What characterizes a sigma mirror plane?

A sigma mirror plane, often denoted as σ, is a symmetry element in which one half of an object is a mirror image of the other half. This plane reflects all points on one side of it to corresponding points on the opposite side, which is central to understanding the molecule's symmetry.

The statement that it contains the z-axis in vertical mirror operations correctly highlights the nature of sigma mirror planes in the context of three-dimensional molecular geometry. When we reference vertical mirror operations, we are typically discussing planes that include the principal axis of the molecule, often the z-axis in a conventional coordinate system. This results in a reflection of the structure across that plane.

In contrast, while a sigma mirror plane can indeed bisect the principal axis, this is not its defining characteristic since it does not apply to all orientations or definitions of sigma planes, particularly in lower symmetry molecules. Similarly, stating that sigma mirror planes only exist in high symmetry molecules overlooks their presence in lower symmetry scenarios.

Lastly, the notion that they represent horizontal reflections is inaccurate as it doesn't define the broad scope of sigma planes which can be oriented in various directions based on the molecule's structure and can include vertical or horizontal reflection planes depending on the context. The key focus here is how the sigma mirror