Shift rang

Figrue BTl 1.1 shows the range of radiolfequencies where resonances may be expected, between 650 and 140 MHz, when Bq = 14.1 T, i.e. when the H resonance frequency is 600 MHz. There is one bar per stable isotope. Its width is the reported chemical shift range (Bl.11.5) for that isotope, and its height corresponds to the log of the sensitivity at the natural abundance of the isotope, covering about six orders of magnitude. The... 

Just as chemical shifts in H NMR are measured relative to the protons of tetramethylsi lane chemical shifts m NMR are measured relative to the carbons of tetramethylsilane Table 13 3 lists typical chemical shift ranges for some representative types of carbon atoms In general the factors that most affect chemical shifts are... 

Table 2.1. H chemical shift ranges for organic compounds...

The C chemical shift ranges for organic compounds in Table 2.2 show that many carbon-containing functional groups can be identified by the characteristic shift values in the C NMR spectra. 

Table 2.3. Chemical shift ranges for organonitrogen compounds...

Figure 2.7. H NMR spectrum of 3,4-dimethoxyben2aldehyde (6) [aromatic shift range, CDCI3, 25 °C,...

In the chemical shift range for alkenes and aromatic and heteroaromatic compounds enol ether fragments (furan, pyrone, isoflavone, 195-200 Hz) ... 

From Cg/ZgNO (problem 4), for example, the empirical formula C Hjo is derived and compared with the alkane formula Cc>H2o, a hydrogen deficit of ten and thus of five double-bond equivalents is deduced. If the NMR spectra have too few signals in the shift range appropriate for multiple bonds, then the double-bond equivalents indicate rings (see, for example, a-pinene. Fig. 2.4). 

Conditions CDCI3, 25 °C, 20 MHz. (a) //broadband decoupled spectrum (b) expanded sp shift range (c) expanded sp shift range (b) and (c) each with the H broadband decoupled spectrum below and NOE enhanced coupled spectrum above. 

Conditions CDCI3, 25 °C, 100 MHz ( C), 400 MHz H). (a-e) C NMR spectra (a,b) //broadband decoupled spectra (c,d) NOE enhanced coupled spectra (gated decoupling) with expansion (e) of the multiplets in the sp shift range (f) //NMR spectrum with expanded multiplets. 

Conditions CDCI3, 25°C, 200 MHz ( //), 50 MHz ( C). (a) NMR spectrum with expanded multiplets (b) NOE difference spectrum, irradiated at Sff = 1.87, (c) C NMR partial spectra, each with H broadband decoupled spectrum below and NOE enhanced coupled speetrum (gated decoupling) above (d) CH COSY diagram ( empty shift ranges omitted). 

Apart from the A-methyl group, three double-bond equivalents and three multiplets remain in the chemical shift range appropriate for electron rich heteroaromatics, Sh = 6.2 to 6.9. A-Methyl-pyrrole is such a compound. Since in the multiplets at Sh = 6.25 and 6.80 the Jhh coupling of 4.0 Hz is appropriate for pyrrole protons in the 3- and 4-positions, the pyrrole ring is deduced to be substituted in the 2-position. 

The //NMR spectrum contains five signals with integral levels in the ratios 1 1 1 1 3 four lie in the shift range appropriate for aromaties or heteroaromaties and the fifth is evidently a methyl group. The large shift values (up to Sh = 9.18, aromaties) and typical coupling constants (8 and 5 Hz) indicate a pyridine ring, which accounts for four out of the total five double-bond equivalents. 

The C NMR spectrum of the metabolite shows 16 signals instead of 8 as expected from the elemental composition determined by high-resolution mass spectrometry. Moreover, aromaticity of the 2,6-xylenol is obviously lost after metabolism because two ketonic carbonyl carbon atoms (5c = 203.1 and 214.4) and four instead of twelve carbon signals are observed in the shift range of trigonal carbon nuclei (5c = 133.1, 135.4, 135.6 and 139.4) in the C NMR spectra. To conclude, metabolism involves oxidation of the benzenoid ring. 

First the five protons (integral) of the //NMR spectrum (Sfj = 7.50 - 7.94) in the chemical shift range appropriate for aromatics indicate a monosubstituted benzene ring with typical coupling constants 8.0 Hz for ortho protons, 1.5 Hz for meta protons.). The chemical shift values especially for the protons which are positioned ortho to the substituent Sn = 7.94) reflect a -M effect. Using the CH COLOC plot it can be established from the correlation signal hclS = 66.AI7.94 that it is a benzoyl group A. 

The spectra of Figure 3 illustrate two further points. All the C Is peaks in Figure 3a are of equal intensity because there are an equal number of each type of C atom present. So, when comparing relative intensities of the same atomic core level to get composition data, we do not need to consider the photoionization cross section. Therefore, Figure 3c immediately reveals that there is four times as much elemental Si present as Si02 in the Si 2p spectrum. The second point is that the chemical shift range is poor compared to the widths of the peaks, especially for the solids in Figures 3b and 3c. Thus, not all chemically inequivalent atoms can be distin-... 

Difluommethyl moieties have a weU-defmed chemical shift range from -110 to -129 ppm and couphngs of approximately 57 Hz ( 7hf) 17 Hz Fluorme... 

The rehability of these analytical methods may be questionable when chemical shift differences of derivatives are of the same magnitude as variations encountered from solvent, concentration, and temperature influences. Reported fluorine chemical shift ranges for tnfluoroacetylated alcohols (1 ppm), p-fluorobenzoylated sterols (1 ppm), and p-fluorobenzoylated ammo acids (0.5 ppm) are quite narrow, and correct interpretation of the fluonne NMR spectra of these denvatized mixmres requires strict adherence to standardized sampling procedure and NMR parameters. 

Tile CH carbon shift ranges between S 62.4 and 95.1 ppm and is typical of carbons carrying two electronegative substituents. Tliis could be a hint for the observed overall ease of substitution reaction at this center. However, electronic and structural properties of these substituents cause no characteristic differences in the S values. 

---