Chemistry, Third edition

20 · LIGHT AND SPECTROSCOPY

Nuclear magnetic resonance

spectroscopy

InNuclear magnetic resonance(NMR) the absorption of radio waves by atomic

nuclei surrounded by a powerful magnet produces spectra which are used to help

decide the structural formulae of compounds.

Nuclei which have an odd number of protons or an odd number of neutrons (or

both of these) behave like tiny bar magnets; they have a magnetic moment(they are

also said to have spin). Just as when a small bar magnet is placed in the field of a second

magnet it will be attracted or repelled, in an applied magnetic field such nuclei will

have their magnetic moments aligned with the field or opposed to it. These arrange-

ments will be of different energies as shown in Fig. 20.26, the difference in energy

between the two will depend upon the strength of the external field.

NMR depends upon the fact that energy in the radiofrequency region of the spec-

trum can be absorbed by the sample and cause the energy of the nuclei in the lower

energy state to jump to a higher energy; the frequencies at which the nuclei absorb

radiation are referred to as resonance frequenciesor (simply) as resonancesorsig-

nals. The earliest (and still most important) nuclei to be studied by NMR were

hydrogen nuclei (i.e. protons) and it is proton NMR (symbolized as^1 H-NMR) that

we will deal with here.

Chemical shifts

Protons are sensitive to their chemical environment – electrons moving near them

produce their own magnetic field, that changes the external field experienced by the

proton. Protons in different chemical environments therefore experience slightly

different magnetic fields and absorb at different frequencies.

Figure 20.27 shows the low-resolution NMR spectrum of ethanol, CH 3 CH 2 OH.

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Fig. 20.26Different alignments of bar magnets lead to different energy states.

Fig. 20.27Low-resolution NMR

spectrum of ethanol, CH 3 CH 2 OH.