Three-dimensional structures organic compounds

Unlike the one-dimensional NMR techniques to which this book is largely devoted, those 2D 19F NMR techniques to be briefly discussed as follows will not generally be required for day-to-day structure elucidation by the working organic chemist. However, there will inevitably be situations where these techniques are indispensable in determining the detailed three-dimensional structure of compounds that contain fluorine, and at such times it may be necessary for the synthetic chemist to turn to an NMR specialist for assistance. 

Finally, mechanochemical synthesis offers a solvent-free route for the preparation of MOFs by ball milling of starting components [23-28]. Sufficient amounts of pure material for broad-range testing can be easily obtained from this method. Furthermore, the solid-solid reaction typically produces quantitative yields and leads directly to products in powder form. Hence, the materials are ready for various applications without the need for time-consuming treatments. Mechanochemistry as an approach for the synthesis for one-, two-, and three-dimensional metal-organic compounds is currently employed by several groups, who mainly focus on the synthesis of new structures. 

In the previous chapters, we discussed electron distrihution in organic molecules. In this chapter, we discuss the three-dimensional stracture of organic compounds. The structure may be such that stereoisomerism is possible. Stereoisomers are compounds made up of the same atoms bonded by the same sequence of bonds but having different three-dimensional stracmres which are not interchangeable. These three-dimensional structures are called configurations. 

Molecular structure determines the gross activity of an organic compound, as it is responsible for the molecular volume, water solubility, vapor pressure, density, and electrical charge of the compound. The three-dimensional structure of an organic... 

The second type of stereoisomerism encompasses all other cases in which the three-dimensional structures of two isomers exhibiting the same connectivity among the atoms are not superimposable. Such stereoisomers are referred to as diastereomers. Diastereomers may arise due to different structural factors. One possibility is the presence of more than one chiral moiety. For example, many natural products contain 2 to 10 asymmetric centers per molecule, and molecules of compound classes such as polysaccharides and proteins contain hundreds. Thus, organisms may build large molecules that exhibit highly stereoselective sites, which are important for many biochemical reactions including the transformation of organic pollutants. 

An important field where the cyclopropanation reaction finds growing application is the construction of dendrimers possessing fullerenes either as functional cores [26-33] or branches [34-38]. Dendrimers can serve as building blocks for the construction of organized materials with nanosize precision due to the well-defined three-dimensional structure they possess. An issue of great importance is to incorporate photoactive and/or redox-active units at the center of the dendrimer in order to establish these types of materials as molecular devices. An example of an organofullerene material that has the potential to serve as a core building block for the construction of dendrimeric compounds... 

How then can we account for the high degree of internal order routinely found within globular proteins We believe that combinations of the wide variety of electrostatic interactions reviewed above determine the precise three-dimensional structure of the interior of a protein. We argue that the sum of these interactions produces, at least in part, the enthalpy change on protein folding that is independent of the hydrophobic effect. Crystal structures of small organic compounds provide a useful model of protein interiors, and we now discuss some recent theoretical studies of these systems. 

Many applications have been reported in the field of biomolecular NMR spectroscopy which use RDCs for the refinement of three-dimensional structures. The approach is quite powerful and can also be applied to smaller molecules whenever the conformation of a molecule is important, as for example in the case of rational drug design. Traditionally, NMR in liquid crystals is applied on a multitude of small organic compounds to obtain their fully characterized structure. Most examples are measured on all kinds of aromatic systems as reported in refs. 204—212 other recent examples deal with substituted alkanes, aldehydes216,217 or bridged systems like norbomadiene.218 In general, these very detailed studies can be applied to molecules with up to 12 protons. 

An understanding of the three-dimensional structures of molecules has played an important part in the development of organic chemistry. The first experiments of importance to this area were reported in 1815 by the French physicist J. B. Biot, who discovered that certain organic compounds, such as turpentine, sugar, camphor, and tartaric acid, were optically active that is, solutions of these compounds rotated the plane of polarisation of plane-polarized light. Of course, the chemists of this period had no idea of what caused a compound to be optically active because atomic theory was just being developed and the concepts of valence and stereochemistry would not be discovered until far in the future. 

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