Bulk magnetic susceptibility shift

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. 

Fischer has proposed useful and important methods for factoring the isotropic shifts of uranocenes into contact and pseudocontact components (15) values were reported for uranocene, 1,-1, 3,3, 5,5, 7,7 -octamethyluranocene, and 1 1 -bis(trimethyl-si lyl) uranocene using a non-zero value of Xj Fischer arrived at values of yjj2 and y 2 at several temperatures from the ratio of the geometry factor and the isotropic shift for methyl protons in bis(trimethylsilyl)-uranocene, and bulk magnetic susceptibility data, assuming no contact contributions to the isotropic shift of the methyl protons. From the published data of Fischer, the value of y( - y2 at 30°C is 8.78 BM2. 

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. 

The hypothesis (1-3) that all native celluloses are a composite of two crystalline allomorphs, designated and Ig, has been further tested using C solid state NMR. In particular, two alternate origins of sharp resonance features were considered in addition to the usual origin, the crystalline unit cell. The first source is ordered layers on crystal surfaces the second is possible anistropic bulk magnetic susceptibility (ABMS) shifts associated with well defined fibril patterns (tertiary morphology). 

Constant-time methods are also used in liquid state imaging and in NMR microscopy to avoid distortions from chemical shifts and local variations in the bulk magnetic susceptibility. 

The differences (d) of chemical shifts of PEO with and without the potassium iodide are plotted as a function of the chain length (n) in Figure 2. The concentration of PEO and potassium iodide were kept constant for all measurements, and the bulk magnetic susceptibility correction (see later) was also applied. Again, a sharp change is observed atn = 7. 

Fig. 2. Plots of the difference of chemical shifts (with bulk magnetic susceptibility correction) of PEO (1 %) in methanol with and without the potassium iodide (0.05 M) as a function of the chain...

Measurements of chemical shift were made on 1% cyclohexane, which was mixed with methanol and used as a solvent for preparing the previous samples. This was done because no association between cyclohexane and potassium iodide takes place in the present systems. The results are shown in Figure 4. No detectable difference was observed for A values of the PEO obtained in either pure methanol or methanol containing 1% cyclohexane. Therefore, it is reasonable to use the A values of the cyclohexane in various samples to correct the bulk magnetic susceptibilities. 

Whenever matter is placed in a magnetic field, electromagnetic interactions alter magnetic field lines, which are stretched or accumulated. The magnetic susceptibility of the bulk medium influences the individual magnetic resonance frequencies so that there is an extra frequency shift called the BMS shift. 

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