Spectra of solid samples, however, consist of broad lines and are less easily interpreted.
The amount of structural information given by an NMR spectrum is greatly enhanced by two factors.
Firstly, the exact position of resonance is determined by the chemical environment of the nucleus. Both
the PMR and carbon-13 spectra of an organic compound may therefore show several absorption bands,
each corresponding to a particular nucleus or group of nuclei. Secondly, a given band may be split into
several peaks as a result of interactions between neighbouring nuclei. The two effects give rise to what
are termed the chemical shift and spin-spin coupling or splitting. A third useful feature of the spectrum
is that the integrated area of an absorption peak is directly proportional to the number of nuclei
responsible for the signal. This facilitates structural correlations and provides a means of quantitative
analysis. These factors will now be discussed in the context of PMR.
Chemical Shift
According to equation (9.24) at a given frequency all protons will absorb energy at the same value of
the magnetic field B. However, the field experienced by a particular nucleus differs in magnitude from
that of the applied field because of shielding effects by neighbouring electrons. It is because of varying
degrees of shielding that protons in different chemical environments absorb at different values of the
applied field. Differences between such absorptions are referred to as chemical shifts.
All protons are subjected to diamagnetic shielding by the electrons bonding them to another nucleus.
Under the influence of the applied field these electrons circulate around the field direction and in so
doing they create a small localized magnetic field in opposition to the applied field. The magnitude of
the opposing field is directly related to the electron density around the proton which in turn is
determined by the electronegativities of neighbouring nuclei. The more electronegative the atom to
which a proton is bonded, the less the shielding and the smaller is the applied field required to achieve
the resonance condition. For example, Figure 9.29 shows the relative absorption positions for protons
attached to C, Si, N and O. It is conventional to display the spectrum with increasing applied field from
left to right. Quantitatively, chemical shifts are measured
Figure 9.29
Chemical shift positions for protons attached
to different atoms.