文档介绍：Sigma bonds and bond rotation
Groups bonded by only a sigma bond can
undergo rotation about that bond with respect
to each other. The temporary molecular shapes
that result from rotation of groups about single bonds are called conformations of a molecule. Each possible structure is called a conformer. An analysis of the energy changes associated with a molecule undergoing rotation about single bonds is called conformational analysis.
When we do conformational analysis, we will find that certain types of structural formulas are especially convenient to use. One of these types is called a Newman projection formula and another type is a sawhorse formula. Sawhorse formula are much like other three-dimensional formulas we have used so far. In conformational analyses, we will make substantial use of Newman projections.
To write a Newman projection formula, we imagine ourselves taking a view from one atom (usually a carbon) directly along a selected bond axis to the next atom (also usually a carbon atom). The front carbon and its other bonds are represented as and those of the back carbon as .
The rotation around the single bond in ethane, while not obviously hindered, does generate conformational isomers having different potential energies. As shown bellow, as the dihedral angle between the ethane hydrogen atoms changes from 60 (a staggered conformation) to 120 (an eclipsed conformation), the potential energy of the molecule increases by about 3 kcal/mole. As the methyl group continues to rotate towards 180, the potential energy again drops and rises again as the next eclipsed structure is formed.
This can be contrasted, however, with rotation around the central carbon-carbon bond in butane, shown below, in which two methyl groups clearly overlap during a single rotation (the van der Waals radii of the methyl hydrogen atoms clearly overlap).
The effect of rotation on the potential energy of butane around the central carbon-carbon bond is more significant, as sh