Low magnetic fields

Fig. 7. Field dependence of the moment of earbon nanoiubes at the temperatures shown at low magnetic fields (after Here-mans el al.[26]).

In low magnetic fields (less than 100 G) the effect of Ag is small and therefore transition probabilities between the a and nuclear spin... 

In Figure 3.3.2, the strong dependence of the 3H relaxation time of n-heptane on coke content was shown for low magnetic field strengths although less pronounced, this 7 dependence still holds for high fields [2]. For large catalyst pellets... 

Figure 4.14 View of the structure of Dy(hfac)3(NIT—C6H4—0—Ph) chain developing along the b crystallographic axis. The two possible alignments with antiparallel favoured at low temperature and low magnetic field (a), and parallel favoured at high temperature and strong magnetic field (b), of...

It has been postulated [39] and demonstrated previously that Hetero-PHIP for nuclei such as 13C, 29Si, and 31P can result in a signal enhancement (SE) > 10 [8, 40, 41], particularly if the reactions are carried out at low magnetic fields (i.e., under ALTADENA conditions). 31P INEPT(+n/4) experiments have also been carried out to transfer the initial proton polarization to 31P [8]. 

It is worthwhile pointing out that it is desirable to acquire all spectra under identical conditions, such as hydrogen pressure, elapsed time prior to the acquisition of the spectrum, temperature, and amount of catalyst used. For this reason it is desirable to use a set-up which permits the spectra to be recorded in totally standardized fashion, which does not depend on any individual human factor . Such a system would allow the reaction to be conducted at a low magnetic field and would thereafter transfer the solution automatically (notably quickly and adiabatically ) into the NMR spectrometer for subsequent analysis. 

Likewise, the initial proton polarization may be transferred to other magnetically active heteronuclei, most attractively to those associated with a low y-value of their nucleus (i.e., to 15N, 29Si), and similarly difficult ones, using heteroatom PHIP at low magnetic fields [8, 45]. 

In yet another recent investigation, Kruk and Kowalewski considered the case when the static ZFS was smaller than the transient ZFS and the latter term should be considered as the unperturbed Hamiltonian at low magnetic fields (125). The validity conditions for the theory derived in that case were rather difficult to realize in experimentally relevant situations. The aqueous solution of Ni(II), a difficult case treated previously by the slow-motion theory (92,93), was however found to be possible to describe in a reasonable way using the new approach (125). 

Models for the outer-sphere PRE, allowing for faster rotational motion, have been developed, in analogy with the inner sphere approaches discussed in the Section V.C. The outer-sphere counterpart of the work by Kruk et al. 123) was discussed in the same paper. In the limit of very low magnetic field, the expressions for the outer-sphere PRE for slowly rotating systems 96,144) were found to remain valid for an arbitrary rotational correlation time Tr. New, closed-form expressions were developed for outer-sphere relaxation in the high-field limit. The Redfield description of the electron spin relaxation in terms of spectral densities incorporated into that approach, was valid as long as the conditions A t j 1 and 1 were fulfilled. The validity... 

A first example is represented by the Mn(III)/Mn(II) redox switch. The complexes of Mn(II) and Mn(III) with the water-soluble tetraphenylsulpho-nate porphyrin (TPPS, Chart 13) display significantly different ri values at low magnetic field strength (lower than 1 MHz), but very similar values at the fields currently used in the clinical practice (> 10 MHz) (141). However, the longer electronic relaxation rates of the Mn(II) complex makes its relaxivity dependent on the rotational mobility of the chelate. In fact, upon interacting with a poly-p-cyclodextrin, a 4-fold enhancement of the relaxivity of [Mn(H)-TPPS(H20)2] at 20 MHz has been detected, whereas little effect has been observed for the Mn(III)-complex. The ability of the Mn(II)/Mn(III)... 

In order to better illustrate these points, consider, for example, a magnet such as the one described in Section III.D. It reaches the maximum field of 1.143 T, corresponding to 48.7 MHz of Larmor frequency, with a current of 400 A. It follows that for a very low magnetic field corresponding to, let us say, 1 kHz, one has to set a current of just 8 mA. In order to do that, one should be able to control the current with a precision and resolution of about 20 ppm of the maximum value. The required absolute precision is therefore of the same order of magnitude as the current offsets and thermal drifts of even the best analog electronic components. 

This gap was filled by the development of NMR spectroscopy. It is Bovey s merit to have demonstrated the potentiality of this technique in the macromo-lecular field and, in particular, its sensitivity to the stereochemical environment. The applications of this technique, the methods of investigation, and the type of problem that can be solved are closely connected with the progress made in NMR instmmentation in the last quarter century. From low magnetic field spec-... 

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