Impurities, paramagnetic

The above discussion ignores all paramagnetic effects including band paramagnetism. Evidence for a Curie-like contribution is seen at low temperatures in some of the curves displayed in Fig. 5 and could arise, in part, from paramagnetic impurities (see below). 

The paramagnetic impurity which invariably accompanies Wilkinson s catalyst has proved difficult to identify. It is probably the air-stable, green, irans- (RhCl(CO)(PPh3)2j- see K. R. Dunbar and S. C Harfner, Innrg. Chem. 31, 36"76-9 (1992),... 

Induced paramagnetic impurities might play some part. It seems however, that the concentration which they can reach at room temperature is so low that their effect is insignificant. 

Our new measurements within the temperature range 4.2 to 300 K shows that Pu(C0T)2 is temperature independent paramagnetic, Fig. 5. A small paramagnetic impurity was found which corresponded to Vgff = 0.01 y. Extrapolation for l/T- 0 yielded Xjip = (191 10) x 10-6 emu. The diamagnetic correction used, =... 

Unlike high-resolution NMR spectra of bulk solutions where NMR linewidths well below 1 Hz can be obtained routinely, NMR spectra of liquids permeating porous solids in most cases will not exhibit such a high spectral resolution. First of all, the interaction of liquid phase molecules with pore walls of the catalyst and rapid diffusion-driven intrapore transport will lead to a pronounced homogeneous broadening of the observed NMR lines. Smaller pore sizes and the presence of paramagnetic impurities in the solid material usually aggravate the situation and thus should be avoided. Another reason why NMR spectra of liquids in porous... 

The electrochemical cells employed in epr measurements are generally fabricated out of silica since glass contains paramagnetic impurities that can interfere with the signal. The need to minimise the interaction of the sample with the electric component of the radiation in the cavity, and the geometry and size of the cavity itself, have generally led to electrochemists employing flat cells that are c. <0.5mm thick. 

The hyperfine couplings between localized electrons on paramagnetic impurities and nuclear spins often serve as the dominant source of spin-lattice relaxation for... 

In situ EPR spectroelectrochemistry monitors paramagnetic species, usually radicals in solution. The chemical stability of such species can be readily determined by this technique. It was seen that the most conunon problems encountered with in situ electrochemical EPR work emanate from the use of absorbing solvents and polymers containing paramagnetic impurities. 

F NMR of Fluorine-Doped -Alumina. The samples studied 115) were high surface area aluminum oxides doped with fluorine by addition of aqueous HF to alumina and subsequent dehydration. A suflScient number of paramagnetic impurities were present in the samples to give relaxation times of the order of 0.01 second. The BET surface areas of most of the samples examined were within 20% of 250 meterVgram. 

Fio. 18. Spin-lattice relaxation time Ti for silica gel (SG) and silica-alumina (SA) versus paramagnetic impurity content N. The straight line corresponds to the relation Ti oc (irS). 

Magnetic susceptibility studies can be difficult to interpret for multicentered metalloproteins particularly when paramagnetic impurities are present, whereas with MCD spectroscopy the magnetic properties of individual centers can be investigated independently. However, it should be emphasized that MCD studies... 

The most important relaxation processes in NMR involve interactions with other nuclear spins that are in the state of random thermal motion. This is called spin-lattice relaxation and results in a simple exponential recovery process after the spins are disturbed in an NMR experiment. The exponential recovery is characterised by a time constant Tj that can be measured for different types of nuclei. For organic liquids and samples in solution, Tj is typically of the order of several seconds. In the presence of paramagnetic impurities or in very viscous solvents, relaxation of the spins can be very efficient and NMR spectra obtained become broad. 

Two mechanisms have been suggested to account for the reduction in the heat release rate a barrier mechanism, in which the clay functions as a barrier to mass transfer of the polymer, and a radical trapping mechanism, which occurs due to the presence of iron or other paramagnetic impurities as a structural component in the... 

Calcium Carbonate (CaC03) Speleothem, Shell and Coral. - 3.1.1 Paramagnetic Impurities Field-swept ESE and Out-of-phase Measurements. Carbonate dissolution and the back reaction of its deposition are described as follows... 

In the case of spins I—i/l and assuming that the sample does not contain paramagnetic impurities and that other susceptibility effects can also be neglected, the relationship between v g and Vq is given by the following ... 

The sample in ESR spectroscopy is often placed in a quartz tube (glass has paramagnetic impurities), which is inserted into the resonant cavity through holes. The sample is usually a solid or liquid some gas-phase ESR work has been done. 

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