Wedge structures

One of the most unexpected results obtained was for Pg, where the much-studied cubic (0 ) form corresponds to a shallow local minimum in the energy surface. Simulated annealing led, however, to the Czv structure (Fig. 4b), which is much (ca. 1.7 eV) more stable. This wedge structure, which may be viewed as a (distorted) cube with one bond rotated through 90°, is a structural unit in violet (monoclinic, Hittorf) phosphorus [48]. A second isomer of Pg ( >21, Fig. 4a) is also much more stable than the cubic form. There is a striking analogy between the structures of the Pg-isomers and those of the valence isoelectronic hydrocarbons (CH)g. The cubic form of the latter (cubane) has been prepared by Eaton and Cole [49], and can be converted catalytically to the wedge-shaped form (cuneane) [50]. 

Two of the possible conformations of ethane staggered and eclipsed. Interconversion is easy via a 60° rotation about the C—C bond, as shown by the curved arrows. The structures at the left are spacefilling models. In each case, the next structure is a dash-wedge structure, which, if viewed as shown by the eyes, converts to the sawhorse" drawing, or the Newman projection at the right, an end-on view down the C—C axis, in the Newman projection, the circie represents two connected carbon atoms. Bonds on the front carbon go to the center of the circie, and bonds on the rear carbon go oniy to the edge of the circie. 

Novel chiral diene ligands were reported to form highly active complexes with rhodium for asymmetric conjugate addition (236). Feng s ligand of a wedge structure provided a chiral environment around rhodium, giving excellent enan-tiocontrol in the asymmetric arylation of N-tosylarylimines (237) and in asymmetric 1,4-addition of aryl boronic acids (238). 

While one would expect that this approach could be extended to many other material combinations, the rules are not understood. Efforts to form similar stmctures other than oxides, such as titania, or various crystalline materials, have been only partly successful. Possibly, any rapid or localized conversion process also disrupts the liquid crystalline organization. Stupp and co-workers (2000) have produced a range of amphiphiles that assemble into various ribbon and wedge structures, and the authors have explored their catalytic activity. 

Draw the Fischer projection for each of the following dash-wedge structures ... 

The larger group, —CH3, is in the equatorial position. 26-9. There are two chiral centers, at the 2nd and 3rd C atoms (to which 6r atoms are attached), and four stereoisomers. To show this, sketch a dashed line-wedge structure with the two Br atoms on the same side of the molecule next to it sketch its nonsuperimpos-able mirror image. Sketch another structure with the two Br atoms on opposite sides of the molecule and its mirror image, making a total of four stereoisomers. 

The stereochemistry is usually expressed in structure diagrams by wedged and hashed bonds. A wedge indicates that the substituent is in ont of a reference plane and a hashed bond indicates that the substituent is pointing away om the viewer (behind the reference plane). This projection is applied both to tetrahe-... 

A molecule editor can draw a chemical structure and save it, for example as a Molfile. Although it is possible to include stereochemical properties in the drawing as wedges and hashed bonds, or even to assign a stereocenter/stereogroup with its identifiers R/S or E/Z), the connection table of the Molfile only represents the constitution (topology) of the molecule. 

To code the configuration of a molecule various methods are described in Section 2.8. In particular, the use of wedge symbols clearly demonstrates the value added if stereodescriptors are included in the chemical structure information. The inclusion of stereochemical information gives a more realistic view of the actual spatial arrangement of the atoms of the molecule imder consideration, and can therefore be regarded as between the 2D (topological) and the 3D representation of a chemical structure. 

Methane is a tetrahedral molecule its four hydrogens occupy the corners of a tetra hedron with carbon at its center We often show three dimensionality m structural for mulas by using a solid wedge ) to depict a bond projecting from the paper toward you and a dashed wedge (i 111 ) for one receding away from you A simple line (—)... 

Fischer projection formulas can be used to represent molecules with several stereogenic centers and are commonly used for caibohydrates. For other types of structures, a more common practice is to draw the molecule in an extended conformation witii the main chain horizontal. In this arrangement, each tetrahedral caibon has two additional substituents, one facing out and one in. The orientation is specified widi solid wedged bonds for substituents facing out and with dashed bonds for substituents that point in. 

Because of their greater thickness, CAA oxides serve to protect the metal surface from corrosion better than thinner oxides but the important factor for bond durability is the stability of the outer oxide structure when water diffuses to the oxide-polymer interphase. Accordingly, it would be expected that the performance of CAA treated adherends would be similar, although no better, than that of PAA, or BSAA. The wedge test data shown in Fig. 20 and other work [29,77,97,98] support this and demonstrate that when these processes are done correctly the wedge test crack will be forced to propagate entirely within the adhesive. Similar arguments are likely with BSAA adherends, also. 

To illustrate the effects of nonplanarity of the substrate on fluid structure, a hard-sphere fluid exposed to a periodic array of wedges (see Fig. 13) is... 

M. Schoen, S. Dietrich. Structure of a hard-sphere fluid in hard wedges. Phys 7 cv 5(5 499-510, 1997. 

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