Spin-conservation rule

This is an example of the Wigner spin conservation rule (p. 316). Note that spin conservation is something entirely different from symmetry conservation. 

On the other hand, oxidation of a DNA base by a triplet state of the an-thraquinone (AQ5"3) generates a contact ion pair in an overall triplet state, and back electron transfer from this species to form ground states is prohibited by spin conservation rules. Consequently, the lifetime of the triplet radical ion pair is long enough to permit the bimolecular reaction of AQ- with 02 to form superoxide (02 ) and regenerate the anthraquinone. 

Table 6.1 Some energy-transfer processes by the electron-exchange mechanism allowed according to the Wigner spin conservation rule...

Energy transfer by the exchange mechanism will occur provided the spin states before and after overlap obey the Wigner spin conservation rule that is, provided the overall spin states before and after overlap have common components (Table 6.1). 

A reaction between two species (Le., atoms or molecular entities), both of which are in triplet electronic states, resulting in at least two products, one of which is in its ground singlet state and the other in an excited singlet state. Delayed fluorescence often accompanies such processes. See also Annihilation Wigner Spin Conservation Rule Delayed Luminescence... 

WIGNER SPIN CONSERVATION RULE DELAYED LUMINESCENCE TRIPLET-TRIPLET ENERGY TRANSFER TRIPLET-TRIPLET TRANSITIONS TRITIUM TRITON TRITIUM... 

PROTEIN TURNOVER KINETICS WIGNER SPIN-CONSERVATION RULE WOMACK-COLOWICK DIALYSIS METHOD WORK... 

The reaction with 4-hydroxy-TEMPO leading to the ether is particularly interesting as the reaction could be spin allowed (i.e., doublet - - triplet —> doublet) if sufficient interaction between the O—H and nitroxide centers takes place. However, the EPR parameters for 4-hydroxy-TEMPO suggest that the interaction between the two sites is small.The magnitude of the relaxation of spin conservation rules seems unclear, but the kinetic results show virtually no effect. The rate constant for insertion at the O—H bond is 2 x 10 s, which is essentially the... 

Last but not least, it should be noted that the description of ECL processes as a simple superposition of the two or three electron transfer channels is somewhat oversimplified from the mechanistic point of view. In real cases, the electron transfer processes are preceded and followed by the diffusion of reactants from and electron transfer products into the bulk solution, respectively. Moreover, ECL reactants and products are species with distinctly different spin multiplicities, which causes an additional kinetic complication because of spin conservation rules. Correspondingly, the spin up-conversion processes (e.g., between two forms of an activated complex 1 [A- D + ] 3 [A- D + ]) cannot be a priori excluded from the kinetic con-... 

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. 

In this paper we will first review the manner in which spin-free permutation and point group symmetry arise. Some general concepts concerning time-dependent processes will be discussed. Spin-free processes in which spin is conserved will be studied, and spin-free spin conservation rules and examples will be given. Special attention will be given to processes in which spin apparently is not conserved, but is in actuality. In addition, we will treat processes in which spin is not conserved. The role of doublepoint group symmetry and of Franck-Condon factors will be developed. Special emphasis is given to spin-forbidden processes in methylene, benzene, and chromium(III) complexes. 

Equations (5-6) and (5-7) are the spin-free analog of the Wigner spin-conservation rule. The only total permutational symmetries [A] which may arise from molecules with permutational symmetries [Aa] and [AB] are given in the sum of eq. (5-6). Examples for certain cases of [Aa] and [Ab] are given in Table III. Using the correspondence eq. (2-14) between... 

Spin-Free Wigner Spin Conservation Rules ... 

The multiplicity of the CF2 produced in reaction (4-7a) has not been specified, but spin conservation rules require that it be triplet. Subsequent work in the presence of 0276,82-8a has confirmed that this is the case. However, in the absence of 02, the triplet CF2 converts to singlet CF2 before removal.40 In mechanism II the C2F40 intermediate is presumably also a triplet. It does not stabilize, as C2F40 is not a product of the reaction. 

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