Reactant spin multiplicity

Intersystem crossing occurs when the product spin multiplicity differs from the reactant spin multiplicity. Because the spin angular momentum is not conserved in such a reaction, it is referred to as a spin-forbidden reaction. 

One expects the impact of the electronic matrix element, eqs 1 and 2, on electron-transfer reactions to be manifested in a variation in the reaction rate constant with (1) donor-acceptor separation (2) changes in spin multiplicity between reactants and products (3) differences in donor and acceptor orbital symmetry etc. However, simple electron-transfer reactions tend to be dominated by Franck-Condon factors over most of the normally accessible temperature range. Even for outer-... 

These state orderings lead to a situation in which the reactants and products belong to one spin surface, while the intermediates have a different spin multiplicity. The experimental and theoretical evidence indicates that for iron, as a 3d element, the spin-change itself is mediated by spin-orbit coupling and is in fact rate-determining for the occurrence of the overall reaction this scenario can accoimt for all experimental observations. 

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-... 

Reactant states will only correlate with product states of the same spatial symmetry and spin multiplicity. 

Only when one or both reactants are singlet states is there only one spin state for the product. (If one reactant is not a singlet state, then the product must be of the same spin multiplicity as that reactant.) In Sect. 5.2,... 

The scheme indicated in Figure 1 (excitation, charge separation, charge recombination) is most likely to occur when the product state has the same spin multiplicity as the reactant state, or when the product species are held together (e.g., by chemical bonds) for periods that allow intersystem crossing to occur. However, the charge separation-recombination sequence is not the only type of process that photoexcitation can achieve. An example is when the product state is held only loosely by a solvent cage and the entities therein can diffuse apart and have independent reactive identities. Such a scheme would be typified by the sequence... 

It is noteworthy that the rate of reduction of the Zn porphyrin j-radical cation by the copper(I) complex is much faster than direct charge recombination between the porphyrin radicals, although the reaction exoergonicity for Eq. 19 is very much less than that for Eq. 18. The faster rate for Eq. 19 may arise because of the closer proximity of the reactants and/or because electron transfer occurs via a different route. In addition, /ci9 shows some dependence on the porphyrin donor. This effect could be due to a different spin multiplicity of the precursor excited state. 

The cross-section of a given pair of reactants for undergoing a specific channel of chemical reaction is dependent not only on the relative kinetic energy but also on the internal states of the reactants (energy and other properties such as orbital symmetry and spin multiplicity). Thus, in order to speak of the cross-section of a channel unequivocally, it is necessary to specify both the relative kinetic energy and the internal states of the reactants. 

From Ref. 23. Energies calculated relative to the ground state reactant asymptote with corrections for differential zero-point energy effects. In each case the energy reported is for the lowest electronic state, regardless of its spin multiplicity. Geometries optimized at the Hartree-Fock level. 

Chemical reactions between two reactants in different electronic states, leading to two products in electronic states that preserve the total spin multiplicity, can also be called radiationless transitions. Two frequently encountered examples are electron and electronic-energy... 

Furthermore, the short scan times of EPR (usually 500 ms or less) and the ability to measure species in diamagnetic matrices, for example, aqueous solutions, enable the time-resolved monitoring of chemical reactions involving radical reactants or intermediates. In this way, kinetics of such reactions can be studied even if multiple magnetic species are involved, as their characteristic signals typically differ sufficiently to deconvolute the resulting EPR spectra. Commercial pulsed EPR spectrometers are also available, enabling the study of spin dynamics, that is, the relaxation of the excited system via spin-spin and spin-lattice mechanisms. 

Computational and theoretical chemistry has a very important role to play in helping to predict and rationalize the nature of the electronic ground state of TM compounds. Being able to do so is critical in many respects, if one wishes to predict the structure, properties, and reactivity of such compounds. First of all, the predicted structure and properties (for example the spectroscopic features) will of course be very different for different spin states. Of more interest to our research group is the notion that reactivity often crucially depends on the preferred spin state of reactants, products and intermediates. Thus, we have shown elsewhere (for reviews, see [2-4]) that many reactions of TM compounds involve multiple electronic states, of different spin. In these cases, reactivity is strongly influenced by the energy... 

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