The Chemical Shift Scale

As Example 5.3 shows, signals downfield from the reference signal have positive chemical shifts, and those upfield of the reference signal have negative chemical shifts. In the older literature there was some confusion about whether the upfield or downfield signals should carry positive chemical shifts, but the above convention has since been universally adopted. 

At one time an alternate H chemical shift scale called the tau (t) scale was used, defined by the relationship 

Although the x scale is not used much any more, you may occasionally run across it in older literature. So beware  

As Example 5.3 shows, signals downfield from the reference signal have positive chemical shifts, and those upfield of 

If you are wondering why there are more lines for the ring hydrogen signals than the three we expected on the basis of symmetry, you will find out in Chapter 8  

The nuclear Overhauser effect is the enhancement of intensity of an NMR signal generated by one nucleus when it is near another nonequivalent nucleus being simultaneously irradiated. We will discuss this effect in more detail later 

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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. ... 

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

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. 

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).

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... 

This is a simple procedure whereby a reference peak (e.g., TMS in organic solvents) is selected with a cursor (Bruker uses a triangle or vertical arrow, Varian a vertical red line) and given a specific chemical-shift value. Without this reference, the chemical-shift scale... 

Another new tool in our arsenal comes from the technology of NMR imaging (MRI). In MRI, there is only one chemical shift (that of H2O) and the chemical shift scale is used... 

Fig. 36.—Structural-reporter-group Regions of the Resolution-enhanced, 500-MHz, H-N.m.r. Spectrum of a Mixture Containing Compounds 49,50, and 51 in the Ratios of 12 2 3. [The bold numbers in the spectrum refer to the corresponding residues in the structures, and die italic numbers to the compounds in the mixture. Signals of corresponding protons in the various components of this mixture coincide, unless otherwise indicated. The relative-intensity scale, as well as the chemical-shift scale, of the N-acetyl-proton region and the Fuc CH3-proton region (b), differ from those of the other parts (a) of the spectrum, as indicated. The HOD resonance, as well as the H-l signal of Man-3, have been omitted from the spectrum.]...

We have described these protons as A and X with a purpose in mind. A spectrum of two equal doublets is called an AX spectrum. A is always the proton you are discussing and X is a proton with a very different chemical shift. The alphabet is used as a ruler nearby protons (on the chemical shift scale—not necessarily nearby in the structure ) are called B, C, etc. and distant ones are called X, Y, etc. You will see the reason for this soon. 

The separation of resonance frequencies resulting from the different electronic environments of the nucleus of the isotope is called the chemical shift. It is expressed in dimensionless terms, as parts per million (ppm), against an internal standard, usually retramethyLvilane (TMS). By convention, the chemical shift is positive if the sample nucleus is less shielded (lower electron density in the surrounding bonds) than the nucleus in the reference and negative if it is more shielded (greater electron density in the surrounding bonds). The chemical shift scale ((5) for a nucleus is defined as ... 

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