Wigner’s spin rule

When energy transfer takes place via an exchange mechanism, the system obeys Wigner s spin rule.(32) If and xs are the initial spin quantum numbers... 

The spin statistical factor is found from Wigner s spin rules in this case, a — n/3m. Recent work has been discussed by Wilkinson and Tsiamis... 

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. 

When an exchange mechanism is operating, spin selection rules are identical to those for all bimolecular collisions, i.e., the system obeys Wigner s spin rule96 which states If Si and S2 are the initial spin quantum numbers of the colliding species, the resultant spin quantum number of the two systems taken together must have one of the values... 

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. 

In contrast to the dipole-dipole interaction, the electron-exchange interaction is short ranged its rate decreases exponentially with the donor-acceptor distance (Dexter, 1953). This is expected since, for the electron exchange between D and A, respective orbital overlap would be needed. If the energy transfer is envisaged via an intermediate collision complex or an exciplex, D + A—(D-------A)- D + A, then Wigner s rule applies there must be a spin com-... 

In Eq. (4.5) the donor emission spectrum/ and the acceptor absorption spectrum eA are separately normalized to unity, so that the transfer rate is independent of the oscillator strength of either transition. Unfortunately, the constants W and L are not easily determined by experiment. Nevertheless, an exponential dependence on the distance is expected. It should be noted that this type of transfer involves extensive orbital overlap and is guided by Wigner s (1927) spin rule. 

In these reactions a close approach of two reacting partners is necessary. Essentially, Wigner s rule states that the total spins of the a type ( f)... 

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