Ferromagnetically Coupled Systems

If we first consider the three scenarios at the same field, we can immediately see how the paramagnetic material shows the largest magnetocaloric effect, followed by a ferromagnetically coupled system, and then the antiferromagnetic case. 

Fig. 10.11. Magnetic energy levels for a ferromagnetically coupled system SA = SB = 1 with positive zero-field splitting parameters.

Although extended ID systems comprising ferromagnetically coupled gadolinium and copper ions were obtained [46], the exchange interactions were too weak to provide significant effects and the increase of the x l product was observed only at very low temperatures. A much richer scenario was instead... 

The active site in the aerobic, inactive D. vulgaris enzyme shows no EPR signals. The FTIR spectrum indicates the presence of a bridging CO, in addition to the end-on bound CN/CO pair to each iron atom (Fig. 7.2, trace A). We presently assume that both Fe atoms in the bimetallic site are low-spin ferric and that their S = 1/2 systems are anti-ferromagnetically coupled to a diamagnetic state. 

Once the system passes through 7TS1, it forms a high-spin (HS) Fe(III) ion that is ferromagnetically coupled to a substrate alkyl radical. Thus, the second electron transfer from the substrate to... 

Even for dimetallic centers with each metal ion having 5 > V2, Eq. (6.12) provides a value of R m which is one-half of that provided by the analogous equation for the monomer. If we repeat the calculation for the hyperfine shifts (Eqs. (2.6) and (6.9)), at variance with nuclear relaxation, we see that the shifts for the coupled and uncoupled systems are the same as long as / kT. Note that ferromagnetic coupling would have provided the same results because our reasoning was not based on the order of the S levels. 

Naturally, the product of two-electron reduction of the para derivative can be depicted (Scheme 3-54) as an anionic diborataquinoid system. In contrast with the para-dib-orataquinoid dianion, an anionic meta-quinoid system is impossible. Indeed, the meta-sub-stituted isomer depicted in Scheme 3-54 has been characterized as a spin-unpaired triplet species with boron-centered spins (Rajca et al. 1995). This dianion diradical can be viewed as two stable borane anion radicals linked with a ferromagnetic coupling unit, i.e., 1,3-phenylene see Chapter 1. 

Beyond these two example applications a number of others have been described. These concern not only ordinary radicaloid (finite) molecules, but also polymer ends, graphitic comers, carbene containing systems (with ferromagnetic couplings between the singly-occupied a-orbitals and the 7t-network), and even some systems with transition metal centers involved in the exchange network. See Refs. [195,198]. 

By fitting the low temperature susceptibility data to a series expansion for the spin-1/2, ID Heisenberg system [49], we find a 1/T2 dependence of the susceptibility which confirms the quasi-ID behaviour and gives a ferromagnetic coupling constant of 50 K. This plot is shown in Fig. 20a. McConnell s mechanism for ferromagnetic exchange... 

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