Chemical shift scaling

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... 

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...

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... 

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-... 

In general, multiple pulse techniques sufficiently average the dipolar interactions, compress the chemical shift scale, but they do not affect heteronuclear dipolar interactions and the chemical shift anisotropy. A combination of both multiple pulse techniques and magic angle spinning, so-called CRAMPS (Combined Rotational And Multiple Pulse Spectroscopy) is found to yield satisfactory results in the solid state H NMR of solids 186). The limitations of all these techniques, from the analytical point of view, arises from the relatively small chemical shift range (about 10 ppm) as compared with some other frequently studied nuclei. However, high resolution H NMR of solids is useful in studies of molecular dynamics. 

FIGURE 23. Contour plots of the 2D H-29Si correlation experiments on silica gel obtained with a 22.0 ms contact time, a 3.0 s repetition time and a 4.0 kHz sample spinning rate. The vertical axis represents the proton chemical-shift scale and the horizontal axis the 29Si chemical-shift scale. The spectra above and at the side of the figures are the one-dimensional projections. The 2D spectrum was obtained from 64 individual experiments (a) unwashed silica gel, 80 scans for each individual experiment (b) D2O washed sample, 200 scans for each experiment. Reprinted with permission from Reference 137. Copyright 1988 American Chemical Society... 

Figure 4 Dependence of the 15N principal values of the chemical shift in pyridine with the N-H distance for the pyridine-methanol complex. Note the different chemical shift scales for each shift component, SUj <%2, 3 and < so. The N-H distances are in A and the horizontal scales in the figure are the same for all the shift components.

Fig. 3. The chemical shift of the trans proton vs. the mole ratio (CH3)/[(CH3) + (C2H3)] at various temperatures A, — 50° B, —30° C, —4° andD, +36°C. Each curve is offset by 2 cps (one unit on the graph) on the chemical shift scale (in cps, 60 MHz).

Fig. 20. 300-MHz H NMR spectrum and H COSY spectrum of isolated a chains of HbCO A in 0.1 Af phosphate in D20 at pH 5.3 and 29°C. The H chemical shift scale is referred to DSS. [From Dalvit and Ho (1985)].

Of course, these relationships do not take into account the variations in SO4 chemical shift due to the effects of counterions, pH, concentration and temperature. It has not been possible to adopt the IUPAC chemical shift scale exactly, because that would require measuring the chemical shifts of all secondary references reported in the literature with respect to the standard suggested by IUPAC. 

There are many analogies in the structural influences on S, P and Se NMR chemical shifts. Wasylishen et al.36 have compared the relative 33S and 77Se chemical shift scales and found that the correspondence is noticeable. For example, the resonances of the hydrides H2S and PH3 lie at the extreme upheld end of the total spectral range. Substitution of a hydrogen atom with a methyl group provokes downfield shifts of 44.7 and 64.5 ppm, respectively. Further substitution of a second hydrogen atom with a methyl causes downfield shifts by 31 and 64.9 ppm, respectively. 

Two-Dimensional Experiments A full NMR analysis of a carbohydrate, in which each lH and 13C peak in the spectrum is assigned to a particular position in the molecule, requires the use of two-dimensional (2D) NMR. In a 2D spectrum, there are two chemical shift scales (horizontal and vertical) and a spot appears in the graph at the intersection of two chemical shifts when two nuclei ( H or 13C) in the molecule are close to each other in the structure. For example, one type of 2D spectrum called an HSQC spectrum presents the chemical shift scale on the horizontal (v) axis and the 13 C chemical shift scale on the vertical (y) axis. If proton Ha is directly bonded to carbon Ca, there will be a spot at the intersection of the H chemical shift of Ha (horizontal axis) and the 13 C chemical shift of Ca (vertical axis). Because the peaks are spread out into two dimensions, the chances of overlap of peaks are much less and we can count up the number of anomeric and... 

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