Symmetry Tutorial - Improper Rotation

Improper Rotation (S_n)
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 S_n axis) in the molecule. Some example molecules are shown below.

Staggered Ethane (C₂H₆) -- S₆ axis
   

Methane (CH₄) -- S₄ axis