Chemical shift magnetic anisotropy, effect

Pi-complexing is most commonly used to rationalize effects observed in aromatic solvents. The most frequent evidence cited is magnetic anisotropy effects on chemical shifts in the solute molecule. As was the case for hydrogen bonding no quantitative correlations with substantive parameters such as ultraviolet spectral shifts have been attempted. 

A number of workers have reported the NMR spectra of mono-substituted phenyl groups and correlated the shifts vdth molecular parameters. It has been shown for organic compounds that the chemical shift of the para-carbon in monosubstituted benzenes is linearly related to the total TT-electron density at the para position in these compounds. Also the shift separation of the meta- and para-carbons appears to be linearly related to the rr-electron density on the para-carbon due to resonance interaction with the substituent 49, 156). Spiesecke and Schneider have reported a good linear relationship between the para-carbon chemical shift of monosubstituted benzenes and the Hammett, o-para constant, but no such relationship appears to exist for the other carbon chemical shifts, except between the chemical shift for the substituted carbon atom (corrected for magnetic anisotropy effects of the substituent) and the electronegativity of the substituent 210). 

The magnetic anisotropy effect (5J) is related to the diange of the chemical shift, e.g., of a peptide proton as induced by the magnetic anisotropy of the adjacent group. Usually a proton in the plane of the peptide group shows a downfield shift, and a proton above the plane shows an upfield shift. On this basis, the peptide proton of the (i+3)-th residue of the /3-turn (Fig. 2) is placed above the plane of the peptide group at the corner, and thus shows an upfield shift (27). Or vice versa, the presence of a peptide proton signal at upfield could be used as a proof for the / tum structure. 

Since polarization effects vanish with 1/R whereas the McConnell equation (2) contains l/R, it is obvious that polarisation effects can be observed when magnetic anisotropy effects are far beyond 0.1 ppm, hence polarizations should not be neglected when using the McConnell equation for isotropic chemical shifts. 

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. 

While MAS and CP-MAS are often sufficient to resolve chemical sites for nuclei like C, P and Si, this is not the case for other nuclei such as ZAl, and B. The reason is that the first group of nuclei have spin while the second have higher spin. Nuclei with spin greater than are subject to electric quadrupole effects, in addition to chemical shift, and these effects present a significant additional source of line-broadening. Because the quadrupole anisotropy in the presence of a strong magnetic field does not transform simply like a 2nd rank tensor, it cannot be... 

French workers have studied the 1H- and 13C-NMR parameters of disubstituted selenophenes.37 38 The proton chemical shifts are discussed in terms of magnetic anisotropy and electric field effects of the substituents in order to study the conformational equilibrium of the carbonyl group. The relationship between the H- and 13C-chemical shifts and 7t-electron distribution calculated by the PPP method are examined. Shifts and coupling constants are discussed in additivity terms. 

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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