Magnetic Shielding by Electrons

Up to now, we have considered the resonance of a naked proton in a magnetic field, but real protons in organic compounds are not naked. They are surrounded by electrons that partially shield them from the external magnetic field. The electrons circulate and generate a small induced m netic field that opposes the externally applied field. 

Induced magnetic field. Moving a loop of wire into a magnetic field induces a current in the wire. This current produces its own smaller magnetic field, in the direction opposite the applied field. In a molecule, electrons can circulate around a nucleus. The resulting current sets up a magnetic field that opposes the external field, so the nucleus feels a slightly weaker field. 

A proton shielded by electrons. The magnetic field must be increased slightly above 70,459 gauss (at 300 MHz) for resonance of a shielded proton. 

If all protons were shielded by the same amount, they would all be in resonance at the same combination of frequency and magnetic field. Fortunately, protons in different chemical environments are shielded by different amounts. In methanol, for example, the electronegative oxygen atom withdraws some electron density from around the hydroxyl proton. The hydroxyl proton is not shielded as much as the methyl protons, so the hydroxyl proton absorbs at a lower field than the methyl protons (but still at a higher field than a naked proton). We say that the hydroxyl proton is deshielded somewhat by the presence of the electronegative oxygen atom. 

Because of the diverse and complex structures of organic molecules, the shielding effects of electrons at various positions are generally different. A careful measurement of the field strengths required for resonance of the various protons in a molecule provides us with two important types of information  

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To provide an idea of magnitudes, two values can be mentioned of 5 referred to a bare proton, a proton with no magnetic shielding by electrons for cyclohexane, 5 is +27.0, and for molecular hydrogen, 5 is +27.5 (848). 

The two primary causes of shielding by electrons are diamagnetism and temperature-independent paramagnetism (TIP). Diamagnetism arises from the slight unpairing of electron orbits under the influence of the magnetic field. This always occurs so as to oppose the field and was first analysed by Lamb [7]. A simplified version of his fomuila. 

We said previously that differences in chemical shifts are caused by the small local magnetic fields of electrons surrounding the different nuclei. Nuclei that are more strongly shielded by electrons require a higher applied field to bring them into resonance and therefore absorb on the right side of the NMR chart. Nuclei that are less strongly shielded need a lower applied field for resonance... 

The three-dimensional magnetic shielding by the surrounding electrons is an additional interaction that the nucleus experiences in either the solution or the... 

Chemical shift As mentioned before, the field experienced by nuclear spins of the same species, e.g. of protons, is modified due to magnetic shielding by the cloud of electrons around each nucleus. This is accompanied by a small local magnetic field proportional to B0, but opposite to B0. As a result a slightly higher value of Ba is needed to achieve resonance, because... 

The nuclear magnetic resonance spectra of R/sSF4 and of R/SF6 compounds indicate (1) the fluorine nuclei of the R/ groups are more shielded by electrons than those of the —SF5 and =SF4 groups (2) the —SFB groups have four identical fluorine atoms and one (presumably at the apex of a square pyramid) which is different (3) in R/2SF4 there is only one resonance peak for the =SF4 fluorine atoms therefore, the two R/ groups... 

Chemical shielding G So 102-10 Alteration of the magnetic field by the electrons... 

Section 13 4 The energy required to flip the spin of a proton from the lower energy spin state to the higher state depends on the extent to which a nucleus is shielded from the external magnetic field by the molecule s electrons... 

Each electronically distinct 1H or 13C nucleus in a molecule comes into resonance at a slightly different value of the applied field, thereby producing a unique absorption signal. The exact position of each peak is called the chemical shift. Chemical shifts are caused by electrons setting up tiny local magnetic fields that shield a nearby nucleus from the applied field. 

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