Dihedral (Torsion) Angle Explained

When you first learned about molecular shapes in general chemistry, you probably focused entirely on standard bond angles—the angle formed by three atoms connected in a sequence by two bonds.

But once we start looking at three-dimensional molecules and how they twist and rotate, standard bond angles aren't enough. We need a way to describe the spatial relationship between bonds on adjacent atoms. That brings us to the dihedral angle (also called a torsion angle), which measures the angle between two intersecting planes defined by four connected atoms ($A-B-C-D$).

Why do we care? Because as a molecule rotates around a single carbon-carbon bond, the dihedral angle constantly changes. This twist dictates everything from rotational barriers and torsional strain to the molecule's overall energy, stability, and shape.