Normal diamagnets

Weak diamagnetic materials have magnetic susceptibility values close to zero their molar magnetic susceptibility is of the order of Xmoi = —10 x Kr9m3mol 1. Diamagnetic susceptibility is a temperature independent quantity (Fig. 7.1). Some diamagnetic materials (water, benzene) are used to calibrate (check) the experimental equipment. 

Some substances exhibiting diamagnetic behaviour can switch to the paramagnetic state when the temperature is increased. These spin-crossover (mostly Fe(II)) systems will be discussed in more detail later. 

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Only in a tartrate buffer is the rate appreciable and it is significant that only in this buffer does the normally diamagnetic Mo(V) display esr signals g = 1.937 and g = 1.945) indicating the presence of monomeric Mo(V) which is regarded as the active species in the proposed mechanism, viz. 

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. 

The first observation of an abnormal magnetic moment, in the sense intended here, was made by French et al. (90), who found that the normally diamagnetic bis(formylcamphor)ethylenedi-imine nickel(II) had a magnetic moment of 1.9 B.M. in methanol. They suggested that a fraction of the complex molecules were fully paramagnetic in the solution, an idea which is still universally accepted. Willis and Mellor (261) developed this theme and found that the phenomenon covered a wide range of nickel chelates and also of solvents. The moments developed were larger in pyridine than in inert solvents like benzene or chloroform, and the moments in pyridine often corresponded to complete conversion into para-... 

The theory needed to explain why normally diamagnetic compounds should develop singlet and triplet states of very nearly equal energy has been developed by Maki (139-161) Ballhausen and Liehr have come to similar conclusions on much simpler grounds (11). Maki (159) has used purely electrostatic models to determine the splittings of the states of a nickel(II) ion in different symmetries. The results obtained have been tested by attempting to assign the near infrared and visible spectra of suitable complexes with them (160, 161). 

The diamagnetic susceptibility is a measure of the averaged electronic distribution in bulk matter. Careri et al. (1977, 1980) showed that the differential diamagnetic susceptibility per gram of water adsorbed on lysozyme powders reached the bulk water value at 0.2 h. Lysozyme behaved as a normal diamagnetic substance. The diamagnetic susceptibility and the enthalpy of sorption for lysozyme change similarly at low hydration. 

The triplet ground state of molecular oxygen provides a considerable kinetic barrier to the auto-oxidation of normally diamagnetic organic molecules where reactions involving change of spin are generally very slow, and where products formed in triplet states are unstable. This barrier may be circumvented in three ways ... 

Radicals are molecules which are normally diamagnetic, but which for one reason or another (because of chemical reactions or photolysis) have lost or gained one electron. The pairing balance is therefore lost one electron is unpaired and possesses a magnetic moment which in a magnetic field interacts with electromagnetic radiation as described above. 

The observed second order decay may arise by the spin pairing of Au° atoms forming the normal diamagnetic metal, or by the bimolecular reaction of the electron adduct species which also produces the metal. Second order decay of Au° caused by disproportionation reactions, as... 

Fig. 7.1. Temperature behaviour of magnetic suceptibility for normal diamagnetics.

The transition from the normal diamagnetic state to superconductive diamagnetism is characterised by one or more transition temperatures Tc identified by the inflection points on the x versus T curve. A very good identification of the transition temperature is provided by the maximum of the imaginary (out of phase) susceptibility x" measured in an alternating magnetic field (Fig. 7.2). 

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

The maximum oxi tion state possible for any transition element is the group number, N, because only N valence electrons are available for ionization or for forming covalent bonds. These compounds ate normally diamagnetic. Re... 

Suppose that a static magnetic field H is applied to the system. Since nematic and cholesteric liquid crystals are normally diamagnetic, the energy density induced by the magnetic field is given by... 

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