The principle of shielding and deshielding of atoms by their electrons is the reason why protons and other nuclei in the molecule behave differently based on their environment. It is also the reason why protons of different compounds behave differently in the NMR experiment.
Principle of Shielding by Electrons
The nucleus of an atom has some magnetic moment due to which we obtain an NMR signal as was seen in the chapter of Principle of NMR. However, looking at the theory, the nuclei of all protons should show an NMR signal at the same frequency. The answer as to why nuclei in different molecular environments display a different NMR signal lies within the electrons which surround the nucleus.
We must keep in mind that electrons are also charged particles which have a spin, and hence would produce (albeit small) a magnetic moment He. Also, the magnetic vector of the spin of the electrons is generally in the opposite direction to the applied magnetic field. (This is similar to two magnets kept in the vicinity of each other, the magnets would try to orient themselves such that their polarity is opposite i.e north of one magnet is closer to south of the other magnet). The diagram below illustrates this in a better way.
Since the direction is opposite, it is going to oppose the strong magnetic field H0. This secondary magnetic field generated by the electrons is going to “shield” or protect the nucleus from the effects of the strong magnetic field H0. Thus in effect, to obtain resonance, the magnetic field strength H0 would have to be decreased to overcome this induced shielding effect by the electrons. Thus the larmour frequency (explained in chapter on chemical shift) for this nucleus would be calculated by
ω0 = γ . (H0 – He)
ω0 = γ . Heff
Thus on the NMR scale as we go left to right, the shielding effect increases, therefore He keeps on increasing. H0 is a constant for a given NMR instrument as it depends on the strength of the strong magnet in it. As a result on the scale if we move from left to right the larmour frequency or the precession frequency or the frequency at which the signal is obtained keeps decreasing. This can be well observed in the image below.
Thus the larmour frequency for the protons in tetramethylsilane (TMS), which are highly shielded, is observed highly to the right and is considered highly shielded due to the electron pushing effects of the silicon and carbon atoms.
In general, most organic compounds are relatively deshielded in comparison to the TMS standard. Hence the He for these compounds would be lower, thus they would require higher frequencies for their signal, and thus appear to the left of the reference TMS.