Wigner's Spin Conservation Rule
The Wigner spin conservation rule states that in any allowed electronic energy transfer process, the overall spin angular momentum of the system should not change. The rule is applicable whether the transfer occurs between an excited atom or a molecule and another molecule in its ground state or in the excited state. In an electronic transition between the energy states of the same molecule also, spin is necessarily conserved. But the phenomenon is governed by rules for dipole-dipole interaction.
Wigner's spin conservation rule requires that there must be correlation of spins between the reactants and the products. The possible spin of the transition state for reactants A and B with spins SA, and SB can be obtained by vector addition rule as |SA + SB|, |SA + SA - 1|,..., |SA - SA|. In order that there is smooth correlation with the products X and Y, the transition state formed by the products must also have total spin magnitude which belongs to one of the above values. This situation allows the reactants and the products to lie on the same potential energy surface. Such reactions whether physical or chemical, are known as adiabatic.
In these reactions a close approach of two reacting partners is necessary. Essentially, Wigner's rule states that the total spins of the α type (↑) and the β type (↓) in the combined initial and final state of the system remains the same, no matter how we combine them in the final state.
Thus, for example, in a singlet-singlet, triplet-triplet and triplet-singlet transfer of energy by exchange mechanism, the following combinations are possible-
These rules also predict the nature of photoproducts expected in a metal- sensitized reactions. From the restrictions imposed by conservation of spin, we expect different products for singlet-sensitized and triplet-sensitized reactions. The Wigner spin rule is utilized to predict the outcome of photophysical processes such as, allowed electronic states of triplet-triplet annihilation processes, quenching by paramagnetic ions, electronic energy transfer by exchange mechanism and also in a variety of photochemical primary processes leading to reactant-product correlation.
Limitations of Wigner's Spin Conservation Rule
The Wigner's spin conservation rule may break down when the total angular momentum quantum number is no longer a good quantum number, such as in strong magnetic fields, where spin mixing can occur. The propensity for spin conservation can be influenced by the mechanisms of the reactions involved. For particular reactions, especially those involving complex systems or high energy states, the conservation rule may not strictly apply.
Source: Fundamentals of Photochemistry by K.K.Rohatgi-Mukherjee
Test Your Knowledge
Wigner's Spin Conservation Rule MCQs
1. What does Wigner's Spin Conservation Rule primarily state regarding electronic energy transfer?
A. The total orbital angular momentum of the system must change.
B. The overall spin angular momentum of the system should not change.
C. Only the individual spins of atoms must remain constant.
D. The kinetic energy of the system must be conserved.
View Answer
Option B is correct answer.
The Wigner spin conservation rule states that in any allowed electronic energy transfer process, the overall spin angular momentum of the system should not change.
2. To what situations is Wigner's Spin Conservation Rule applicable?
A. Only to energy transfer between an excited atom and a ground state molecule.
B. Only to electronic transitions within the same molecule, governed by dipole-dipole interaction.
C. To energy transfer processes between excited atoms/molecules and other molecules, and electronic transitions between energy states of the same molecule.
D. Only to reactions occurring in strong magnetic fields.
View Answer
Option C is correct answer.
The rule is applicable whether the transfer occurs between an excited atom or a molecule and another molecule in its ground state or in the excited state. In an electronic transition between the energy states of the same molecule also, spin is necessarily conserved.
3. What does Wigner's spin conservation rule require regarding reactants and products?
A. There must be no correlation of spins between them.
B. There must be a correlation of spins between them.
C. Reactants and products must have zero total spin.
D. Only the spin of the reactants needs to be considered.
View Answer
Option B is correct answer.
Wigner's spin conservation rule requires that there must be correlation of spins between the reactants and the products.
4. How can the possible spin of the transition state for reactants A and B with spins SA and SB be obtained?
A. By scalar addition rule: SA + SB.
B. By vector addition rule: |SA + SB|, |SA + SB - 1|,..., |SA - SB|.
C. By multiplying their spins: SA x SB.
D. It cannot be obtained, only observed experimentally.
View Answer
Option B is correct answer.
The possible spin of the transition state for reactants A and B with spins SA and SB can be obtained by vector addition rule as |SA + SB|, |SA + SB - 1|,..., |SA - SB|.
5. Reactions where reactants and products lie on the same potential energy surface are known as what
A. Non-adiabatic reactions
B. Exothermic reactions
C. Adiabatic reactions
D. Endothermic reactions
View Answer
Option C is correct answer.
This situation allows the reactants and the products to lie on the same potential energy surface. Such reactions whether physical or chemical, are known as adiabatic.
6. Wigner's rule states that the total spins of what types remain the same in the combined initial and final state?
A. Protons and neutrons
B. Alpha and beta type spins
C. Only electrons in the initial state
D. Only electrons in the final state
View Answer
Option B is correct answer.
Wigner's rule states that the total spins of the α type (↑) and the β type (↓) in the combined initial and final state of the system remains the same.
7. Which of the following energy transfer combinations are possible under Wigner's rule for exchange mechanism?
A. Singlet-doublet and triplet-quartet
B. Singlet-singlet, triplet-triplet, and triplet-singlet
C. Only singlet-singlet transfers
D. Any combination, as the rule is absolute
View Answer
Option B is correct answer.
In a singlet-singlet, triplet-triplet and triplet-singlet transfer of energy by exchange mechanism, the combinations are possible.
8. Wigner's spin rule is utilized to predict the outcome of which of the following photophysical processes?
A. Only allowed electronic states of triplet-triplet annihilation.
B. Quenching by paramagnetic ions and electronic energy transfer by exchange mechanism.
C. Both A and B, plus a variety of photochemical primary processes.
D. Only the nature of photoproducts in singlet-sensitized reactions.
View Answer
Option C is correct answer.
The Wigner spin rule is utilized to predict the outcome of photophysical processes such as, allowed electronic states of triplet-triplet annihilation processes, quenching by paramagnetic ions, electronic energy transfer by exchange mechanism and also in a variety of photochemical primary processes leading to reactant-product correlation.
9. Under what specific condition may Wigner's spin conservation rule break down?
A. When the total orbital angular momentum is conserved.
B. In the presence of very weak magnetic fields.
C. When the total angular momentum quantum number is no longer a good quantum number, such as in strong magnetic fields.
D. During non-adiabatic reactions.
View Answer
Option C is correct answer.
The Wigner's spin conservation rule may break down when the total angular momentum quantum number is no longer a good quantum number, such as in strong magnetic fields, where spin mixing can occur.
10. What can influence the propensity for spin conservation, especially for particular reactions?
A. The presence of catalysts.
B. The mechanisms of the reactions involved.
C. The temperature of the system only.
D. The size of the molecules involved.
View Answer
Option B is correct answer.
The propensity for spin conservation can be influenced by the mechanisms of the reactions involved. For particular reactions, especially those involving complex systems or high energy states, the conservation rule may not strictly apply.