Chemical shift magnetic contributions

Equation (16) is very important because it imcovers two central ideas. First, the second-order frequency splitting depends inversely on the Larmor frequency, thus the importance of this term diminishes with increasing external magnetic field strength. Second, the k=0 term has no orientation dependence (Do 0 = 1) or, in other words, it is an isotropic term. This means that the isotropic shift observed in the NMR spectriun of a quadrupolar nucleus has, in addition to the usual isotropic chemical shift, a contribution from the quadrupole cou-phng, which is given by... 

In general, the longitudinal relaxation rate of a hydride resonance is dominated by dipolar and chemical shift anisotropy contributions, the former being by far the most important in determining the observed relaxation time.[14] Thus a first look to the T value may provide a useful tool for structural assignments, since it immediately reports on the distances with respect to other magnetically active... 

Considering the symmetry of the magnetic interactions (dipolar and chemical shift anisotropy) contributing to the spin Hamiltonian of the system for the probe,... 

The fourth chapter in this volume, contributed by Helmut Duddeck, is an exceptionally thorough survey of substituent effects on carbon-13 nuclear magnetic resonance (NMR) chemical shifts. Organic chemists and others who are routinely dependent on 13C NMR for structure elucidation and for information about stereochemistry will welcome the summary presented here. Although... 

As indicated in the previous discussion, Mossbauer spectroscopy provides information that when coupled with results using other structural techniques assists in determining the structure of the complex under analysis. The relationships between the various techniques are summarized in Table II. The Mossbauer chemical shift provides information about the 4 electron contribution to the bond between the metal and the ligands in a complex. Similar estimates can be obtained from the results of measurements on the fine structure in the x-ray absorption edge and nuclear magnetic resonance data. The number of unpaired electrons can be evaluated from magnetic susceptibility data, electron spin resonance, and the temperature coeflScient of the Mossbauer quadrupole splitting (Pr). 

All the same, the quantitative determination of the aromaticity and antiaromaticity from the ring current model may be complicated by at least two problems. First, experimentally observable values of magnetic susceptibilities and their exaltations and anisotropies as well as the H-NMR chemical shifts are not necessarily determined exclusively by ring currents hence, all other effects have to be identified and removed. Naturally, for this model to work, the contribution by the ring current must be predominant. Another problem is that the calculated results on ring current intensities for molecules from the diatropic-paratropic border area may vary qualitatively depending on the method of calculation (80PAC1541). 

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