Micro view of NMR

NMR is an instrumental technique that uses photons of radio frequency energy to cause a transition, or change in state, in an atom. Radio is used because transitions at the atomic level are quantized, and the amount of energy needed to cause these transitions happens to fall in the radio region of the electromagnetic spectrum. Quantized energy means there has to be the right amount of energy to cause a change of state, too much or too little and no change occurs.

There are many transitions in atoms. The ones of interest in NMR are quantum spin transitions of protons and neutrons. Quantum spin in protons and neutrons has two states, which are normally equivalent, but when placed in a magnetic field they become non-equivalent, so adding the appropriate sized photon of energy can cause a transition from one to the other. This works fine for single protons and neutrons, but when these are combined into a nucleus the situation gets more complex and we have to rely on a net nuclear spin, called I.

Across the entire periodic table, net nuclear spin values ranging from I = 0 to I = 8 in ½-unit increments can be found. Protons and neutrons each have net spins of ½, but this derives from the elementary quarks and gluons of which they are composed. As a result of this complexity, no simple formula exists to predict I based on the number of protons and neutrons within an atom.

The formula for the number of states = 2I+1, thus a spin 1/2 nucleus such as a single hydrogen atom will have 2 states and 1 transition (when placed in a static magnetic field). For I greater than 1 there are more than two states and thus many transitions. The focus in NMR is on spin 1/2 nuclei since two states with one transition gives good, clean spectra that are easily interpretable. A lot of information about the environment of a nucleus can thus be obtained, making 1H NMR the most useful analytical technique in science.

To obtain a spectrum, a spin 1/2 atom must first be placed into a static magnetic field (B₀) to cause separation of the two states. The amount of separation depends on the gyromagnetic ratio for that atom (a constant) and the intensity of the static magnetic field according to the equation:

v=γ*B₀

The atom is then pulsed with a composite radio wave that includes the frequency v, called the Larmor Frequency. The atom absorbs just the v part of the radio wave (since it is a quantum transition), and a detector in the same axis as the radio wave (usually the same antenna) is then turned on to receive any emitted energy.

Making this happen in an instrument is thus mainly an engineering problem.

Complications

From the equation above, since γ is a constant and B₀ is a constant, there is just one frequency for that atom, which makes sense since there is just one transition, thus there is just one peak expected in the spectrum. In reality though, an atom never exists in isolation, thus it never experiences the pure B₀. The actual frequency v is thus slightly different for atoms of a particular γ based on the actual magnetic field around the atom. These slight differences in frequency are what gives a spectrum of peaks for a molecule rather than a single peak. There is still just one transition for that atom type (if that type is spin 1/2), but the transition energy is different for each atom that is in a different magnetic environment.

List of topics in this section

• Gyromagnetic ratio
• Spins of nuclei