Molecular structures within crystals

Provided that suitable crystals can be cultivated from a solid terpene, these can be used to determine the three-dimensional molecular structure within the crystal by means of X-ray diffraction... 

CONTEXT X-ray diffraction is the most common method for determining molecular structures within a crystal, but other methods are capable of faster, less detailed information about the crystal. For example, electron backscatter diffraction (also called backscatter Kikuchi diffraction), from a scanning electron microscope, measures the diffraction patterns of electrons that scatter off more than one plane in the crystal. From the patterns, the crystallographic point group, the orientation of the crystal, and the exposed Miller indices of the surface can be determined. Copper crystals, which have the advantage of simple structure, have been used to test the strengths and limitations of this method. 

A particular advantage of the calculated energy is that it can be broken down into the specific interactions along particular directions and further partitioned into the constituent atom-atom contributions. This is the key link between molecular structure and crystal arrangement and allows a profile of the important intermolecular interactions to be built up within families of compounds. As a result it is possible to build up an understanding of the interactions which contribute to particular packing motifs. 

From the foregoing it is clear that indentation anisotropy is a consequence of high molecular orientation within highly oriented fibrils and microfibrils coupled with a preferential local elastic recovery of these rigid structures. We wish to show next that the influence of crystal thickness on AMH is negligible. The latter quantity is independent on 1 and is only correlated to the number of tie molecules and inter-crystalline bridges of the oriented molecular network. 

Ito et al. [152] described the crystal structure of 4-[(S)-2-methylbutyl]phe-nyl 4 -hexylbiphenyl-4-carboxylate which shows a smectic A phase and a cholesteric phase. The molecules are arranged in a tilted smectic-like layer structure. Within the layers, the long molecular axes are tilted (30°). However, the compound exhibits no smectic C phase. 

The molecular arrangement within the crystal units cells of nylon is governed by the need to maximize hydrogen bonding between adjacent chains. Hydrogen bonding within crystallites is facilitated by the fact that nylon chains adopt planar zig-zag conformations with dipoles perpendicular to the chain axis to thin the plane of the molecule. Examples of nylon crystallite structures are shown in Figs. 23.8 and 23.9 for nylon 6 and nylon 66, respectively. In the... 

NMR spectrometers in today s research laboratories are sophisticated pieces of instrumentation that are capable of performing a myriad of experiments to analyze questions ranging from the molecular structure of unknown organic compounds to the different crystal forms contained within a solid. While an NMR spectrometer is quite complex, it is comprised of a few key components. An NMR spectrometer in its most basic form consists of the following (i) a magnet, (ii) shims (iii) a RF generator and a receiver—probe, and (iv) a receiver. 

Protein X-ray crystallography gives a snapshot of the structure of a protein as it exists in a crystal. This technique provides a complete and unambiguous three-dimensional (3-D) representation of a protein molecule. It is important to note that the model generated from a crystallographic study is a static or time-averaged view of the molecular structure. Information about molecular motions can be obtained from precise diffraction data however, the motions of molecules within a crystal are usually severely restricted in comparison to the motions of molecules in solution. 

After a listing of some general definitions relating to crystalline polymers (Section 1), the subject is divided into sections dealing, successively, with local structural arrangements at the scale of a few bond lengths (Section 2), morphological aspects (Section 3), molecular conformation within polymer crystals (Section 4) and, finally, kinetic aspects of crystallization (Section 5). An alphabetical index of terms is provided for the convenience of the reader. 

Solvent flatness. On average, protein crystals contain about 50% solvent, which on an atomic scale usually adopts a random, non-periodic structure within the crystal and hence is featureless within the averaged unit cell. Therefore, if we know the location of the solvent regions within a macro-molecular crystal, we already know a considerable part of the electron density (i.e. the part that is flat and featureless), and flattening the electron density of the solvent region can improve the density of our macromolecule of interest. 

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