Three-dimensional objects

The scattered intensity measured from the isotropic three-dimensional object can be transfonned to the onedimensional mtensity fiinction/j(<3 ) by means of the Lorentz correction [15]... 

Our major objectives m this chapter are to develop a feeling for molecules as three dimensional objects and to become familiar with stereochemical principles terms and notation A full understanding of organic and biological chemistry requires an awareness of the spatial requirements for interactions between molecules this chapter provides the basis for that understanding... 

The surest test for chirality is a careful examination of mirror image forms for superimposabihty Working with models provides the best practice m dealing with mol ecules as three dimensional objects and is strongly recommended... 

When using Fischer projections for this purpose however be sure to remember what three dimensional objects they stand for One should not for example test for superim position of the two chiral stereoisomers by a procedure that involves moving any part of a Fischer projection out of the plane of the paper in any step... 

In chemistry, perhaps because of the significance in visualizing molecular strac-ture, there has been a focus on how students perceive three-dimensional objects from a two-dimensional representation and how students mentally manipulate rotated, reflected and inverted objects (Stieff, 2007 Tuckey Selvaratnam, 1993). Although these visualization skills are very important in chemistry, it is evident that they are not the only ones needed in school chemistry (Mathewson, 1999). For example, conceptual understanding of nature of different types of chemical bonding, atomic theory in terms of the Democritus particle model and the Bohr model, and... 

Among crystals with stacking faults the lack of a periodic order is restricted to one dimension this is called a one-dimensional disorder. If only a few layer positions occur and all of them are projected into one layer, we obtain an averaged structure. Its symmetry can be described with a space group, albeit with partially occupied atomic positions. The real symmetry is restricted to the symmetry of an individual layer. The layer is a three-dimensional object, but it only has translational symmetry in two dimensions. Its symmetry is that of a layer group there exist 80 layer-group types. 

A two-dimensional disorder results when rod-like polymeric molecules are mutually shifted with statistical frequency. Translational symmetry then only exists in the direction of the molecules, and not in the transverse directions. The rod is a three-dimensional object with one-dimensional translational symmetry. Its symmetry is that of a rod group. Layer groups and rod groups are subperiodic groups. They are listed in detail in International Tables for Crystallography, Volume E. 

If the critical separation is determined for a large number of relative geometries of the electron and molecule it is possible to obtain a three-dimensional picture of the probability of ionization as a function of the orientation of the molecule. Effectively, the idea of an ionization cross section, the area the target molecule presents to the electron, is extended to a three-dimensional object defined by the critical distances, with ionization occurring when the electron penetrates the surface enclosing this volume. The volume enclosed by the electron impact ionization surface may be used to obtain an estimate for the cross section (volume averaged cross section) ... 

Chirality is a fundamental property of many three-dimensional objects. An object is chiral if it cannot be superimposed on its mirror image. In such a case, there are two possible forms of the same object, which are called enantiomers,... 

Fig. 23. Schematic diagram illustrating the acquisition of a series of titled projections and reconstruction of the three-dimensional object (118,119).

The Fischer projections are two-dimensional representations of three dimensional objects. Further a Fischer projection may be rotated in the plane of paper by 180°, but not by 90° as illustrated in the following examples—... 

Common ideas about dimensionality include that three-dimensional objects have all three-dimensional parameters (length, width, depth) in the same macroscopic size range. Two-dimensional objects have one length which is considerably smaller than the other two (consider a line such that the length and thickness of the line may be determined but the depth is so much smaller that it may essentially be considered as a two-dimensional object). A one-dimensional object has two dimensions much smaller than the third. An example of this is the carbon nanotube, which has a length much longer than the tube diameter. Dimensionality essentially adds... 

A random coil is clearly a three-dimensional object when looked at from long distance. Locally, however, it resembles more a one-dimensional thread. Therefore it is sensible to describe the coil by a fractal dimension that lies closer to 1 (for other architectures somewhere between 1 and 3). Such disordered objects are called fractals [101,102]. 

A tetrahedron is a three-dimensional object with four vertices and four sides, each of which is a triangle. A regular tetrahedron in which all of the sides have the same length, and this is true of methane, has equilateral triangles for each face ... 

The primary structure of proteins is not the whole story. To really understand how proteins work, we have got to understand them as three-dimensional objects. So on to higher dimensions in the next chapter. But first, a few paragraphs about another role for the protein amino acids biosynthesis. 

That is enough for now. We move on to look at proteins as three-dimensional objects in the next chapter. 

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