Small molecules, crystals INDEX

One of the most exciting observations of LEED studies of adsorbed monolayers on low Miller index crystal surfaces is the predominance of ordering within these layers (18). These studies have detected a large number of surface structures formed upon adsorption of different atoms and molecules on a variety of solid surfaces. Conditions range from low temperature, inert gas physisorption to the chemisorption of reactive diatomic gas molecules and hydrocarbons at room temperature and above. A listing of over 200 adsorbed surface structures, mostly of small molecules, adsorbed on low Miller index surfaces can be found in a recent review (/). 

Indexing" and Lattice Parameter Determination. From a powder pattern of a single component it is possible to determine the indices of many reflections. From this information and the 2 -values for the reflections, it is possible to determine the unit cell parameters. As with single crystals this information can then be used to identify the material by searching the NIST Crystal Data File (see "Small Molecule Single Stmcture Determination" above). 

For small molecules mounted on a glass fibre, the crystal can often be described by indexing the faces of the crystal and estimating their distance from a common point. In such a case a numerical integration can be used. However, this approach is not valid when the crystal is mounted in a capillary surrounded by mother liquor. 

Unit cell and the lattice dimensions, while necessary for crystal structure determination, are by themselves valuable in polymer characterization. This is because in contrast to the lattice dimensions in crystals of small molecules, which remain essentially unchanged, these dimensions in polymeric materials vary over a relatively large range. The polymer needs to be obtained in at least a fiber or a biaxially oriented film in the initial identification of the unit cells, and in assigning the reflections to appropriate lattice planes. Once the reflections are indexed, cell dimensions can be calculated... 

Anisotropic molecules show optically isotropic behavior in the bulk when they are disordered and randomly oriented, for instance in solutions or liquid crystal above the transition temperature. Under the influence of a strong beam, the induced dipole moment of the molecules feels a torque that tends to orient the molecule. The reorientation of the molecular dipoles induces a change in the refractive index. The typical values for molecular susceptibilities and the time-responses vary depending on the type of systems. For small anisotropic molecular systems, x 10 esu, with a time response 10 s. However, in the nematic phase, liquid crystal molecules are strongly correlated, resulting in much higher values, x 10 esu,... 

Figure 12.8 shows the (PPE) curves (see 12.3.3) for the rotation of flat molecules, (in the rigid environment approximation). An elongation index is defined as D /D (see 12.2.3.1), the ratio of the two molecular dimensions in the molecular plane. The rotation is severely hindered if the elongation index exceeds about 1.3. A large molecule such as coronene rotates easily in the crystal, due to its very small elongation index. For these molecules, the ease of molecular rotation in the solid depends more on molecular shape than on details of the crystal structure. 

Molar mass (inversely proportional to melt flow index, MFl) has a small influence. Generally in the chain folded crystal structure, the molar mass will not be important. It will mean that a particular molecule has more folds in the crystal. Melting is determined by the crystal thickness. Crystal thickness is determined by crystallization conditions and the presence of branches that sterically prevent crystallization of some segments. 

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