Selection Rule for UV-Visible Spectra

Contents

• Selection Rule for UV-Visible Spectra
• 1. Dipole Moment
  
• 2. Symmetry Rule
  
• 3. Laporte Rule
  
• 4. Multiplicity Rule
  
• Forbidden Transitions

Selection Rule for UV-Visible Spectra

Allowed and Forbidden Transition in UV-Visible Spectroscopy

Transition Probability

It is not always necessary that the excitation of an electron takes place from a bonding orbital or lone pair to an antibonding or non-bonding orbital when a compound is exposed to UV or Visible light.
It can be shown by Extinction coefficient-
ε_max = 0.87 x 10²⁰ P.a
Where P = transition probability with value from 0 to 1
a = target area of the absorbing system (Chromophore).
The transitions with ε_max value more than 10⁴ are usually allowed transitions and generally arises due to π → π* transitions. The forbidden transition is arises due to the excitation of one electron from the lone pair present on the heteroatoms to an antibonding π* orbital. The value of ε_max for forbidden transitions are generally below 10⁴.
Theoratically, many transitions are possible but all these transitions are not actually observed. Hence, the following set of selection rules came into practice-
1. Dipole Moment
2. Symmetry Rule
3. Laporte Rule
4. Multiplicity Rule

1. Dipole Moment

Transitions must involve a change in dipole moment. Greater the change in dipole moment, greater is the intensity of absorption.

2. Symmetry Rule

An electronic transition is allowed between the similar symmetric orbitals. A π → π* transition occurs with high probability as both π and π* orbitals lie in the same xz plane. Hence, it gives a high intensity absorption band (~10⁴ - 10⁵) at 185 nm in HCHO. But the n → π* transition in HCHO gives a relatively weak band (ε_max ~100) at 270 nm which means that the transition is symmetry forbidden. and orbitals lie in xz plane while p_y, orbital containing nonbonding electrons is in yz plane and hence perpendicular to the π* orbital. Since the regions in space of two orbitals overlap so poorly, the possibility of the transition between them is low.

The fact that it occurs at all is due to the vibration of atoms which includes some twisting which in turn increases overlap.
The electronic excitation caused by uv-vis absorption is generally from singlet ground state (S_o) to singlet excitation state (S₁) with the retention of spins. The S₁ state may pass over to more stable excited triplet state (T₁) after spin conversion followed by emission of light. Electrons are farther apart in S₁ state due to minimum mutual repulsion, making it more stable. However, direct S_o → T₁ state is symmetry forbidden.

3. Laporte Rule

According to this rule, any allowed electronic transition must have Δl = ± 1

l Value	0	1	2	3
Subshell	s	p	d	f

Hence, d-d-transitions are Laporte forbidden since the Δl = 2-2 = 0 but the appearance of low intensity absorption band (ε_max < 50) is due to the mixing of d-orbitals with p or f orbitals. Or, we can say that g → g or u → u transitions are Laporte forbidden.
In ethylene σ(g) → σ*(u) and π(u) → π*(g) are only allowed transitions.

4. Multiplicity Rule

Transitions undergoing change in the number of unpaired electrons (n) or spin multiplicity (n + 1 or 2S + 1) are forbidden. Any d-d transition in high spin d⁵(Mn⁺²) complexes is doubly forbidden (Laporte & spin), still a weak absorption band (ε_max < 1) occurs due to weak coupling of spin orbital angular momenta.
Hence Mn(II) complexes are off-white or pale flesh coloured or feebly coloured.

Forbidden Transitions

Forbidden transitions does not mean that it never occurs but it is highly improbable and consequently it appears in many compounds as weak band or of low intensity. Allowes and forbidden transitions have molar extinction coefficient (ε_max) value greater and less than 10⁴ respectively.

εmax	20,000	180
Nature of Transitions	Allowed	Forbidden

Allowed and forbidden transition is governed by the geometry of ground as well as excited state molecular orbitals, symmetry of the molecule as a whole and orientation of electronic dipole of the incident light that might induce excitation.