Conservation, spin

Previous expositions of photochemical laws have distinguished ptominentiy between states of singlet and triplet multiplicity (1). This distinction continues to be important with respect to photophysics of smaH organic molecules, but among inorganic and organometaHic compounds, states of other multiplicities, eg, doublet and quartet states (23), play an important role. Spin conservation characterizes electronic molecular excitations and localized... 

The diamagnetic nature of most O2 complexes might facilitate reactions to form diamagnetic products which would otherwise be hindered by the requirement of spin conservation ... 

There does not seem to be any selection rule such as conservation of spin or orbital angular momentum which this reaction does not satisfy. It is also not clear that overall spin conservation, for example, is necessary in efficient reactions (5, 16, 17, 20). Further, recent results (21) seem to show a greatly enhanced (20 times) reaction rate when the N2 is in an excited vibrational state (vibrational temperature 4000 °K. or about 0.3 e.v.). This suggests the presence of an activation energy or barrier. A barrier of 0.3 e.v. is consistent with the low energy variation of the measured cross-section in Figure 1. 

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

All of these reactions are exothermic but some of them violate the law of spin conservation since S2 is a triplet molecule while all species on the right side of the above equations are singlet molecules in the ground state. Therefore, a radical-chain reaction has been assumed. S3 may also be formed from S2 by picking a sulfur atom up from a di- or polysulfide in the a-layer. 

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. 

Whereas the intermediate produced by a thermal process has to be in the singlet state initially because of spin conservation, the ground state in this case appears to be the triplet (2). 

Spin conserving reactions are significantly faster than spin disallowed reactions. 

By absorption of light a molecule is promoted to a higher electronic state. The monomolecular physical processes for the dissipation of the excess energy are outlined in Fig. 5 in a so called Jablonski diagramm. In principle one has to differentiate between radiative and non-radiative deactivation on the one side and on the other side one has to consider if the multiplicity of the system is conserved or not. Radiative deactivation, i.e. deactivation accompanied by emission of light, is termed fluorescence if the transition occurs with spin conservation and phosphorescence, if spin inversion occurs. 

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). 

Equation (4.79) shows that Ro, and consequently the transfer rate, is independent of the donor oscillator strength but depends on the acceptor oscillator strength and on the spectral overlap. Therefore, provided that the acceptor transition is allowed (spin conservation) and its absorption spectrum overlaps the donor fluorescence spectrum, the following types of energy transfer are possible ... 

Fig. 21 Schematic representation of strategies for spin alignment in D/A salts or complexes by application of spin conservation in different electron configurations of interacting molecular orbitals, (a) Typical D/A interaction between two closed-shell D and A, (b and b ) McConnell s proposal, (c) Breslow s extension, (d) Torrance s model, (e) Wudl s model, and (f) Chiang s model for further doping.

As a result of a sequence of chemical or photochemical reactions a pair of radicals is produced. This pair usually has the same spin as the precursor from which it originates, in accordance with the spin conservation law. 

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... 

Fig. 2 Schematic diagram of spin-conserved tunneling via the Julliere model. The top and bottom panels indicate parallel and antiparallel alignment of the magnetization of the top and bottom electrodes, respectively...

In addition to energetic considerations, however, there are other factors such as spin conservation that also determine the importance of various sets of products. As discussed in Chapter 3.A, since the ground state of 03 is a singlet, dissociation into either two singlet states (e.g., reaction (5)) or into two triplet states is expected to predominate. However, as discussed shortly, both hot-band absorption by rovibra-tionally excited 03 and by a spin-forbidden process are believed to contribute significantly to the atmospheric photochemistry of 03. 

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