Non-adiabaticity due to spin forbidden processes

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. 

Finally, when the reaction barrier is estimated, the rate constant will be given by 

For the self-exchange reaction of Co(phen)3 + the frequency factor will be r = (10 )(6.3xl0 )(10 ) = 6.3x10 sec , and 1 M , as usual. With the structural data in Appendix HI, we can estimate AG = 15 kJ mol and obtain a rate constant 14 M sec , which compares very well with the experimental data at room tern- 

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