Dreiding molecular models

Brief examination of Dreiding molecular models reveals that unrestricted heteronins incorporating one or more trans double bonds will... 

It should be remembered once more that the use of molecular models ("ball-and-stick". as well as Dreiding or similar) is always essential for fully understanding the conformational and configurational changes involved in multistep syntheses of complex organic compounds... 

In his constant search for better methods of teaching he made a 60-minute color-sound movie Techniques of Organic Chemistry, and developed a set of precise plastic molecular models, which are larger than, but have the same relative dimensions as, Dreiding models. Unlike the latter, however, the Fieser models have been so inexpensive to manufacture that even undergraduate students have been able to afford a set. 

Examination of Dreiding and Corey-Pauling-Koltung molecular models reveals that the trans-ligand (b) cannot chelate a metal ion without severe steric interactions while the cis-(a) can do so easily. The fact that the nickel (II) complexation-cyanide decomplexation sequence leads to enrichment of the major isomers of VI and VII leads us to postulate that the major isomers are the cis-(a) species. We have equilibrated Via and VIb thermally to a ratio of 58 42 thus the macrocyclization gives a non-equilibrium mixture favoring the cis isomer, analogous to PPh and AsMe macrocyclizations that we have described recently (1, 7 ) ... 

High quality computer graphics is now widely available and is used as an aid in visualizing molecules, molecular processes, and intermolecular interactions. In some cases molecular graphics has replaced the building of physical (e. g., Dreid-ing) models but the potential applications are much greater. In order to visualize molecules they need to be constructed in a reliable manner and molecular mechanics is an ideal tool for doing so. 

Molecular models were constructed from Dreiding Stereomodels. 

As everyone knows who has handled mechanical molecular models, such as Dreiding models, bond distances and angles alone do not, in general, completely define the three-dimensional structure of a molecule, i.e. models constructed with fixed values of of bond distances and angles are generally not rigid. In order to make them so, we need to fix some of the torsion angles as well. How many ... 

The solution to any problem with a stereochemical aspect requires access to molecular models. Of these, there are two main kinds. The first is the skeletal or framework model such as those devised by Dreiding or Kendrew. These indicate the centres of bonds that join atoms, and are useful to find conformations suitable for interaction between two molecules. The other type of model, space-filling (e.g. CPK, or Courtauld), shows both the shape of the molecule and the volume that it occupies. This kind is very useful for showing the overall shape, surface and volume of a molecule. With practice, a chemist can learn to see a conformational drawing as a three-dimensional skeletal shape, and eventually as a space-filling molecule. There are also the CCS models, which are fundamentally skeletal models that can be quickly converted to space-filling types and back again (Clarke, 1977). 

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