Term symbol of homopolar molecules
Given the electronic configuration of a molecule, we can define its term symbol as follows:
- indicates the value of the total spin;
- is the symbol that represents the eigenvalue of , fixed by the value of ;
- , from the German gerade (even) or ungerade (odd), indicates whether the molecule has a defined parity with respect to its centre;
- indicates the rotational symmetry within respect to a molecular plane: we use when the molecule is symmetrical, when it's not.
Values of has to be chosen according to these rules, in analogy with atomic orbitals:
As for atomic orbitals, in a diatomic molecule we can have two electrons in the orbital, in the orbital six due to the three-fold degeneracy and so on. In the molecular case however, the energy disposition of the molecular orbitals is non trivial, but it has to be derived from case to case.