NMR Theories

Any successful theory must explain the observed facts. In NMR the fact is that when a sample of a molecule such as ethanol is placed in the center of a strong magnetic field and pulsed with a narrow band of a specific radio frequency energy (rf), a reproduceable spectrum of peaks will be produced. This spectrum has the following properties that must be explained:

1. only hydrogens are seen when hydrogen settings are used for the rf
2. there are only three peaks
3. the areas under the peaks are directly proportional to the concentrations of the hydrogens in ethanol (i.e. the areas are 3:2:1)
4. two of the peaks are split into multiplets, while the third is a singlet
5. the intensity of the spectrum depends on the power of the rf pulse
6. the distances between the peaks changes depending on the strength of the magnetic field
7. the sizes of the multiplets do not change with the strength of the magnetic field

There are two main ways that these facts can be explained: One way looks at the absorbance of photons by atomic nuclei, while the other looks at induction by the magnetic fields of a sample. Unfortunately, the NMR community mixes up the two ways, using whatever viewpoint is the most convenient for each aspect of the technique. This is extremely confusing since the theories are so different. This wiki will try to keep the two theoretical viewpoints separate yet still explain all aspects of NMR.

List of topics in this section

• Micro level - at the level of the nucleus

• Macro level - at the level of the sample

• NMR of solids - it is still fundamentally NMR, but the technical challenges of having a solid vs a liquid need a whole section to describe the theory

External Resources

There are many sites that still give good summaries of NMR theory, but they are disappearing fast. Latest link Check May 2020