Bulk susceptibility shift

Fig. 16. Single voxel STEAM spectra of the SOL muscle (top) and the TA muscle (bottom). Different fibre orientation in those muscles results in clearly different patterns of the lines in the spectra In SOL (feathered muscle with oblique fibres), IMCL and EMCL signals show lower frequency separation than in TA (spindle-shaped muscle) due to bulk susceptibility effects. Furthermore, in SOL the Cr2 doublet merges into one resonance, the Cr3 triplet is less resolved, and TAU is shifted towards TMA.

The reference compound can be added to the sample solution (internal reference) or kept separate from the sample in a sealed capillary (external reference, Fig. 2.36). If an external reference is necessary, a correction term accounting for the difference between the bulk susceptibilities of reference ( R) and sample solution ( s) must be added to the observed shift, dob5 ... 

Neither CS2 nor TMS are ideal standards. The 13C signals of CS2 and carbonyl carbons overlap, as do the 13C signals of cyclopropane and some methyl carbons with TMS (Fig. 3.3). Furthermore, the 13C resonance of TMS has been shown to suffer from solvent shifts of the order of + 0.1 to 1.5 ppm in common NMR solvents, even at infinite dilution [74]. This must be considered if 13C shifts relative to TMS of one compound in different solvents are to be compared. There are two alternative methods to overcome this problem one is to use cyclohexane as the internal reference cyclohexane was shown to have 13C solvent shifts lower than + 0.5 ppm [74], The other alternative is to use TMS as an external reference (Sections 1.9.3 and 2.8.5) and to make bulk susceptibility shift corrections according to eq. (1.44). 

In general, two different solvent effects on NMR spectra can be distinguished (a) shifts due to a difference in the bulk volume magnetic susceptibility x of the 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, is not of chemical interest, some form of correction for it is applied. For two... 

Only shifts observed in excess of this amount may then be attributed to intermolecular interaction effects. Use of an internal standard provides an automatic compensation for the bulk susceptibility effect, but for comparison of shifts measured in this way in different solvents it must be kept in mind that the standard itself may be subject to solvent effects. These are minimized in and NMR spectroscopy by the use of tetra-methylsilane (TMS) as an internal standard. 

For completeness, I mention the ° Pd Knight shift measured in micrometer-sized powders with 0 < x < 0.025 (64). The shift changes linearly with the bulk susceptibility as AKIAx = 41 mol emu In pure Pd, this value... 

The relatively small changes in chemical shift with pressure can be qualitatively explained in terms of changes in bulk susceptibility as most spectra in SCFs are obtained without internal lock. Large chemical shift ranges are a characteristic of metal NMR spectroscopy, and the changes observed on changing temperature, pressure or solvent are no more exaggerated in supercritical solvents than those observed with conventional solvents. 

Table 3 presents Mg line width and chemical shift data obtained as a function of the concentration of MgBr2 and MgCl2 [41]. Also included in Table 3 are the viscosities of the solutions investigated. The chemical shifts, when corrected for bulk susceptibility effects, vary over a total range of less than 1 ppm. 

Chemical shifts ( 0.1 ppm) relative to the signal for Mg(C104)2. extrapolated to infinite dilution. More positive values refer to lower shielding. Bulk susceptibility corrections have been made. 

Data from ref. 61 l4N spectra at 4-3345785 MHz ( 0-5 Hz), sample temperature 30 2°C concentric spherical sample containers are used in order to eliminate bulk susceptibility effects on shifts and signal shape external standards are used, CH3NO, (neat liquid) and, for signals within 15 ppm of that of neat CH,N02, tetranitromethane (neat liquid) reported shifts represent values obtained from iterative fitting of theoretical and experimental lineshapes using a differential saturation method, ref. 63, reported errors are standard deviations for the fitting of at least 200 data points, and represent 68% confidence limits for shifts which are recalculated from values referred to C(N02)4, the error of the shift of C(N02)4 relative to 0H,NO2 is included. 

In order to provide a means for the precise recalculation of nitrogen chemical shifts reported since 1972, it is necessary to have accurate values of the differences in the screening constants between neat CH3N02 and the large number of reference compounds which have so far been used. Table VII shows the results of precise, 4N measurements (61) which have been carried out in concentric spherical sample and reference containers in order to eliminate bulk susceptibility effects on the shifts. Since the technique adopted (61, 63) involves the accumulation of a large number of individually calibrated spectra with the subsequent use of a full-lineshape analysis by the differential saturation method, (63) the resulting random errors comprise those from minor temperature variations, phase drifts, frequency instability, sweep nonlinearity, etc. so that systematic errors should be insignificant as compared with random errors. 

Ref. 67, l5N natural-abundance spectra, concentric cylindrical sample tubes, no correction for bulk susceptibility effects referred to what is reported as ca. 10 m HisNO, from data reported for this standard sample relative to (CH3)4N Cle in subsequent papers (34, 36) its shift is ca. +6-1 ppm from neat CH3N02 this corresponds to a HN03 concentration of between 1 m and 2 m, as reckoned from data in Table VII errors quoted are 0-2 ppm, but comparison of results with those from ref. 36, suggests 1 ppm as more realistic limits. 

The first measurements of the dependence of proton chemical shifts 8 on temperature were performed using ethanol, and observing the shift of the OH proton with respect to the methyl and methylene protons. The first extensive study" of hydrogen-bonding mediated chemical shifts showed that, in the gas phase at very low pressures, the chemical shift measured at a particular temperature is essentially liquid phase, two factors alter the chemical shift from the value measured in the gas phase. The first factor is the change in bulk susceptibility caused by the change in density. The second factor is the association shift , which is the difference between the calculated and measured chemical shifts. The calculated value is given by ... 

In Table VII are the relative H chemical shifts of water in Nafion at several water contents. The experiments were conducted on specially prepared Nafion spheres in order to eliminate bulk susceptibility effects. These spheres behaved the same as the corresponding Nafion films and powders in limited 23Na NMR experiments. The H linewidths are sufficiently narrow to allow accurate measurement of the chemical shift. With decreasing water content, the resonance shifts upfield, suggesting the breakup of water-hydrogen bonding as for NaCl. The relative shift of pure water and water in saturated Nafion is not known at this time. The increased linewidth indicates decreased water mobility, as seen for the sodium ions. Additional experiments using model electrolytes and a chemical shift standard are warranted. 

Once convinced that the Li NMR technique gave a reasonable value for Kui> spectra of paramagnetic solutions of Liblred were obtained (Fig. 5b). Here, the chemical shift, 8obs, increases linearly with the concentration ofLijlred- The Sobs value is a linear combination of contributions from the bulk susceptibility (Sbulk> a concentration-dependent result of macroscopic interactions) and the contact shift. Sc, that arises from spin-density transfer from the paramagnetic center, V(IV), in Iredl... 

The bulk susceptibility arises from (6) and must be allowed for if an external reference is used. The effect of the bulk susceptibility on chemical shifts may in principle be calculated and may also be obtained experimentally by a simple technique. ... 

In 1960 Bothner-By observed that for several simple organic compounds, a change from the gaseous to the liquid state was accompanied by a shift to low field of the protons in excess of that calculated using the classical bulk susceptibility correction. It was found possible to calculate the observed shift jS/ of a solute proton i in a solvent j by use of the empirical relationship—... 

As discussed in sect. 3.1 the internal field has two components, the dipolar and the contact (hyperfine) field. In the polycrystalline average (and if the bulk susceptibility is isotropic), the dipolar contribution averages to zero. The Knight shift then allows a determination of the contact field magnitude. For more details on muon Knight shifts, the reader is referred to Schenck (1985) chapter 4.1, or Schenck (1999). 

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