Contributions to the Chemical Shift
Broadly speaking, we have established that protons in inequivalent positions in a molecule will have different resonance frequencies. This explains the appearance of the NMR spectrum of ethanol:
(Note that this representation would be a very low resolution spectrum – normally the three lines shown above would be split into different components from which further information about the structure of the molecule can be deduced.)
The CH3 protons in ethanol are all in the same magnetic environment, so form one group of signals at δ ≈ 1. The two protons of the CH2 group have a different environment, so experience a different degree of shielding and form a group at δ ≈ 3 . The OH proton is in a different environment again and resonates at δ ≈ 4.
In this simple molecule it is possible to correlate the chemical shift with the electron-withdrawing (deshielding) power of the O atom. The OH hydrogen is directly attached to it so is strongly deshielded. The CH2 protons are adjacent to the O atom, so are also deshielded by its electron-withdrawal. The CH3 protons are more remote from the O atom, so are less affected.
In general, the presence of an electronegative atom adjacent to a proton will reduce the electron density of the proton and thus deshield it. This effect falls off quickly as the number of intervening bonds increases, and is usually negligible if there are four or more intervening bonds.
It is also possible to show that more electronegative substituents are more strongly deshielding. However, there are various other more subtle effects which may obscure or even reverse this trend. We must must therefore be aware that while it is a useful rule of thumb, it is by no means always correct.
Note also that the intensities of the three lines are in the same ratio as the number of protons each line represents.
This is a general feature of 1H NMR spectra – the areas under absorption lines are in the ratio of the number of nuclei represented by each line.
There are various other effects which can contribute to the shielding or deshielding of a particular nucleus:
An applied field can induce currents in the electron density of nearby groups of atoms (the so-called neighbouring group effect). These currents generate magnetic fields which can either shield or deshield an atomic nucleus, depending upon its location relative to the group. These effects are commonly observed in unsaturated and, particularly, aromatic compounds, as electrons in π orbitals can be made to circulate (and thus generate a magnetic field) more easily than those that are more tightly bound in σ orbitals.
The solvent can also influence the degree of shielding of nuclei in the sample material. For example, if the solvent has any strong interaction with the sample molecule, such as hydrogen bonding, then it can cause large chemical shifts in the resonances of some protons in the sample molecule.