Rapid Review

Nuclear Magnetic Resonance (NMR)

Each proton in the nucleus of an atom spins, including the ones that are a part of the neutrons. We know from physics that when a charge moves (or spins) it generates a magnetic field. If both the number of protons and the number of neutrons are even, the number of protons spinning in one direction equals the number spinning in the opposite direction. Thus, the magnetic fields generated cancel each other out and the nucleus is effectively non-magnetic. However, if either the number of protons or the number of neutrons is odd, the magnetic fields generated from each proton cannot cancel each other out and the nucleus is effectively magnetic. Examples of magnetic nuclei are ¹H, ¹³C, ¹⁴N, and ³¹P.

If a compound containing, say, ¹H is placed in a very strong magnetic field, the magnetic nuclei tend to align themselves with the magnetic field. Further, if the compound is simultaneously irradiated with electromagnetic radiation (specifically radio waves) of a certain frequency, the radiation will be absorbed and the nuclei will resonate. A NMR spectroscope can apply the magnetic field and the electromagnetic radiation to a sample, and it can detect when magnetic resonance is occurring. The result is a NMR spectrum, which provides detailed information about a compound's structure. The following are some features of NMR spectra.

Chemical Shift

From our knowledge of electromagnetic induction, when an external magnetic field is applied, an e.m.f. and current are induced to produce a magnetic field opposite to the external one. This often happens with the electrons in a compound under study. As a result the net magnetic field experienced by the nuclei is less than the external magnetic field. This is called shielding of protons and results in a shift of the NMR absorption signal towards the higher magnetic field end of the NMR spectrum (upfield shift). The amount of shift compared to a signal from a known reference substance indicates, for example, whether the hydrogen atoms are attached to a benzene ring or to a carbon atom bonded in turn to a chloride atom, and so forth.

Thus, the chemical shifts convey information about the environments of the atoms being studied, which provides qualitative information about the molecular structure.

Integration of Peak Areas

The area under the NMR spectrum at the peak areas is proportional to the number of the corresponding structures in the molecule under study. Thus, the ratio of certain structures to other structures in the molecule can be determined (i.e. quantitative information).

Spin-spin Splitting

Spin-spin splitting results from magnetic influences of hydrogen atoms on atoms adjacent to those bearing the hydrogen atoms causing the signal being considered. For example,

a doublet is caused by one nearby proton,
a triplet is caused by two nearby protons, and
a quartet is caused by three nearby protons.

Note: Equivalent protons (i.e. protons that are in identical environments and, therefore, are chemically equivalent) have exactly the same chemical shift and do not cause each other to signal split.