Shielding and Deshielding of Protons

For example, the aromatic protons of 1,4-dimethylbenzene absorb at higher frequency (S 7.05) than the various alkyl protons of 1,4-dimethylbenzene and 1,1,2-trichloroethane (Figs. 9.3 and 9.4). 

We can see how this happens if we consider the electrons around the proton in a r bond of a C—H group. In doing so, we shall oversimplify the situation by assuming that r electrons move in generally circular paths. The magnetic field generated by these t electrons is shown in Fig. 9.13. 

The small magnetic field generated by the electrons is called an induced field. At the proton, the induced magnetic field opposes the external magnetic field. This means that the actual magnetic field sensed by the proton is slightly less than the external field. The electrons are said to shield the proton. 

A proton strongly shielded by electrons does not, of course, absorb at the same frequency as a proton that is less shielded by electrons. 

Ifc electrons were not present the signal might appear here. 

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Shielding and deshielding of protons (a) shielding of proton due to induced diamagnetic electron circulation b) deshielding of protons in benzene due to aromatic ring currents. 

The following generalizations apply to shielding and deshielding of the protons in a molecule ... 

When in the structural vicinity of a fluorine substituent, phosphorous chemical shifts can be significantly affected by the presence of fluorine substituents but not in the way one might think. In contrast to the intuitive expectation based upon the inductive effect of fluorine, which results in the deshielding of protons and carbons in their vicinity, the effect of proximal fluorine upon the chemical shifts of phosphorous compounds is to shield the phosphorous nuclei. Scheme 2.10 provides a few examples. 

A spectacular example of shielding and deshielding by ring currents is furnished by some of the annu-lenes. At about -60°C, the protons outside the ring of [18]-annulene are strongly deshielded (S 9.3) and those inside are strongly shielded (8 - 3.0, i.e., more shielded than TMS). 

Annulene shows two signals in its NMR spectrum, one at 8.9 (12 H) and one at-1.8 (6 H) ppm. Using a similar argument to that offered for the chemical shift of benzene protons, explain why both shielded and deshielded values are observed for 18-annulene. 

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