ABSTRACT
Nuclear magnetic resonance (NMR) was discovered by Bloch, Purcell and Pound in 1945, as a development of the use of nuclear spins in low-temperature physics. Its initial use was for the accurate measurement of nuclear magnetic moments. However, increases in instrumental precision led to the detection of chemical shifts (B1.11.5) and then of spin-spin couplings (B1.11.6). This stimulated use by chemists. There have been spectacular improvements in sensitivity, resolution and computer control since then, so that NMR equipment is now essential in any laboratory for synthetic chemistry. Within moments or hours, it can determine the structure and, if desired, conformation of most medium-sized molecules in the solution phase. For this reason, a large pharmaceutical company will typically generate several hundred NMR spectra in one working day. NMR is also widely used in biochemistry for the much more challenging problem of determining the structures of smaller proteins and other biomolecules. The rates and extents of molecular motions can also be measured, through measuring the rates of energy transfer to, from and between nuclei (relaxation, B1.13). Outside of chemistry, it is used in a different mode for medical and other imaging, and for the detection of flow and diffusion in liquids (B1.14). It can also be used for clinical and in vivo studies, as the energies involved present no physiological dangers.
