals, called sp3 orbitals because they are made of one s and three p orbitals, result in a tetrahedral-shaped arrangement. When there are three sp orbitals, made of one s and two p orbitals, called sp orbitals, the molecular has a triangular-planar shape. If there is bonding in two sp orbitals, made of one s and one p orbital, a linear molecule results. 

Pauling also knew that oxygen atoms in crystals often formed an octahedron, a cube-shaped arrangement, with other elements. Instead of marbles in a box, Pauling began... 

For example, the 2-dimethylaminomethyl-5-methylphenyl copper tetramer [CuC6H3(2-CH2NMe2)(5-Me)]4 contains p2-ligands of the type shown in structure 45a and a butterfly-shaped arrangement of its four metal atoms, whereas the ps-ligand environment is found in 2-dimethylaminophenyl copper compounds 45b. In both types of compounds, pairs of copper atoms are bridged by (hypercoordinate) carbon atoms of the type already noted in Al2Me4Ph2... 

Figure 2. The equilibrium structure of (HCCH)i3 calculated [167] with a polarizable model potential. The central molecule is seen essentially end on. The 12 surrounding molecules are in three layers. The upper and lower layers have three molecules and resemble the structure of the cyclic trimer of acetylene. The middle layer of six acetylene molecules has a pinwheel like arrangement. Puckering of the rings in the layers yields T shaped orientations between acetylenes in different layers, along with the essentially T shaped arrangements for adjacent molecules within each layer. The number of favorable (T shaped) quadrupole quadrupole interactions among acetylenes is thereby enhanced.

Positive sodium ions attract negative chioride ions to form a cube-shaped arrangement in sodium chioride. In this arrangement, six chioride ions surround every sodium ion, and six sodium ions surround every chioride ion. The forces hoiding each ion in piace are ionic bonds. ... 

L. Omelin included a V-shaped arrangement of 16 triads in the 4th etfition of his Handbuch dtr Chemie. 

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