Magnetic field dependencies

The first step for any structure elucidation is the assignment of the frequencies (chemical shifts) of the protons and other NMR-active nuclei ( C, N). Although the frequencies of the nuclei in the magnetic field depend on the local electronic environment produced by the three-dimensional structure, a direct correlation to structure is very complicated. The application of chemical shift in structure calculation has been limited to final structure refinements, using empirical relations [14,15] for proton and chemical shifts and ab initio calculation for chemical shifts of certain residues [16]. 

Fig. 6. The magnetic field dependence of the high- and low-temperature MR, respeetively the solid lines are caleulated. The inset shows a sehematic of the eontact eonfiguration for the transport measurements.

Song et al. [16] reported results relative to a four-point resistivity measurement on a large bundle of carbon nanotubes (60 um diameter and 350 tm in length between the two potential contacts). They explained their resistivity, magnetoresistance, and Hall effect results in terms of a conductor that could be modeled as a semimetal. Figures 4 (a) and (b) show the magnetic field dependence they observed on the high- and low-temperature MR, respectively. 

Fig. 4. Magnetic-field dependence of the magnetoconductance of an MWCNT at different temperatures [10].

Measurements of a pyrite sample with a two mode resonator16 yielded the magnetic field dependence of microwave transmission (Fig. 43) from... 

Figure 15.10 (a) AFM image and schematic illustration of electrode modified with nanocluster of QqN and MePH. (b) Magnetic field dependence on Q-values. 

Tjandra, N., Bax, A. Measurement of dipolar contributions to /ch splittings from magnetic-field dependence of J modulation in two-dimensional NMR spectra./. Magn. Reson. 1997, 124, 512-515. 

This model also explains the unexpected magnetic field dependence of the relaxation time in a temperature range in which Raman processes are normally expected to be predominant [32]. 

Taraban, M.B., Leshina, T.V., Anderson, M.A., and Grissom, C.B., Magnetic field dependence of electron transfer and the role of electron spin in heme enzymes horseradish peroxidase, J. Am. Chem. Soc.,... 

The story is even more complicated than we have suggested, because carbon can relax by more than one mechanism. Protons rely on dipole-dipole relaxation, which also works well for protonated carbons but badly for non-proton-ated carbons. But carbon also for example makes use of spin-rotation relaxation, which is particularly active for methyl groups. And the magnetic field dependence of the various mechanisms also differs. We realize that relaxation is a very difficult subject, and if you want to know more then there are plenty of textbooks available ... 

We wish also to mention the discovery of an unusual magnetic field dependence of specific heat in some multicomponent glasses in the 0.3 tK range [46]. A theoretical explanation of the phenomenon can be found in ref. [[47] pp. 17-46]. 

Fig. 9. The magnetic field dependence of the /xSR frequencies in Si with the field aligned along the (100) direction. The solid (dashed) curves are predicted if none (one) of the nearest-neighbor nuclei on the symmetry axis is 29Si. From Kiel et al. (1988b). 

Figure 7.11. The magnetic field dependencies of transport Jc at 77 K (H // c) for a two-layer O.S-pm ED-TBSBCCO/Ag/ED-TBSBCCO/Ag/LAO and a single-layer 2.6-pm ED-TBSBCCO/Ag/LAO film. [Reproduced with permission from Ref. 106. Copyright 2000 Elsevier.]...

Figure 12.2 Magnetic field dependence of the energy levels of ortho- and para-H2. Parahydrogen (p-H2) is a singlet that is unaffected by the magnetic field, whereas orthohydrogen (o-H2) is a triplet. Its energy levels split, showing the Zeeman effect.

Soule D. Magnetic field dependence of the hall effect and magnetoresistance in graphite single crystals. Physical Review. 1958 112(3) 698-707. 

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