Low-field end

Single vinylic fluorine substituents absorb over quite a wide range of chemical shifts, with fluoroallene at the high field end (-169 ppm) and P-fluoroacrylate derivatives at the low field end (-75 ppm) (Scheme 3.36). 

The ring current effect is the main reason that protons attached to aromatic rings typically appear at the low field end of the NMR spectrum since they are in the deshielded zone of the aromatic ring. 

Figure 16-17 Electron paramagnetic resonance spectrum of the Fe-S protein putidaredoxin in the natural form (32S) and with labile sulfur replaced by selenium isotopes. Well-developed shoulders are seen in the low-field end of the spectrum of the 77Se (spin = l/2)-containing protein. From Orme-Johnson et al.29S Courtesy of W. H. Orme-Johnson.

The shifts of the protons of alkanes and cycloalkanes fall in the range 0.9-1.5 ppm with C—H protons coming at the low-field end of this range and —CH3 protons coming at the high-field end (see Table 9-4). 

The following table lists the 15N chemical shifts (in ppm) for common standards. The estimated precision is better than 0.1 ppm. Nitromethane, according to Levy and Lichter,1 is the most suitable primary measurement reference, but has the disadvantage of lying in the low-field end of the spectrum. Thus, ammonia (which lies in the most upfield region) is the most suitable for routine experimental use.1-6... 

Each alcohol has a saturated carbon atom next to oxygen, all close together. Then there are carbons next door but one to oxygen they are back in the 0-50 p.p.m. region but at its low field end— about 30-35 p.p.m.. Notice the similarity of these chemical shifts to those of carbons next to a carbonyl group (Table 3.5 on p. 63). In each case we have C-C-O and the effects are about the same. Two of the alcohols have carbon(s) one further away still at yet smaller chemical shift (further upfield, more shielded) at about 20 p.p.m., but only the n-butanol has a more remote carbon still at 15.2. The number and the chemical shift of the signals identify the molecules very clearly. 

This is demonstrated in Fig. 1 which illustrates the proton spectrum of l,4-bis(2-oxopropyloxy)-benzene. Spectra are conventionally scanned from low field to high field with the low field end on tiie left. The aromatic protons are the least shielded followed by the methylene protons and then the methyl protons which are the most shielded. 

In high resolution we find that the peaks are often split multiplets (Fig. 24.18). For example, in the acetaldehyde case, the possible spin quantum numbers of the three methyl protons are, , —j, — these spin states have statistical weights of 1, 3, 3, 1. These four spin states exert four different effects on the remaining proton in the —CHO group. Consequently, in high resolution, four lines appear at the low-field end of the spectrum. The possible spin orientation of the aldehydic proton is and — This proton exerts two different effects on the methyl protons, resulting in the split of the methyl proton resonance into two lines. Even after splitting into a number of lines, the total area ratio remains at 1 3 between the aldehydic proton peaks and the methyl proton peaks. 

Another trend is that sp -hybridized carbons generally absorb from 0 to 90 8, while sp carbons absorb from 110 to 220 8. Carbonyl carbons (C=0) are particularly distinct in NMR and are always found at the low-field end of the spectrum, from 160 to 220 8. Figure 11.8 shows the NMR spectra of butan-2-one and p-bromoacetopbenone and indicates the peak assignments. Note that the C=0 carbons are at the left edge of the spectrum in each case. 

Figure 4 75.4 MHz proton-decoupled spectra of 30% menthol in deuteriochloroform, (A) recorded with all signals within the spectral window, and (B)-(D) with the transmitter displaced to high field in 500 Hz steps. Spectra (C) and (D) show the aliasing of the high-field signals to reappear at the low-field end of the spectrum.

At 500 MHz, moderate-sized (more than six residues) oligosaccharides lie within the spin-diffusion limit. However, for smaller molecules, as the value of the function (OqT (where (Oq is the Larmor frequency of protons, and is the correlation time of the molecule) approaches 1 then the value of the NOE tends towards 0. Cross-peak intensities of NOESY spectra of smaller oligosaccharides (2-5 residues) may thus become too small to measure accurately. In such cases, the rotating frame Overhauser effect spectroscopy (ROESY, originally referred to as CAMELSPIN) experiment is commonly used to measure NOE values. To reduce the appearance of HOHAHA-like cross-peaks, a low power spin-lock field should be used, and the transmitter carrier offset to the low-field end of spectrum. The offset dependency of cross-peak intensities should also be removed by 90° pulses at either end of spin-lock period. 

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