ethoxy group. The sharp singlet at δ2.1, integrating to 3 protons, is shown from tables to be due to an
isolated methyl group attached to a carbonyl. The broad resonance at δ7.9, which can be removed by
treatment with D 2 O, integrates to one proton and is due to the secondary amide group.
The spectrum of divinyl-β,β'-thiodipropionate is shown in Figure 9.49. The typical ABX pattern at
lowfield is related to the vinyl group, the four peaks at δ7.2 corresponding to the X proton. The geminal
A and B protons are well separated between δ4.5 and δ5.0 and the symmetrical but complex A 2 B 2
pattern around δ2.7, together with an integral corresponding to four protons, indicates two adjacent
methylenes. Tables confirm that the ABX pattern has chemical shifts corresponding to a vinyl group
adjacent to an ester group.
Figure 9.50 shows the spectrum of thymol. The doublet at δ1.3 together with the septet at δ3.2 (integral
ratio 6:1) are the methyls and methine respectively of an isopropyl group. The outer peaks of the septet
are often lost in baseline noise and are better seen by increasing the sensitivity. The sharp resonance at
δ4.75, disappearing on treatment with D 2 O, is that of the phenolic OH, and the complex pattern between
δ6.5 and δ7.5 is typically aromatic. Tri-substitution is confirmed by an integral corresponding to three
protons. The sharp singlet at δ2.3, also corresponding to three protons, is in the expected position for a
methyl group attached to an aromatic ring. Additional examples of PMR spectra are given in the section
on the combined use of spectral data (p. 441).
Quantitative Analysis
NMR has been used comparatively little for quantitative analysis although peak areas are directly
proportional to concentration. The principal drawbacks are the expensive instrumentation and a lack of
sensitivity. The latter can be improved with the aid of computers to accumulate signals from multiple
scans or by using a pulsed (Fourier transform) technique. Relative precision lies in the range 3–8%.
Applications of NMR Spectrometry
The technique is currently not used as widely as UV, visible and infrared spectrometry partly due to the
high cost of instrumentation. However, it is a powerful technique for the characterization of a wide
range of natural products, raw materials, intermediates and manufactured items especially if used in
conjunction with other spectrometric methods. Its ability to identify major molecular structural features
is useful in following synthetic routes and to help establish the nature of competitive products,
especially for manufacturers of polymers, paints, organic chemicals and pharmaceuticals. An important
clinical application is NMR imaging where a three-dimensional picture of the whole or parts of a
patient's body can be built up through the accumulation of proton spectra recorded over many different
angles. The technique involves costly instrumentation but is preferable to