A dihedral angle is obtained when two planes pass through three atoms and two bonds, of which one bond is common to both planes.
In the below example, plane 1 passes through X-C-C, containing three atoms and two bonds. Similarly, the plane 2 passes through C-C-Y. The separation between the two planes or the angle of intersection is denoted using a dihedral angle (ϴ) in degrees (o).
The term dihedral angle is used copiously in the study of conformational isomers in stereochemistry. It reveals the angle by which two adjacent carbons’ substituents eclipse or depart when one of the carbon atoms rotates, enabled by the free rotational property of the single bonds.
So, the dihedral angle tracks the transiting substituents to identify conformations where the bonds eclipse (eclipsed conformations obtained at 0o, 120o, 240o) or stagger half-way (staggered conformations at 60o, 180o, 360o).
The dihedral angle is an important tool for studying the energetics of certain conformations to understand their stability and reactivity further.
Related Comparison - Differences between Dihedral Angle and Bond Angle