Particular face

Here is the distance between the molecule i in the gas phase (or, for a complex molecule, the centre of its atom i) and the centre of an atom j in the solid. If a particular face of a crystalline solid is being considered, the various values of r, can be expressed in terms of a single quantity z here z is the distance between the centre of the gas molecule (or a given atom or group thereof) and the plane through the centres of the atoms in the outermost layer of the solid. 

AEyl chloride reacts with sodamide in Hquid ammonia to produce benzene when sodamide is in excess, hexadiene dimer is the principal product, with some trimer and tetramer (C24, six double bonds). AEylation at carbon atoms alpha to polar groups is used in the preparation of a-aEyl-substituted ketones and nittiles. Preparation of P-diketone derivatives, methionic acid derivatives, and malonic ester, cyanoacetic ester, and P-keto-ester derivatives, etc, involving substitution on an alpha carbon between two polar carbonyl groups, is particularly facEe. 

The enzyme-catalyzed interconversion of acetaldehyde and ethanol serves to illustrate a second important feature of prochiral relationships, that ofprochiral faces. Addition of a fourth ligand, different from the three already present, to the carbonyl carbon of acetaldehyde will produce a chiral molecule. The original molecule presents to the approaching reagent two faces which bear a mirror-image relationship to one another and are therefore enantiotopic. The two faces may be classified as re (from rectus) or si (from sinister), according to the sequence rule. If the substituents viewed from a particular face appear clockwise in order of decreasing priority, then that face is re if coimter-clockwise, then si. The re and si faces of acetaldehyde are shown below. 

The similarity of the results obtained for finite elusters and the infinite slab allows to eonclude in favour of the validity of the eluster model of adequate size (6 or 8 molybdenum atoms). In addition to the chemisorption of organic molecules on solid surfaces which is generally considered as a localized phenomenon, the interaction between molybdenum oxide and an adsorbate can also be represented by a loeal eomplex formed by a finite eluster and the adsorbed molecule. Indeed, the study of the evolution of the electronic properties as a funetion of the cluster size shows that, for a eluster eontaining 6 or 8 molybdenum atoms, most of the electronic properties converge towards limit values. This eonvergence is sensitive to the direction of the cluster growth. On the other hand, the electronic properties of the (001), (010) and (100) faces are not identieal, the type of surface atoms being different these results allow to predict that the characteristics of the chemisorption step will depend on the particular face on which it takes place. 

From a comparison of various spot electron diffraction patterns of a given crystal, a three-dimensional system of axis in the reeiproeal lattice may be established. The reeiproeal unit cell may be eompletely determined, if all the photographs indexed. For this it is sufficient to have two electron diffraction patterns and to know the angle between the seetions of the reeiproeal lattice represented by them, or to have three patterns which do not all have a particular row of points in common (Fig.5). Crystals of any compound usually grow with a particular face parallel to the surface of the specimen support. Various sections of the reciprocal lattice may, in this case, be obtained by the rotation method (Fig.5). 

In contrast, an enzymic reduction utilizing NADH will be executed stereospecifically, with hydride attaching to one particular face of the planar carbonyl. Which face is attacked depends upon the individual enzyme involved. For example, reduction of pymvic acid to lactic acid in vertebrate muscle occurs via attack of hydride from the Re face (see Section 3.4.7), and produces the single enantiomer (S )-lactic acid. Hydride addition onto the alternative Si face is a feature of some microbial dehydrogenase enzymes. 

NADH. These experiments were pioneering with respect to contemporary enzymology, especially with regard to early recognition that coenzymes are held within enzyme active sites in stereochemically preferred ways. One typically utilizes NADH that contains a tritium or deuterium atom in the 4R or 45 position, and the success or failure of substrate deuteration/tritiation indicates the stereochemistry. Westheimer has tabulated the known examples of dehydrogenases that exhibit specificity for a particular face of NADH. Creighton and Murthy have reproduced this tabulation in their comprehensive review on the stereochemistry of enzyme-catalyzed reactions at carbon. 

Figure 12 shows the size analyses from one run. Crystal content and solution analysis measurements were In good agreement. The size shown In Figure 12 Is the volume equivalent size, which Is acceptable for crystal growth where the crystals retain the same elongated shape as they grow. However when needle-1Ike crystals dissolve, there Is no longer preferable transfer on particular faces, so the shape changes. If allowance Is not made for this. Incorrect dissolution rates will result. Thus for purposes of...

Impurity ions adsorb selectively on a particular face and cover the whole surface, leading to retardation of the growth rate R of the face and a change of Tracht. 

The indices of crystal faces are traditionally expressed by the points at which the lattice plane parallel to the face cuts the a-, h-, and c-axes, respectively. Indices are always expressed as integers. A particular face is expressed by parentheses (), and crystallographically equivalent faces are denoted by curly brackets. Miller indices use the reciprocal number of the unit length at which the respective axes are cut, and are widely adopted. 

It has been observed by many authors that the catalytic activity of several crystals is related to some particular faces of the crystal (50). 

Others have examined the geometry of the chromate and dichromate ions and compared then to models of the silica surface (12). Again, the uncertainty is high because we have very little idea what the silica surface really looks like. Hydroxyl spacings derived from a particular face of cristobalite or tridymite may have little relevance to amorphous, high surface area silica gel. 

As was shown to be the case in an earlier review of the vibrational spectroscopic literature on chemisorbed CO (81), it is very profitable to consider the more complex spectroscopic results obtained on finely divided catalysts in the light of conclusions reached from work on simplified experimental situations that involve particular faces of single crystals of the metal in question (17). We therefore first consider the principles involved in the interpretation of spectra obtained under these conditions. 

When larger areas of a particular face are desired, flat surfaces may be cut parallel to a particular crystal plane, and methods have been developed as described later, whereby measurements may be carried out on a single face and the effect of edges and other faces may be eliminated. A particularly convenient specimen is a spherical crystal with plane surfaces cut on it parallel to the particular faces which it is desired to study. The actual measurements can be made on the flat faces, and the pattern on the spherical surface can be used as a sensitive test for possible contaminants and improper control of experimental conditions. 

A region about the 011 faces, extending outward in an elliptically shaped area to approximately the 012 and 133 faces, in which the oxide was oriented completely parallel to the metal. Within this region there was only one orientation on any particular face. This region was one which oxidized slowly. 

It should be noted that results on the major faces of iron Indicate that a close-packed [110] direction of the oxide is parallel to the second most closely packed [100] direction of the metal. The relationship, however, does not hold for oxide on some of the high index faces as shown by Bardolle s work. Bardolle (53) has proposed the general rule that FeO forms on the metal surface in such a way as to make the <111> directions of great atomic density of the metal correspond as closely as possible with the <110> directions of great density of iron Ions in the oxide. K is not possible, however, to predict the orientation of oxide on any particular face from this rule alone. 

The electron density on the two faces of the alkene may be selectively modified by adjacent substitution. For example, the repulsion between the lone pairs of an adjacent ether and the n-system may increase the electron density on the face opposite to the ether. Hyperconjugative assistance from adjacent axial C-H bonds may favour attack from a particular face of an alkene. 

Small Ab values imply a greater likelihood of incorporation of the impurity on a particular face. In certain cases, Ab can be negative, which implies that impurity binding is more favorable than host incorporation. If Ab is very large, then it is unlikely that the impurity will be incorporated. The Ab values are system-specific, and so it is not always entirely clear as to what constitutes a large or a small value. 

Although most of the work has been done on polycrystalline porous films, more recent research has shown ways of producing single crystal films which are well defined and exhibit diffraction patterns characteristic of particular faces. Careful choice of substrate, substrate temperature and evaporation rate are required. In general single crystal films are most easily formed on substrates which are themselves well defined crystals, e.g. silver on mica, and germanium on calcium fluoride . ... 

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