Chemistry Reference
In-Depth Information
3
The Point Groups Used with
Molecules
3.1
Introduction
In this chapter we will review the common point groups found in chemistry and arrive at
a simple methodology to assign any molecule to a point group. Example molecules will
then be discussed in Chapter 4. The properties of each point group are encapsulated in the
character tables listed in Appendix 12. Each table contains the point group symbol and a
listing of the symmetry operations on the top line of the table. The meaning of the rest of
the symbols in these tables will become clear in later chapters. Here, we just point out that,
once a molecular point group assignment has been made, the operations that are allowed
for the molecule can be checked by reference to the point group character table.
3.2 Molecular Classification Using Symmetry Operations
The fact that molecules have a three-dimensional structure and shape was shown by Louis
Pasteur in 1848 in some critical experiments on crystalline salts of tartaric acid that formed
part of his doctoral studies. Tartaric acid is a naturally occurring compound that is extracted
from grape juice and sometimes crystallizes as potassium bitartrate from solution in wine.
Pasteur concentrated on the related compound sodium ammonium tartrate. The two forms
of tartrate were chemically identical, but a solution of potassium bitartrate would rotate
the plane of polarization of plane polarized light to the right whereas a solution of sodium
ammonium tartrate would not. Pasteur studied the crystal structures of tartaric acid salts
and found the crystallites themselves were chiral, i.e. the facets of the crystals occur in
two forms that are mirror images of one another, so that the two crystallite forms can-
not be superimposed. In the pure potassium bitartrate, only 'right-handed' facets were
Molecular Symmetry
David J. Willock
c
2009 John Wiley & Sons, Ltd
