Effect of paramagnetic impurities

By contrast, the problem of degassing is less crucial. As no effect upon the T values of HOCH2CH2NH2 is observed by saturating the 

36 s and 12 s respectively, in the absence and in the presence of cryptate. Therefore, the addition of relaxation reagents enables more efficient pulsing and observation of otherwise hardly detected nitrogens, such as those of 2,6-t-butylpyridines, can be obtained in more reasonable time (D 10). 

It should be noted, however, that the addition of noticeable amounts of Cr(acac) is likely to introduce non-negligible chemical shift perturbations. These effects occur not only through intermolecul-ar interactions but also through magnetic susceptibility variations since an external reference is usually employed. When quantities up to 0.08 M are added in order to shorten the T-j values significantly, chemical shift variations of about 0.5 to 1.5 ppm are frequently introduced. 

Moreover, as such reagents act as quenchers for the nuclear Overhauser enhancement, the risk of cancelling the nitrogen signal is present (Sect. 2.5) and they should be used with caution. 

Paramagnetic relaxation reagents capable of specifically interacting with a basic site of the investigated substrate can also be 

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Smernik, R. J., and Oades, J. M. (2000a). The use of spin counting for determining quantitation in solid state C-13 NMR spectra of natural organic matter 1. Model systems and the effects of paramagnetic impurities. Geoderma 96,101-129. 

The narrow linewidths of liquids allow one to observe weak spin interactions such as the indirect spin-spin (/) coupling and the isotropic chemical shift (7>). In the solid state, however, linewidths are broadened considerably by much stronger interactions, involving mainly the dipole-dipole interaction (typically 10-100 kHz), chemical-shift anisotropy ( 1 kHz), the quadrupole interaction ( 250 kHz) and the effect of paramagnetic impurities. For these reasons the / and <5 interactions are generally not observed in solids.2... 

The conversion has been used to demonstrate the existence of radicals in the photochemical decomposition of CHsI, HI, 02116, " and NHs. The chief difficulty with the method involves the inability to distinguish between different radicals and also the effects of paramagnetic impurities. 

The zeolite specimen used in the present studies was prepared from zone-refined aluminum and silicon its ferric ion impurity content of < 6 ppm is low enough to ensure that only intrinsic NMR relaxation is observed (11) and that the effects of paramagnetic impurities, which troubled previous studies (4, 6, 8, 10), are completely suppressed. The... 

From the comparison of the measured and calculated temperature dependences of the relaxation time (see Fig. 20), it follows that the inelastic phonon scattering is the most essential mechanism of the spin-lattice relaxation for Ge. It is evident that only at low temperatures T < 30K) some other mechanisms (the most probable one is the relaxation due to a small amount of paramagnetic impurities) become dominant. At T > 300/C some additional mechanism of relaxation may also exist. The interaction of the nuclear quadrupole moment with vibrations of the nearest four Ge atoms brings about the main contribution to the spin-lattice relaxation rate. The effective modulation of the EFG by the nearest bond charges is greatly reduced because of strong correlations between their displacements. As the main result of the present investigation of spin-lattice relaxation,... 

In 1956, Selwood published the results of studies by the spin-echo method, in which the response of the nuclear magnetization to a series of pulses is measured, for methanol, ethanol, water, and n-hexane adsorbed on y-a l UBiina, silica-alumina, MnO, and CuO, all presumably diamagnetic materials ( ). He found for the proton relaxation time marked reductions over those in the liquid phase, but pointed out that many uncertainties remained, such as the effects of traces of paramagnetic impurities and of the shapes of pores in which the adsorbate molecules were located ... 

Probably the best approach makes use of gated decoupling in order to suppress the NOE effect, long relaxation times, and an effective relaxation reagent with a nonspecific effect for all the nitrogens present. Even traces of paramagnetic impurities in sample, which act preferentially on certain molecular structures, can make quantitative analysis impossible. 

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). 

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 ... 

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