Curie law paramagnetic

The NMR spectra [27] of [Fe(HB(pz)3)2] exhibit shifted resonances as would be expected for a paramagnetic complex, but the shifts are intermediate between those observed for fully diamagnetic complexes and the comparable resonances in the fully high-spin [Fe(HB(3,5-(CH3)2pz)3)2] complex. In contrast to the increasing chemical shifts, either in the positive or negative direction, that follow the Curie law, observed at lower temperatures for [Fe(HB(3,5-(CH3)2pz)3)2] and [Fe(HB(3,4,5-(CH3)3pz)3)2], the chemical shifts of [Fe(HB(pz)3)2] decrease as the temperature is decreased, as would be expected for an increase in the percentage of the low-spin complex. 

The partial orientation of the elementary dipoles in a paramagnetic solid is counteracted by thermal agitation, and it would be expected that at high temperatures the random motion of the atoms in the solid would cancel the alignment due to the magnetic field. The paramagnetic susceptibility would therefore be expected to vary with temperature. The temperature dependence is given by the Curie law ... 

Frequently the magnetic behavior of a paramagnetic solid does not follow the Curie law exactly but is rather better fitted by the Curie-Weiss law ... 

The average magnetization I is given by (15), where Bj a) is called a Brillouin function, and a by (16). When a 1, Bj(a) may be expanded, and, if we take only the first term, then (17) results. The paramagnetic susceptibility X is inversely proportional to the absolute temperature T. This relation is called the Curie law, and the proportionality constant C is the Curie constant. 

In the context of ESR spectroscopy, the Curie law may be stated in its simplest form as / = C/T, where I is the intensity of an absorption line, T is absolute temperature, and C is a constant. A modified form of the law (Curie-Weiss law), / = C/(r — 9), sometimes is needed when the plot of I versus 1/Thas a non-zero intercept. In both cases, the plot should be linear if the paramagnetic species responsible for the signal is not engaged in an equilibrium with other species of different multiplicity. The most common candidate for such other species is a singlet, with spin of zero. 

For an ideal paramagnet which obeys the Curie law at low magnetic field strengths the solution of the problem at gflH x kT is available in closed form. It is convenient to formulate the situation in terms of the magnetization, M = ymH, rather than of x- Then... 

Transition-metal (dn) complexes with open shells belong to the class of paramagnetic materials their magnetic susceptibility is positive (the sample is attracted to the magnetic field) and is temperature dependent. At high enough temperatures and in small fields, the molar magnetic susceptibility normally obeys the Curie law... 

The determination of n from measurement of peff is the most familiar application of magnetic susceptibility measurements to inorganic chemists. To the extent that the spin-only formula is valid, it is possible to obtain the oxidation state of the central atom in a complex. Thus an iron complex with a peff of 5.9B.M. certainly contains Fe(III) (high-spin d5) and not Fe(II). The diamagnetism of AgO rules out its formulation as silver(II) oxide, because Ag2+ has an odd number of electrons (d9) and should be paramagnetic it contains Ag(I) and Ag(III), in equal amounts. There are, however, a number of pitfalls, especially if reliance is placed on a single measurement at room temperature. The Curie law is rarely obeyed within the limits of experimental error. This means that the measured peff is somewhat temperature-dependent. A number of factors can be responsible for deviations from ideal Curie (or even Curie-Weiss) behaviour, and/or from the spin-only formula. 

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