Modern NMR
spectrometers acquire multinuclear spectra (13C, 31P,
29Si, etc) rather routinely. However, it remains more
difficult to obtain say a 13C-NMR spectrum than a 1H-NMR
spectrum for two main reasons. One, the natural abundance of many NMR
active isotopes is low, so there are fewer NMR-active nuclei per mole
of compound to absorb energy. Second, the inherent signal intensity
per nucleus is generally very low. (i.e. For equal numbers of 1H
and 13C nuclei, the signal intensity for 13C is
roughly 1/4 that of 1H.) When combined with the fact that
the natural abundance of 13C is roughly 1% of that of
1H, this means that the signal intensity of 1H is
over 400 times greater than that of 13C. Consequently NMR
spectroscopists have sought ways to increase the signal intensity of
carbon and other X-nuclei. All of the methods they have developed
involve a phenomenon known as polarization transfer whereby
magnetization is transferred via spin–spin couplings.
