Improper Rotation (Sn)

An improper rotation is performed by rotating the molecule 360°/n followed by reflection through the plane perpendicular to the rotation axis. If the resulting configuation is indistinguishable from the original, we say there exists an n-fold improper rotation axis (or Sn axis) in the molecule.

Each of the following molecules contains one or more improper axes:

  • Staggered ethane contains an S6 axis
  • Methane contains three S4 axes

Improper rotation axes: