Wigner s spin conservation rule

In any allowed electronic energy transfer process, the overall spin angular momentum of the system should not change. This statement is known as Wigner s spin conservation rule. 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... 

Wigner s spin conservation rules govern energy transfer between two molecules, photodissociation energetics, T—T annihilation energetics and other photophysical and photochemical processes. 

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 j-SB j, Sa FSb — 1, . .., Sa —Sb. 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. 

Show that the collision between two triplet states can produce two singlet ground states, obeying Wigner s spin conservation rule. Hint Keep the spin quantum number of the electrons the same, but exchange them so that the products are... 

All contact reactions involving spin species occur with a probability determined by the necessity for the conservation of spin multiplicity, [37, 38]. For example, reaction (8) which shows a triplet-triplet annihilation occurs with a probability of 1/9, whilst the recombination of all ionic species in reactions (2), (4) and (6) occurs with a probability of unity. These rules (known as Wigner s spin conservation rules) are shown more formally in Fig. 8.11. For reactions involving spin-exchange [i.e. reactions (15), (16) and (17)] Wigner s sign rule for a doublet + singlet or doublet + triplet are used to calculate the probability of reaction. 

Operator equations have been employed by George and Ross (1971) to analyse symmetry in chemical reactions. In order to preserve the identity of electronic states of reactants and products, these authors worked within a quasi-adiabatic representation of electronic motions. By introducing a chain of approximations, going from separate conservation of total electronic spin to complete neglect of dynamics, they discussed the Wigner-Witmer angular momentum correlation rules, Shuler s rules for linear molecular conformations and the Woodward-Hoffmann rules. 

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