Aromatic carbons shifts

Relative intensities are measured on the basis of peak integration of the alkyl-substituted aromatic carbons shift values from TMS. 

Annelation increases the complexity of the spectra just as it does in the carbocyclic series, and the spectra are not unlike those of the aromatic carbocycle obtained by formally replacing the heteroatom by two aromatic carbon atoms (—CH=CH—). Although quantitatively less marked, the same trend for the longest wavelength band to undergo a bathochromic shift in the heteroatom sequence O < NH < S < Se < Te is discernible in the spectra of the benzo[Z>] heterocycles (Table 17). As might perhaps have been anticipated, the effect of the fusion of a second benzenoid ring on to these heterocycles is to reduce further the differences in their spectroscopic properties (cf. Table 18). The absorption of the benzo[c]... 

So to a large extent, 1-D 13C NMR interpretation is a case of matching observed singlets to predicted chemical shifts. These predictions can be made by reference to one of the commercially available databases that we ve mentioned, or it can be done the hard way - by a combination of looking up reference spectra of relevant analogues and using tables to predict the shifts of specific parts of your molecule (e.g., aromatic carbons). We have included some useful 13C shift data at the end of the chapter but it is by necessity, very limited. 

Table 6.7 gives characteristic chemical shifts for the aromatic carbons in benzene derivatives. To a first approximation, the shifts induced by substituents are additive. So, for example, an aromatic carbon which has a -NO2 group in the para position and a -Br group in the ortho position will appear at approximately 137.9 ppm [(128.5 + 6.1(p-N02) + 3.3(o-Br)]. 

Table 6.7 Approximate Chemical Shifts (6) for Aromatic Carbons in Benzene Derivatives Ph-X in ppm relative to Benzene at 6 128.5 ppm (a positive sign denotes a downfield shift)...

In an organic solid representative broadenings are 150 ppm for aromatic carbon chemical shift anisotropy and 25 kHz (full width at half-height) for a rather strong carbon-proton dipolar interaction. At a carbon Larmor frequency of 15 MHz, the shift anisotropy corresponds to 2.25 kHz. In high magnetic fields the forms of the respective Hamiltonians are... 

Further evidence for isotropic chemical shifts was found in variable temperature studies. The aromatic carbon ortho to the oxygen resolved into two peaks at low temperatures. The aromatic carbons meta to the oxygen have two peaks, but these peaks are not as well resolved. As the temperature was raised, the splittings in both coalesced into one peak, indicating that there is rapid sampling from two magnetic environments. This is indicative of the motion of the phenyl group with respect to the backbone. At lower temperatures, the methyl resonance broadens and eventually... 

Box 4.15 Effect of EWG and EDG on the Chemical Shifts of Aromatic Carbon Atoms... 

Bearing in mind that chemical shifts are usually not dependent on the solvent or concentration, but may be partly dependent on the anion37, quaternization of alkyl-phosphines causes an upfield shift (1-7 ppm) for the near carbons of the alkyl chain, the effect becoming only slightly detectable on C(y) carbons37,94. Where triarylphosphines are quaternized, the chemical shift of the phenyl ipso carbon Cf moves upfield whereas that of the Cpara carbon moves downfield, as expected from the polarization of the 71-electron density in the phenyl ring36,37. For the aromatic carbon atoms, the increments... 

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