Activation energy spin crossover

From these considerations it is clear that complexes in spin equilibrium do not exist at the crossover point between high-spin and low-spin configurations represented on a Tanabe-Sugano diagram. The two states are electronic isomers with geometric and electronic structures well separated on either side of the crossover point. The energy required to reach the crossover point represents at least part of the activation energy for the spin state interconversion. 

Cobalt (III) complexes appear to have B and C values of about 500 and 3800 cm. , respectively. These typical values give a crossover point of 16,500 cm. , which has been used in Table III to estimate the spin-free activation energies of the other species, the spin-free activation energies being the amount by which 2A exceeds 5 B + 8 C. That the position of the spin-free crossover is not too low is indicated by the fact that the hypothetical spin-paired [Co Fe]" ion has a A value (Table IV) about equal to the crossover point estimated from the Racah parameters, B and C. The spin-paired activation energies, estimated at 0.4A, are also given in Table III for comparison purposes. 

The low probability of this reaction ( 1 reaction per 10 collisions) is then attributed to the spin forbidden nature of the N2 0 formation step. This reaction would require a significant amount of activation energy, but Kaufman and Koski have pointed out the possible lowering of the crossover point between the 2 and the curves owing to the attractive polarization force between O and N2. ... 

The HS LS relaxation is basically a unimolecular process, and in diluted mixed crystals corresponding relaxation curves are single exponential. In Figure 5, HS LS relaxation rate constants for several spin-crossover complexes doped into inert host lattices as well as for some LS complexes are plotted as Lhl on a log scale vs. l/T. Figure 5 includes data obtained by optical spectroscopy, by Mossbauer line shape analysis, and by Mossbauer emission. Above 50K, these curves show the classical behavior of a thermally activated process, as is expected based on the energy barrier between the two states, and in agreement with results from... 

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