Spin change

It is likely that these compounds will receive increasing attention as inorganic chemists, biochemists, and crystallographers (437) wrestle with the problems associated with spin changes in biological systems... 

The primary step is the absorption of a quantum, which results in an excitation of the Ti-orbital. This singlet orbital can undergo intersystem crossing with spin change to a triplet stage ((t, (t), which has been found in brominated arenes (ref. 29). The photolytic reaction in methanol is much slower compared to hexane (ref. 27). This is shown for 1,2,3,4-TBDD, for which the decay constants ha/e been determined in both solvents ... 

Boillot M-L, Zarembowitch J, Sour A (2004) Ligand-Driven Light-Induced Spin Change (LD-LISC) A Promising Photomagnetic Effect. 234 261-276 Boukheddaden K, see Bousseksou A (2004) 235 65-84 Boukheddaden K, see Varret F (2004) 234 199-229 Bourissou D, see Bertrand G (2002) 220 1-25... 

A certain ambiguity arises in the proper choice of the thermodynamic parameter p, since entropy changes due to solvent orientation are neglected. The available experimental data (cf. Sect. 4) indicate, however, that the free energy of reaction for systems showing a spin change is close to zero. The numerical analysis has been therefore performed for the specific case p = 0, for which value the rate constant in Fig. 15 has been computed as a function of S and h lkgT. 

The allowed transitions occur between levels Ex and E3, and between levels Ei and E4 as indicated by the dashed lines. These transitions are governed by selection rules which require that the electron spin changes by one unit while the nuclear spin remains unchanged. Under certain rather restricted conditions these selection rules no longer apply and forbidden transitions occur. 

Ligand-Driven Light-Induced Spin Change (LD-LISC) ... 

The discussion above has been directed principally to thermally induced spin transitions, but other physical perturbations can either initiate or modify a spin transition. The effect of a change in the external pressure has been widely studied and is treated in detail in Chap. 22. The normal effect of an increase in pressure is to stabilise the low spin state, i.e. to increase the transition temperature. This can be understood in terms of the volume reduction which accompanies the high spin—dow spin change, arising primarily from the shorter metal-donor atom distances in the low spin form. An increase in pressure effectively increases the separation between the zero point energies of the low spin and high spin states by the work term PAV. The application of pressure can in fact induce a transition in a HS system for which a thermal transition does not occur. This applies in complex systems, e.g. in [Fe (phen)2Cl2] [158] and also in the simple binary compounds iron(II) oxide [159] and iron(II) sulfide [160]. Transitions such as those in these simple binary systems can be expected in minerals of iron and other first transition series metals in the deep mantle and core of the earth. 

Drickamer and Frank (1973) Spin changes in iron complexes [208]. 

The 4,4 -bipy derivative undergoes incomplete, thermally induced SCO at 4.6 kbar and 4.8 kbar with estimated T1/2 values ca. 100 K and 150 K, respectively. Interestingly, the system becomes essentially LS at room temperature at a pressure of 5.4 kbar. Similar behaviour is displayed by the bpe derivative. It is important to point out that 85% of the spin change takes place within the range of 2 kbar at room temperature suggesting that these 3D networks show strong cooperativity. 

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