Overview

¹³ C NMR spectroscopy tells us about the environments of the carbon atoms in a molecule. The technique of NMR can be applied only to atoms which have nuclear spin; when they are placed in a strong magnetic field, the nuclear spins can take up different orientations which are at different energy levels. For nuclei with nuclear spin of 1/2, such as ¹H, ¹³C, ¹⁹F and ³¹P, the nuclei orient themselves in one of two ways ^[*]. When the nucleus is irradiated with radiation of the appropriate frequency (radiofrequency), some of the nuclei in the lower energy state absorb radiation and are raised to the higher energy state. When irradiation stops, they relax to the equilibrium distribution. This relaxation process is quite slow.

The frequency of the radiation absorbed depends on the magnetic field applied to the nucleus. The field is normally made up of three components:

1. The externally applied field. This is an extremely powerful magnetic field. When a field of 23.49 kilogauss is applied toa carbon nucleus, it absorbs radiation at 25.14 MHz. The same field, applied to a hydrogen nucleus, would make it absorb radiation at 100 MHz. Since most machines run ¹³ C and ¹H NMR spectra, the radiation absorbed by the latter is often used to describe the power of the machine. Thus, a machine with a field of 23.49 kilogauss would be described as a 100 MHz spectrometer. To attain high, stable magnetic fields, superconducting...