1. In general, the larger the value of n, the farther the nucleus the electron will be. As such, this electron will be shielded from the full charge of the nucleus by the effect of the inner electrons.
2. Electronic transitions are governed by a) symmetry and b) change in multiplicity.
When a center of symmetry is present, electron transitions between "gerade" (g) and "ungerade" (u) ("even" and ""odd") orbitals are allowed, but gerade to gerade and ungerade to ungerade are forbidden. This is called the "parity selection rule."
u �?nbsp; g allowed
g �?nbsp; u allowed
u �?nbsp; u forbidden
g �?nbsp; g forbidden
s and d orbitals are gerade.
p orbitals are ungerade.
All wavefunctions in a molecule with a center of symmetry are g or u.
Therefore, s �?p, p �?s, p �?d, and d �?p are allowed,
but s �?s, p �?p, d �?d, s �?d, d �?s are forbidden.
The multiplicity of a state is 2S + 1 where S = Σms. When S = 0 (all electrons paired) the multiplicity is 1 (a singlet state, also labeled S). When S = 1 (two unpaired electrons) the multiplicity is 3 (a triplet state, T). The transition if allowed if there is not a change in multiplicity, and forbidden if there is a change in multiplicity.
S �?nbsp; S and T �?nbsp; T transitions are allowed,
S �?nbsp; T and T �?nbsp; S transitions are not allowed.
Remember that "not allowed" more realistically means "not probable" most of the time. Spin-orbit and vibrational coupling enable forbidden transitions to occur though these transitions are unfavorable otherwise.
3. The ground state has the lowest potential energy. In diatomic molecules, this would correspond to the optimum bond distance between the atoms. It would correspond to the most favorable geometric arrangement of the atoms in molecules in general. In theoretical studies of reaction mechanisms, the "ground state potential energy surface" of the molecules are calculated.
I had to go to one of my old textbooks (Physical Methods in Chemistry, by Russell S. Drago) for most of this, as it's been a while since I studied spectroscopy principles! Are these for a spectroscopy class? If so, what textbook are you using? (Just curious.)
Steve
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