Paramagnetic compound

There has been little mention of paramagnetic molecules in this chapter, for the simple reason that their nuclear magnetic resonances are normally so broad as to be useless. Nevertheless, some paramagnetic compounds are useful reagents in high-resolution NMR studies, and there are also occasions when the study of paramagnetic species could be profitable and the consequences of the paramagnetism may be helpful. There are some new concepts for us to consider, and in this brief account all we can do is indicate what can be achieved. 

A1NMR spectra of a range of compounds of the formula [M(AlMe4)3], with M = La, Nd, Pr, Sm, Y and Lu. Redrawn with permission from [41a]. Copjfright 2007 John Wiley Sons. 

The line broadening in spectra of paramagnetic compounds is caused by short electronic spin-lattice relaxation times and/or hyperfine electron-nuclear coupling. Consequently, it is usually the case that materials giving useful EPR spectra have nuclear magnetic resonances so broad as to be unobservable. The two methods are therefore to a large extent complementary. 

Nd (2 and 3 unpaired electrons) cause dramatic paramagnetic shifts of the A1 resonances, but no marked broadening effect is observed. 

Finally, we should note that the shifts in resonance frequencies caused by paramagnetic substances can be used to measure the magnetic susceptibilities of those materials [76]. The method, commonly known as Evans method, involves the use of two equally concentrated solutions of an inert substance, typically 2% of f-butanol, one also containing the paramagnetic material. One solution is placed in a capillary, inside the other. Two resonances for the butyl groups are then observed, and the shift between them is attributed to the magnetic material, and is in fact directly proportional to the differenee in volume suseeptibilities for 

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The ferroelectricity usually disappears above a certain transition temperature (often called a Curie temperature) above which the crystal is said to be paraelectric this is because thermal motion has destroyed the ferroelectric order. Occasionally the crystal melts or decomposes before the paraelectric state is reached. There are thus some analogies to ferromagnetic and paramagnetic compounds though it should be noted that there is no iron in ferroelectric compounds. Some typical examples, together with their transition temperatures and spontaneous permanent electric polarization P, are given in the Table. 

Violet, easily hydrolysed, PdFp is produced when Pd [Pd F(sl s refluxed with SCF4 and is notable as one of the very few paramagnetic compounds of Pd. The paramagnetism arises from the configuration of Pd which is consequent on its octahedral coordination in the rutile-type structure (p. 961). The dichlorides of both Pd and Pt are obtained from the elements and exist in two isomeric forms which form i.s produced depends on the exact experimenial conditions used. The more usual a form of PdCb is a red material with... 

Since X-ray determinations of structure were too time-consuming to be widely used in the l9-2()s and 1940s and. in addition, square-planar geometry was a comparative rarity, any paramagnetic compound, which on the basis of stoichiometry appeared to be 4-coordinate, was presumed to be tetrahedral. 

Suggest a reason why copper(II) compounds are often colored but copper(I) compounds are colorless. Which oxidation number results in paramagnetic compounds ... 

In the low-field condition, the quantization axis is defined by the EFG main component In this situation, and rj can both be determined from powder spectra when recorded in an externally applied field. Figure 4.14 shows simulated spectra as is often encountered in practice such as in applied-field measurements of diamagnetic compounds or fast-relaxing paramagnetic compounds at high temperatures. The simulated traces differ in detail from a single-crystal spectrum as shown in Fig. 4.13, but their features still correlate in a unique manner with rj and the sign of... 

Powder spectra of paramagnetic compounds measured with applied fields are generally more complicated than those shown in Fig. 4.14. Large internal fields at the Mossbauer nucleus that are temperature- and field-dependent give rise to this complication. If, however, the measurement is performed at sufficiently high temperature, which is above ca. 150 K, the internal magnetic fields usually collapse due to fast relaxation of the electronic spin system (vide infra, Chap. 6). Under... 

The procedure of spin counting is then to use the EPR spectrum of another paramagnetic compound as an external standard (which we will label K to avoid confusion with the spin S) of known concentration (cK) to obtain the unknown (U) concentration (cv) of the paramagnetic compound of interest as... 

Europium (II) sulfide is a black powder possessing a sodium chloride lattice7 and an unknown melting point. Crystals of the sulfide have a golden hue by reflected light. The compound exhibits ferromagnetism8 with a Curie temperature of 17°K. Above this temperature, europium (II) sulfide behaves as a typical paramagnetic compound.2... 

Beside classical MRI, based on the measurement of water abundance in tissues and variation of its relaxation rates, other techniques such as chemical exchange saturation transfer (CEST) and its improved form using paramagnetic compounds (PARACEST) are currently investigated to improve the potential of MRI diagnosis. 

In the paper published in 1900, he reported that hexaphenylethane (2) existed in an equilibrium mixture with 1. In 1968, the structure of the dimer of 1 was corrected to be l-diphenylmethylene-4-triphenylmethyl-2,5-cyclohexadiene 3, not 2 [38]. Since Gomberg s discovery, a number of stable radicals have been synthesized and characterized, e.g., triarylmethyls, phenoxyls, diphenylpicryl-hydrazyl and its analogs, and nitroxides [39-43]. The radical 1 is stable, if oxygen, iodine, and other materials which react easily with it are absent. Such stable radicals scarcely initiate vinyl polymerization, but they easily combine with reactive (short-lived) propagating radicals to form non-paramagnetic compounds. Thus, these stable radicals have been used as radical scavengers or polymerization inhibitors in radical polymerization. 

The reaction in Equation (102)115 produces a paramagnetic compound in which a Tl2+ ion bridges between two [Pt(C6F5)4]2 groups (Structure 122).115 The Pt-Tl-Pt angle is 179(1)°. The Pt-Tl distances are considerably shorter than those in the related Tl+ Pt/G6F5 complexes mentioned above. 

The chemical shift measured on the bulk water signal and normalized by the mole fraction of bound water, Pm, is given by Eq. (10), where m and mA are the observed chemical shifts with and without the paramagnetic compound and ffios is the shift originating from water molecules outside the first coordination shell. 

Group VA Donor Ligands. The paramagnetic compounds, [Co(CO)2(PR3)]3 (R3 = Bu5, Bu2 Ph, or Phj), have been isolated during stoicheiometric hydro-formylation reactions of linear a-olefins " viz ... 

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