Stereoisomerism torsional angles

The operation that interconverts stereoisomeric planes can be viewed as a ir rotation of the plane that breaks the bond(s) between the plane and the extraplanar atom(s) and then restores it (or them) in a sterically different way. The axes of this rotation pass through the center of the unsaturated system. As they are infinite in number (46) they cover the plane and in a sense define it. Although several of these axes coincide with bonds, the operation differs profoundly from that characteristic of a line of torsion. Rotation of the plane causes a change in bonding, whereas torsion about a line alters the torsional angles between the ligands that are attached to the terminal atoms of the bond that defines the line. 

Conformation vs. Conformer - The term conformation involves a description of a stereomodel by helicity and magnitude of torsion angles without regard of energy content. In contrast, conformcrs are stereoisomeric molecular species at energy minima (see Section 1.1.3.6.). 

Fig. 9.20. Stereoscopic views of three structures along the stereoisomerization path of Ph3PO. Top C3-symmetric conformation with torsion angles 40°, 40°, 40° middle mirror-symmetric transition state with torsion angles 90°, 10°, -10° bottom C3-symmetric conformation with torsion angles -40°, -40°, -40°...

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