CHM 401


Symmetry and Point Groups

Symmetry is one way to indicate geometrical aspects of a molecule (or object)

Symmetry elements are geometric operations that leave the appearance of a molecule identical (in all respects) to the starting point:


Types of symmetry elements

1. Inversion i(x,y,z) → (–x,–y,–z)


2. Rotations Cnrotates the molecule by 360°/n; by convention, rotations are clockwise; successive rotations are denoted by superscripts:

C62 = C3

C32 = 120° rotation counterclockwise

0° rotations are denoted by E.

Infinitely small rotations are denoted by C


3. Mirror planes σ reflection through a plane; the plane is often denoted by a subscript

a mirror is a composite symmetry element : σxy = C2zi

mirrors through bonds are usually denoted σv

mirrors between bonds are usually denoted σd

mirrors perpendicular to the highest n rotation axis are usually denoted σh


4. Improper rotations Sn a rotation followed by a mirror perpendicular to the rotation

Sn = Cnσh this can exist independent of the rotation or the mirror, e.g. S4 in CH4

note: S2 = σC2 = i


Point Group : the collection of all of the symmetry operations present in a molecule

Point Groups have mathematical significance, but we will take them simply as geometrical descriptions of molecules

To find point groups, we only need to identify certain key symmetry elements