Shift scales

In an applied magnetic field B0 the sample and reference materials, with shield-ings crs and crR, respectively, have resonance frequencies vs and %. It is conventional (and in accord with IUPAC recommendations) for the chemical shift to be designated by the symbol 8 and for the scale to increase to higher frequency from the reference. Thus 8 is defined by 

During the early days of NMR applications to chemistry, it was far easier instrumentally to hold the applied rf constant in frequency at v0 and to obtain the spectrum by sweeping the magnetic field through the various resonance lines sequentially. The resonances of sample and reference are then found at the values of the applied magnetic field given by 

To obtain consistency with the definition in Eq. 4.5, 8 for field sweep is defined by 

Some older literature (especially for nuclides other than hydrogen) uses a definition of 8 in which vs and nR are reversed, so that the 8 scale is reversed in sign. Also, some early NMR chemical shifts are reported on the r scale in which r = 10 — 8. These arcane points are mentioned here only to clarify statements in older literature we shall adhere to the definition and conditions of Eq. 4.5. 

The 8 scale is clearly dimensionless, but with the factor of 106, 8 has the units of parts per million (ppm) and a value should be stated, for example, as 8 = 4.13 ppm. Column headings in tables may be expressed as 8/ppm. In some instances when small differences in chemical shifts are being discussed, it may be more convenient to use a multiplicative factor of 109, with the units of parts per billion (ppb). 

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Similar experiments exist to correlate the resonances of different types of nucleus, e.g. C with H, provided that some suitable couplings are present, such as It is necessary to apply pulses at both the relevant frequencies and it is also desirable to be able to detect either nucleus, to resolve different peak clusters. Detection tlirough the nucleus with the higher frequency is usually called reverse-mode detection and generally gives better sensitivity. The spectrum will have the two different chemical shift scales along its axes... 

HH COSY Correlation via HH coupling which has square symmetry because of equal shift scales in both dimensions (6// versus 8f) provides all detectable ////coimectivities of the sample... 

FIGURE 4.16 Proton NMR spectra of several amino acids. Zero on the chemical shift scale is defined by the resonance of tetramethylsilane (TMS). (Adaptedfrom Atelrkh Library of NMR Spectra. ... 

Fig. 6. The P-NMR Chemical shift scale for pyridyl phosphane based systems...

Core electrons are highly relativistic and DFT methods may show systematic errors in calculating the charge density at the nucleus because of the inherent approximations. Fortunately, this does not hamper practical calculations of isomer shifts of unknown compounds, because only differences of li//(o)P are involved. In practice, the reliability of the results depends more on the number of compounds used for calibration and how wide the spread of their isomer shift values was. The isomer shift scale for several Mossbauer isotopes has been calibrated by this approach, among which are Au [1], Sn [4], and Fe [5-9]. For details on practical calculation of Mossbauer isomer shifts, see Chap. 5. 

The zero-point on the chemical shift scale is the 8-value for CFC13. 

The successful application of 170 NMR of zeolite acid sites has been discussed in Sect. 2.1.4 in the context of the 170 chemical shift scale. 

Solid-state 13C NMR spectra were obtained on a General Electric Omega PSG 400 spectrometer operating at 100.6 MHz for l3C nuclei using a Doty Scientific 5 mm high speed MAS probe. The various NMR experiments performed in this study are shown schematically in Figure 1. The carbon chemical shift scale (11) and 90 degree... 

Figure 3.18 Chemical shift scales and chemical shifts of some compounds. (Adapted with permission of Nelson Thornes Ltd. from Figure 2.12 of Akitt, J. W. NMR and Chemistry, 3rd ed 1992.)...

PMR spectrum of />ethoxyacetanilide in CDC13. Spectrum obtained at a magnetic field of 1.4092 tesla and a radiofrequency of 60 MHz. The chemical shift scale is the delta scale relative to the methyl... 

Fig. 5.—13C-N.m.r. Spectra of Bovine Nasal-Cartilage Recorded at 4° (A) and 56° (B). [Solvent, DjO. The chemical-shift scale is based on external CS2 (8C = 0).]...

FIGURE 5. Proton NMR spectra of solutions prepared from (S)-a-phenylethylamine [(S)-22] (10 pL) (upper spectrum) and a mixture of (R)- and (S)-a-phenylethylamine (R)- and (S)-22] (7 and 5 J-L, respectively) (lower spectrum) in 0.3 mL of a carbon tetrachloride solution of tris[3-(ferf-butylhydroxymethylene)-(i-camphorato]europium(III) (96). The chemical shift scale applies only to the lower spectrum. Reprinted with permission from Reference 82. Copyright (1970) American Chemical Society... 

Perlow and Ruby (31) and Ramma Reddy, Barros, and De Benedetti (2, 32) have compared the isomer shifts of a set of compounds with the isomer shifts from an identical set of compounds. This procedure allowed removal of the calculated values of the electronic densities of the compounds. The two isomer shift scales were linearly related by a constant factor, and thus they did not observe any noticeable effect of the valence state of the electronic cloud on the nuclear matter. Ramma Reddy, Barros, and De Benedetti (32) obtained ... 

Calibration of the Isomeric Shift Scale with Isomeric and NMR Chemical Shift Data. The alkali iodide isomeric shifts (5) relative to ZnTe are displayed in Table I and in Figure 3. 

Other Iodine Compounds. By studying the isomer shift and the quad-rupole hfs for a series of compounds, Perlow and Perlow (29) have calibrated the isomer shift scale which is independent of Equation 7. The number of p holes, hp, for the compounds of I2, KICI4 H2O, KIC12 H2O, and ICl has been inferred from the measured value of the quadrupole coupling constant, eqQ, and from the symmetry of the iodine lattice site. Since they found that the isomer shifts and hp are linearly related for these compounds, it appears that 5 electrons do not participate directly in the bonding, and the sp hybridization is negligible. Because of this linearity, they have been able to determine dh/dhp, which leads to a calibration of the isomer shift scale. 

Fig. 5 a. Spin-polarized spectra of 1 photolysis of diphenyldiazomethane in toluene. The chemical shift scale is in hertz downfield from the toluene methyl resonance 86)... 

We assumed that the receptor protons can be subdivided into two classes, PI (and PI ) and P2 (and P2 ), where P2 and P2 stand for protons that experience rf irradiation directly (fast exchange on the chemical shift scale is assumed). The primes indicate protons within the complex. PI and PI stand for all the remaining protons in the receptor in its free and bound states, re-... 

State-independent contributions to the Lamb shift scale as 1/n and vanish in the difference Z = n Pi(nP) — EL nS), which may be calculated more... 

Sequence 3 examines chain-end effects on DNA dynamics. It contains the coumarin probe one base pair removed from the end of the sequence. This chain end sequence has the same base pair sequence as the normal sequence, but with entire sequence, including the coumarin, shifted towards one terminus. The Stokes shifts from the two sequences are displayed on a relative Stokes shift scale in Fig. 2b. 

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