Normal paramagnets

For normal paramagnets organic radicals, transition metal complexes and lanthanide compounds serve as examples. As the p-block elements may have a maximum of 3 unpaired electrons, of-block 5, and /-block 7, these numbers 

Some organic radicals can survive their spin even after combination with metal centres. An example is the nitronyl-nitroxide radical. Under certain circumstances the spin state of the central atom, say Cu(II), can be increased by ligand radicals. 

The molar magnetic susceptibility of a normal paramagnet (a free radical with spin s = 1/2) is of the order of Xmoi = 16 x 10-9 m3 mol- at 293 K. It would be Xmoi= 61 x 10-9m3mol-1 at 77 K and Xmoi = 1122 x 10-9m3mol-1 at 4.2 K. 

In fact, deviations from the Curie law are positive (ferromagnetic) or negative (antiferromagnetic), as is seen from Fig. 7.3. These can be phenomenologically ascribed through the empirical Curie-Weiss law 

5 Example Curie constant, C (10 6Km3 mol-1) Effective magnetic moment, /xcfr//xB 

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E—The elements that are normally diamagnetic are those in the same columns of the periodic table as Be, Zn, and He. All others are normally paramagnetic. 

Devaux and McConnell9 took advantage of the fact that in fluid membranes such as egg phosphatidylcholine, the resonance spectra of spin labels such as V and VI depend strongly and monotonically on the label concentration c when c 3= 5 mole %. The normalized paramagnetic resonance spectra S0(H, c) of a series of samples, all of uniform concentration c, were determined experimentally.9 The observed time-dependent spectra are then obtained from the equation... 

Aj(Oh) transition, whereas compounds I and III behave as normal paramagnetic complexes174). Spin state lifetime measurements on these and other iron(II) spin crossover systems have been carried out by Sutin et al.17S) and Beattie et al.176) the results are given in Chap. 9. 

The experimental results will be considered first of all. Comparing the values for K, found in the Zintl phases with the ones in the pure metals one notices that the AB phases show a normal paramagnetic behaviour. For the AB compounds K, is distinctly smaller than in the case of pure metals. Kj is even negative for NaTl. 

Antiferromagnetism is very much temperature dependent. For antiferromagnetic substances there is a characteristic temperature, known as Neel temperature or Neel point above which the antiferromagnetic substance behave as a normal paramagnetic substance. 

Diamagnetism —normal diamagnet —superconductor Paramagnetism —normal paramagnet... 

The treatment for Mo(0) above has been a detailed one relative to what follows below and there has been some overlap with previous reviews. The sections which follow for the oxidation states Mo(I) to Mo(VI) are meant to be supplementary to the very comprehensive treatment of Minelli et al The bulk of the new data presented here are for compounds of Mo(II) and Mo(VI). NMR investigations for the normally paramagnetic Mo(I) d , Mo(III) d and Mo(V) d metal centres have been somewhat restricted to diamagnetic spin-paired dimers and related extended species (Mo2) . There... 

Most metals show paramagnetic behaviour. It is rather small and independent of temperature and is quite different from the Curie law behaviour of a normal paramagnetic ion. The cause of the weak paramagnetism was explained by Pauli in terms of the Fermi-Dirac statistics of electrons in solids. It is known as Pauli paramagnetism. 

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