Electron transfer, paramagnetic transition

Some electron exchanges of cobalt complexes involve changes in spin configuration, from low-spin to high-spin such that S S V2, and this can lead to another source of non-adiabaticity. The spin forbidden factor, Xs> is entirely empirical within the present formalism, as well as within the TM formalism. For the complexes Co(phen)3 + + experimental evidence, also supplemented by quantum mechanical calculations, suggest that Xs 10. and as a rule of thumb we will employ this value for spin-forbidden electron transfers in transition-metal complexes. Where no heavy metals or paramagnetic species are involved, we expect to have Xs 10 , as shown in Chapter 15 for aromatic hydrocarbons. 

T. J. Kemp, University of Warwick Noting the very low quantum yield for intramolecular electron transfer in low temperatures displayed by your porphyrin-quinone model compound, would it not be possible to shock-freeze a solution undergoing irradiation at a higher temperature (and giving a workable concentration of paramagnetic species) in order to determine a low-temperature spectrum with the particular aim of observing a possible Am = 2 transition ... 

Mott transition, 25 170-172 paramagnetic states, 25 148-161, 165-169 continuum model, 25 159-161 ESR. studies, 25 152-157 multistate model, 25 159 optical spectra, 25 157-159 and solvated electrons, 25 138-142 quantitative theory, 25 138-142 spin-equilibria complexes, 32 2-3, see also specific complex four-coordinated d type, 32 2 implications, 32 43-44 excited states, 32 47-48 porphyrins and heme proteins, 32 48-49 electron transfer, 32 45-46 race-mization and isomerization, 32 44—45 substitution, 32 46 in solid state, 32 36-39 lifetime limits, 32 37-38 measured rates, 32 38-39 in solution, 32 22-36 static properties electronic spectra, 32 12-13 geometric structure, 32 6-11 magnetic susceptibility, 32 4-6 vibrational spectra, 32 13 summary and interpretation... 

A second way to overcome this spin conservation obstacle is via reaction of 302 with a paramagnetic (transition) metal ion, affording a superoxometal complex (Fig. 4.1, Reaction (3)). Subsequent inter- or intramolecular electron-transfer processes can lead to the formation of a variety of metal-oxygen species (Fig. 4.2) which may play a role in the oxidation of organic substrates. 

These reactions turned out to be preparatively very useful since they allow the performance of radical cyclization reactions but produce an organozinc halide as a final product (Scheme 9-33) [65-70]. The treatment of an unsaturated alkyl halide 36 (X = Br, I) with a palladium(O) or nickel(O) complex produces, via a one-electron transfer [72], a paramagnetic nickel(I) or palladium(I) salt MLj,(X) (M = Ni, Pd) and a radical 37 which undergoes a smooth cyclization reaction and produces, after recombination with the transition-metal moiety, the nickel(ll) or palladium(II) species 38. After transmetallation with a zinc(II) salt, a stable organozinc cyclopentylmethyl derivative of type 39 is produced. 

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