Molecular Symmetry and Chirality
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The σ operation reflects all the atoms of the molecule across a plane to a point an equal distance on the other side of the plane.
If the configuration of a molecule is unchanged as a result of this reflection, the molecule has a σ plane; in effect, one half of the molecule is the mirror image of the other.
A σ parallel to and containing the principal Cn axis is called a vertical plane, σv.
A σ parallel to the principal Cn axis and bisecting 2 C2' axes (or bisecting angles between atoms) is called a dihedral plane, σd.
A σ perpendicular to the principal Cn axis is called a horizontal plane, σh.
H2O has a 2 σv planes.
cis-Dibromodichloroplatinate(II) also has a two σv planes. Note that the principal C2 axis is in the molecular plane, so the second plane is not a σh.
1,3,5-Trichlorobenzene has three σv planes, as well as a σh.