Model paramagnet

The aromatic shifts that are induced by 5.1c, 5.If and S.lg on the H-NMR spectrum of SDS, CTAB and Zn(DS)2 have been determined. Zn(DS)2 is used as a model system for Cu(DS)2, which is paramagnetic. The cjkcs and counterion binding for Cu(DS)2 and Zn(DS)2 are similar and it has been demonstrated in Chapter 2 that Zn(II) ions are also capable of coordinating to 5.1, albeit somewhat less efficiently than copper ions. Figure 5.7 shows the results of the shift measurements. For comparison purposes also the data for chalcone (5.4) have been added. This compound has almost no tendency to coordinate to transition-metal ions in aqueous solutions. From Figure 5.7 a number of conclusions can be drawn. (1) The shifts induced by 5.1c on the NMR signals of SDS and CTAB... 

The angular overlap model for the description of the paramagnetic properties of transition metal complexes. A. Benici, C. Benelli and D. Gatteschi, Coord. Chem. Rev., 1984, 60,131 (204). 

The incompletely filled d-subshell is responsible for the wide range of colors shown by compounds of the d-block elements. Furthermore, many d-metal compounds are paramagnetic (see Box 3.2). One of the challenges that we face in this chapter is to build a model of bonding that accounts for color and magnetism in a unified way. First, though, we consider the physical and chemical properties of the elements themselves. 

This example shows that dipolar interactions can produce unexpected effects in systems containing polynuclear clusters, so that their complete quantitative description requires a model in which the dipolar interactions between all the paramagnetic sites of the system are explicitly taken into account. Local spin models of this kind can provide a description of the relative arrangement of the interacting centers at atomic resolution and have been worked out for systems containing [2Fe-2S] and [4Fe-4S] clusters (112, 192). In the latter case, an additional complication arises due to the delocalized character of the [Fe(III), Fe(II)] mixed-valence pair, so that the magnetic moments carried by the two sites A and B of Fig. 8B must be written... 

Hyperfine coupling constants provide a direct experimental measure of the distribution of unpaired spin density in paramagnetic molecules and can serve as a critical benchmark for electronic wave functions [1,2], Conversely, given an accurate theoretical model, one can obtain considerable information on the equilibrium stmcture of a free radical from the computed hyperfine coupling constants and from their dependenee on temperature. In this scenario, proper account of vibrational modulation effects is not less important than the use of a high quality electronic wave function. 

Often the electronic spin states are not stationary with respect to the Mossbauer time scale but fluctuate and show transitions due to coupling to the vibrational states of the chemical environment (the lattice vibrations or phonons). The rate l/Tj of this spin-lattice relaxation depends among other variables on temperature and energy splitting (see also Appendix H). Alternatively, spin transitions can be caused by spin-spin interactions with rates 1/T2 that depend on the distance between the paramagnetic centers. In densely packed solids of inorganic compounds or concentrated solutions, the spin-spin relaxation may dominate the total spin relaxation 1/r = l/Ti + 1/+2 [104]. Whenever the relaxation time is comparable to the nuclear Larmor frequency S)A/h) or the rate of the nuclear decay ( 10 s ), the stationary solutions above do not apply and a dynamic model has to be invoked... 

Linnett used the concept that an octet of valence shell electrons consists of two sets of four opposite-spin electrons to show that in diatomic and other linear molecules the two tetrahedra are not in general formed into four pairs as we have discussed for F2 and the CC triple bond in C2H2. This idea is the basis of the double-quartet model, which Linnett applied to describe the bonding in a variety of molecules. It is particularly useful for the description of the bonding in radicals, including in particular the oxygen molecule, which has two unpaired electrons and is therefore paramagnetic This unusual property is not explained by the Lewis structure... 

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