Paramagnetic species temperature

Mo(V) paramagnetic species is also an argument to exclude an interaction between the Mo site and Fe-S center I. These studies were further complemented by detailed study of the observable splitting and its temperature dependence, EPR saturation, and the effect of differential reduction of the Fe-S centers. A magnetic interaction was also seen in xanthine oxidase, between various Mo(V) EPR species and one of the Fe-S centers. A study on the... 

ESR characterization was performed in situ in order to avoid any contact of the pretreated solids with air. Spectra, recorded as the first derivative of the absorption, were obtained at room temperature or 77K using a Varian E9 spectrometer working in the X band. The g values were measured relative to a DPPH reference (g = 2.0036). The sample tubes were filled with the solid to a height greater than the depth of the resonant cavity and the number of paramagnetic species was calculated by double integration of the recorded spectra normalized to that of Varian Strong Pitch sample (g = 2.0028, 3. lO spins, cm" ). 

Figure 9. ESR signal for various reduced Figure 10. Number of paramagnetic species as solids a function of the reduction temperature.

K did not produce tiny new paramagnetic species, despite FTIR observations confirming appearance of IR features attributable to adsorbed NjO (2234 and 1256 cm in Fig. 4a) upon contact with N2O at 300 K. Stepwise decreases in magnitude of those IR features were, however, observed in each of a sequence of FTIR spectra taken after separate NjO adsorptions at increasing adsorption temperatures (TJ up to 573 K (Fig. 4b-d). From these FTIR observations it could be inferred that increased T, for contact between N,0 and vacuum-outgassed CeOj resulted in increased fractional decomposition of the N O introduced. FTIR spectra did not show bands due to peroxide species after N,0 adsorption. 

Up to this point the paramagnetic species have been considered as isolated entities on the surface or in the bulk phase. It is clear, however, that many supported catalysts are actually clusters of ions. If these ions are paramagnetic, such as Cr203 at temperatures above about 30°, then another term must be included in the spin Hamiltonian to account for the exchange energy. This term is written as... 

Although relaxation measurements have been widely used in nuclear magnetic resonance studies of solid catalysts and adsorbed molecules, they have not found such favor in similar ESR work. Relaxation phenomena, however, do play a very important role in any magnetic resonance experiment, whether or not this aspect of the problem is studied. In fact, the temperature at which most ESR experiments are conducted is dictated by the relaxation process. Furthermore, even qualitative data on relaxation times can be used as supporting evidence in the identification of a paramagnetic species. 

T. J. Kemp, University of Warwick Noting the very low quantum yield for intramolecular electron transfer in low temperatures displayed by your porphyrin-quinone model compound, would it not be possible to shock-freeze a solution undergoing irradiation at a higher temperature (and giving a workable concentration of paramagnetic species) in order to determine a low-temperature spectrum with the particular aim of observing a possible Am = 2 transition ... 

The generation of a paramagnetic species by irradiation of a 10 2 M solution of nitrobenzene in thoroughly degassed tetrahydrofuran at room temperature is efficiently quenched 2V) by 10 i mole l i perfluomaphthalene Et—56.6 kcal mole" ). 

The relation between the spin density distribution in the products of electron transfer to organic molecules and the behavior of these products in further reactions is considered here. This relation is important, but has not been investigated much so far. It would be especially useful to know the reason for the particular sequence of chemical changes in the groups present simultaneously iu the molecule. Let us compare the results of the oue-electrou aud multielectron reductions of 4,4 -dinitro dibenzoyl. The one-electron reduction leads to a paramagnetic species whose ESR spectra depend on temperature and concentration. As shown (Maruyama and Otsuki 1968), such electrou trausfer products can exist in the monomeric or dimeric form. The monomer is present at a high dilution. The dimer forms at an increased concentration or at a low temperature (see Scheme 3.50). 

With ESR spectroscopy, open-shell species can be observed and characterized as long as their total spin differs from zero. With variable-temperature ESR spectroscopy, it is possible to deduce whether the observed multiplicity is a thermally populated excited state or is the ground state [69]. From such experiments, the T-S splittings of a variety of biscarbene and bisnitrenes have been determined. ESR spectroscopy is very sensitive to paramagnetic species, and because it does not see any singlet impurities or by-products, it is relatively easy to pick out the desired signals. At the same time, analysis of ESR spectra is not trivial and special simulations are required for their interpretation. 

Cuprous chlorides, ionization, 9 297 Curie temperature dependence, of paramagnetic species in NMR, 32 15 Curium... 

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. 

The chelates described above also develop paramagnetism when they are melted the temperature variation of the magnetic moments and the spectra show distinct similarities to the solution properties 222, 223), indicating that the mechanism for the formation of paramagnetic species is very similar in both cases. 

As the temperature is further lowered, the natural processes that maintain the Boltzmann distribution (relaxation processes) may be no longer able to keep up with the rate of transitions induced by the microwave radiation. Power saturation leads to a decrease in signal at low temperatures and high levels of microwave power. Because the rate and temperature dependence of relaxation processes is very different in different systems, different paramagnetic species saturate at different levels of power and are best observed at different temperatures. Organic radicals are best observed at relatively high temperature and low levels of power transition metals, especially in systems in which S > 7, are usually observed at cryogenic temperatures because of their rapid relaxation rates. 

Laboratory experiments on the lifetime of paramagnetic species in irradiated ice, dry ice (solid C02), solid SO and CH4 have been made during the consideration of the ambient temperature of outer planets. Materials used for ESR dating, dosimetry, microscopy and assessment of the environment in earth and planetary science are summarized at the end of the chapter. 

Nitric oxide is the simplest stable molecule with an odd number of electrons. Because of its unpaired electron, it is paramagnetic at room temperature (the lowest lying energy level, X2U/z, is diamagnetic), ionizes relatively easily, and is highly reactive with other paramagnetic species, such as NOz and Oa. It is an efficient scavenger of free radicals and triplet states and has been used frequently for this purpose in photochemical and thermal studies. 

Planar-tetrahedral equilibria of nickel(II) complexes were the first spin-equilibria for which dynamics were measured in solution. It had been known that such complexes were in relatively rapid equilibrium in solution at room temperature, for their proton NMR spectra were exchange averaged, rather than a superposition of the spectra of the diamagnetic and paramagnetic species. At low temperatures, however, for certain dihalodiphosphine complexes, it is possible to slow the exchange and observe separate resonances for the two species. On warming the lines broaden and coalesce and kinetics parameters can be obtained. Two research groups reported such results almost simultaneously in 1970 (99,129). Their results and others reported subsequently are summarized in Table V. 

Fig. 7. Optical absorption and magnetic circular dichroism spectra of oxidized hydrogenase from M. thermoautotrophicum (AH strain), nickel concentration 120 pM. (a) Optical absorption spectrum, at room temperature the absorption is predominantly due to iron-sulfur clusters, (b) MCD spectra recorded at 1.53, 4.22, and 8.9 K, in a magnetic field of 4.5 T MCD is predominantly due to Ni(III), which is the only paramagnetic species in the oxidized enzyme. Reproduced, with permission, from Ref. 57.

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