Chemical shift magnetic susceptibility, effect

Magnetic effects result from aromatic diatropic and paratropic antiaromatic ring currents and are reflected in H NMR chemical shifts, magnetic susceptibility exalation, and nucleus-independent chemical... 

Only the anisotropy of the magnetic susceptibility influences the isotropic mean value, whereas the complete susceptibility tensor contributes to the chemical shift anisotropy [see Eq. (36)]. The influence of the susceptibility of a spherical symmetric charge distribution on the isotropic chemical shift of a nearby nucleus will be zero, but there is a contribution to the chemical shift tensor. Especially in solid state proton chemical shift investigations this effect is quite remarkable and can be observed when studying proton chemical shift anisotropies. 

Fyfe et al. [61] studied p-xylene and y-butyrolactone in hectorite in 1981 using H and C NMR with no MAS. The relatively good resolution of these spectra indicates a high degree of mobility of the adsorbates in all cases. The C chemical shifts are nearly identical to those in solution spectra of intercalants, suggesting that magnetic susceptibility effects are not large, at least in this clay. 

There came back an enthusiastic report of a substantial chemical shift difference between the adsorbate and the material in the liquid phase, to be followed shortly by another communication which stated that the difference disappeared when a magnetic susceptibility correction was applied to the results. This illustrates a problem which arises whenever one attempts to measure a chemical shift value in a heterogeneous system, the problem of a suitable reference and of eliminating magnetic susceptibility effects which are particularly troublesome in such systems of dubious geometry. Differences between chemical shifts in the same molecule, if they can be resolved, are... 

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. 

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). 

Solvent effects on nuclear magnetic resonance (NMR) spectra have been studied extensively, and they are described mainly in terms of the observed chemical shifts, 8, corrected for the solvent bulk magnetic susceptibility (Table 3.5). The shifts depend on the nucleus studied and the compound of which it is a constituent, and some nuclei/compounds show particularly large shifts. These can then be employed as probes for certain properties of the solvents. Examples are the chemical shifts of 31P in triethylphosphine oxide, the 13C shifts in the 2-or 3-positions, relative to the 4-position in pyridine N-oxide, and the 13C shifts in N-dimethyl or N-diethyl-benzamide, for the carbonyl carbon relative to those in positions 2 (or 6), 3 (or 5) and 4 in the aromatic ring (Chapter 4) (Marcus 1993). These shifts are particularly sensitive to the hydrogen bond donation abilities a (Lewis acidity) of the solvents. In all cases there is, again, a trade off between non-specific dipole-dipole and dipole-induced dipole effects and those ascribable to specific electron pair donation of the solvent to the solute or vice versa to form solvates. 

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