Torsion angles What are they

By contrast, a Fischer projection is drawn with the atom under consideration at the center of a cross. The upper and lower substituents, drawn with vertical bonds, lie below the plane of the paper A and D ir. 

FIGURE 12.1. Ways of depicting molecular conformation, (a) Perspective view and (b) Newman projection. 

An alternative way of analyzing molecular shape, besides drawing a Newman projection, is to analyze the amount of twist about the bond that is necessary to cause some or all of the bonds from the near (upper) and far (lower) atoms to be totally eclipsed. This leads us to a definition of torsion angle. 

The mathematical way of describing conformation is by the calculation of the torsion angles about each bond the input data are the atomic coordinates. A torsion is the extent of the twist of substituents about a bond. It is a way of looking along a bond and describing the disposition of the substituents attached to the atoms at both ends. It will indicate if the bonds to the substituent groups are eclipsed, staggered, or intermediate between these. 

The method used to measure a torsion angle is to measure how much a bond has to be twisted to cause two substituents on it to be eclipsed 

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