Effect of Bulk Magnetic Susceptibility

There are two variables to consider, K and a (the shape factor). If the material being studied and the reference material are molecularly dispersed in a single sample tube (i.e., an internal reference), then there is only one value of k, and both sample and reference molecules experience the same magnetic field. Hence, the effect of magnetic susceptibility can be ignored. 

For an external reference, however, the sample and reference are physically separated, and Eq. 4.10 applies to each bulk component  

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Kubo A, Spaniol TP, Terao T (1998) The effect of bulk magnetic susceptibility mi solid state NMR spectra of paramagnetic compounds. J Magn Resmi 133 330-340... 

Table 66 Bulk Magnetic Susceptibilities and Effective Magnetic Moments of Agn Complexes at —298 K...

The T2 relaxation times of 50 /rsec and 40 msec given in the preceding discussion correspond to line half-widths of 6.4 kHz and 8 Hz, respectively. Whipple et al. (265) concluded that the line widths of several hundred Hz which are obtained in practice must be due to bulk magnetic susceptibility effects. This type of line broadening is removable by MAS (273) and they were the first to obtain high resolution spectra with linewidths similar to those expected from the T2 values. 

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. 

Solvent effects on nuclear magnetic properties and on the related spectroscopies are well known. In general, two main different effects on NMR spectra can be distinguished (a) shifts due to a difference in the bulk magnetic susceptibility x of tk solute and the solvent (b) shifts arising from intermolecular interactions between solute and solvent molecules. Since the bulk susceptibility effect depends on the shape of the sample and, therefore some form of correction for it is usually applied. Of greater importance is the second component related to solute-solvent interactions here, in particular, we shall describe some of the QM models developed so far within the PCM solvation methods to describe such effect. 

Cams is the contribution of anisotropy of a magnetic susceptibility tensor for graphitized carbon atoms in the bulk of the material a is the extent of effective graphitization of the material... 

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