Molecular Symmetry and Group Theory
Lecture 5 Chemical bonding
Applications of symmetry and group theory:
Chirality a molecule may be chiral if it does not possess an improper rotation axis. As S1 ≡ σ and S2 ≡ i, a chiral molecule can possess no symmetry elements other than E and proper rotation axes Cn.
This excludes all molecules except those with point groups C1, Cn or Dn.
Dipole moments a molecule may not have a dipole moment:
This excludes all molecules except those with point groups C1, Cn or Cnv.
- perpendicular to an axis of rotation;
- perpendicular to a mirror plane;
- or in any direction at all if the molecule possesses an inversion centre.
Orbital degeneracies inspection of the appropriate character table can tell us which p or d orbitals will be degenerate in a given geometry e.g. px and py in BF3 (D3h)
Hybridization Workshop 5
Molecular orbital theory Workshop 5
Molecular vibrations Workshop 6
The application of group theory to chemical problems can be summarized in the following three steps:
- use an appropriate basis to generate a reducible representation of the point group
- reduce this representation to its constituent irreducible representations
- interpret the results
We can use group theory to decide which atomic orbitals on a central atom can hybridize to give an appropriate set of orbitals for a given molecular geometry.
We know that the hybridization of the C 2s and 2p orbitals gives four tetrahedral sp3 hybrids, but how could we tell, for instance, that it is this set of orbitals that gives this particular geometry (Exercise 1)? Or which orbitals would have to combine to give a different geometry (Exercise 2)?
Molecular Orbital (MO) Theory
There are three major requirements for the formation of molecular orbitals from atomic orbitals:
We can only judge whether the third requirement is met through the application of group theory (Exercises 36).
- The atomic orbitals must have similar energy
- They must have significant overlap
- They must have appropriate symmetry