Paramagnetic states

The transition from a ferromagnetic to a paramagnetic state is normally considered to be a classic second-order phase transition that is, there are no discontinuous changes in volume V or entropy S, but there are discontinuous changes in the volumetric thermal expansion compressibility k, and specific heat Cp. The relation among the variables changing at the transition is given by the Ehrenfest relations. 

NMR spectra have been reported for the Rieske-type ferredoxins from Xanthobacter strain Py2 (88) and of toluene 4-monooxygenase from Pseudomonas mendocina (T4MOC) (88a) as well as for the water-soluble Rieske fragment from the bci complex of Paracoccus deni-trificans (ISFpd) (89). The spectra of these proteins are similar, which is consistent with the close structural relationship between the three proteins. In the reduced (paramagnetic) state, all three proteins show several hyperfine-shifted resonances between +83 and -16 ppm at 400 MHz or between 110 and +25 ppm at 300 MHz (Table X). 

It appears that Cluster C catalyzes the chemistry of CO oxidation and transfers electrons to Cluster B, which donates electrons to external acceptors such as ferredoxin. Since a crystal structure of this protein does not exist, the proposed structure of Cluster C is based on spectroscopic measurements. In some cases, the EPR spectrum of a metal center is diagnostic of the type of center. However, the EPR spectra of Cluster C are unusual. The paramagnetic states of Cluster C (Credi and Cred2) have g-values that are atypical of standard [4Fe-4S] clusters (Table III) and are similar to those in a variety of structurally unrelated systems including a t-oxo bridged ion dimer), a [Fe4S4] ... 

In order to figure out the FWs, the nuclear structure was refined from unpolarized neutron data taken at 30 K, in the paramagnetic state, on a 4-circle diffractometer. Furthermore, a set of 248 flipping ratios was measured with polarized neutrons at 1.6 K, with the spin density long range ordered by a 4.65 T applied magnetic field. 

Le Pape, L., Lamotte, B., Mouesca, J.-M., and Rius, G. 1997. Paramagnetic states of four iron-four sulfur clusters. 1. EPR single-crystal study of 3+ and 1+ clusters of an asymmetric model compound and general model for the interpretation of the g-tensors of these two redox states. Journal of the American Chemical Society 119 9757-9770. 

There has been one recent report of a paramagnetic hydrogen state, with indirect evidence that it would be associated with the bond center. ESR experiments by Gorelkinskii and Nevinnyi (1987) and by Gordeev et al. (1988) showed the existence of a paramagnetic state due to H in Si, called the AA9 center. They also showed that the characteristics of AA9 are similar to those of Mu. Since Mu is now known to be associated with the bond center (a fact not appreciated by the Russian group), this provides indirect evidence for bond-centered hydrogen. 

Above a temperature called the Curie temperature, Tc, all ferromagnetic materials become paramagnetic. The transition to a paramagnetic state comes about when thermal energy is greater than the magnetic interactions and causes the dipoles... 

Figure 2.9 The B-Tphase diagram of MnP [13] with the magnetic field along the b-axis. Three different magnetically ordered phases - ferro, fan and screw - are separated by first-order phase transitions. The transitions to the disordered paramagnetic state are of second order and given by a dashed line.

A novel variation of the above technique can be used in some cases. By choosing an appropriate source so that the maximum resonance in the paramagnetic state occurs at or near zero relative velocity, one eliminates the need of a Mossbauer spectrometer completely, and the transition temperature can be determined by measuring the count rate transmitted through a stationary absorber and emitted by a stationary source as a function of temperature—see, for example. Refs. 18, 20. 

Mott transition, 25 170-172 paramagnetic states, 25 148-161, 165-169 continuum model, 25 159-161 ESR. studies, 25 152-157 multistate model, 25 159 optical spectra, 25 157-159 and solvated electrons, 25 138-142 quantitative theory, 25 138-142 spin-equilibria complexes, 32 2-3, see also specific complex four-coordinated d type, 32 2 implications, 32 43-44 excited states, 32 47-48 porphyrins and heme proteins, 32 48-49 electron transfer, 32 45-46 race-mization and isomerization, 32 44—45 substitution, 32 46 in solid state, 32 36-39 lifetime limits, 32 37-38 measured rates, 32 38-39 in solution, 32 22-36 static properties electronic spectra, 32 12-13 geometric structure, 32 6-11 magnetic susceptibility, 32 4-6 vibrational spectra, 32 13 summary and interpretation... 

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