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What Is Chemical Shift?

Nuclear magnetic resonance (NMR) is one of the most powerful tools available to modern organic chemists. With NMR data, determining the structure of small molecules becomes a relatively quick (if not entirely painless) process. One of the concepts you need to interpret NMR data is the chemical shift, which gives you some important information about the hydrogen or carbon atoms in a molecule.

Spin
- The nuclei of atoms have a property called spin. The spin value for a nucleus depends on the number of protons and neutrons it contains. Some nuclei have a nuclear spin of 0, others of 1/2, still others of 1 and so on. If a nucleus has a nonzero spin value, it's called NMR-active, and it acts a little like a tiny magnet, creating its own magnetic field. And just like a magnet, when an external magnetic field is applied, it can align itself with the field or against it.
Absorption
- If a nucleus is oriented so its magnetic field runs counter to the external magnetic field, it will be higher in energy. Consequently, at any given time, most of the NMR-active nuclei in a sample will be aligned with the external magnetic field because this is a lower energy state. You can change this, however, by bombarding the sample with radio waves at different frequencies. If the radio waves have an energy exactly equal to the difference between the two orientations possible for an NMR-active nucleus, it will be absorbed; otherwise, it will pass right through.
Electrons
- The external magnetic field doesn't just affect the atomic nuclei, but it also causes the electrons around the nucleus to move. This current creates a magnetic field that opposes the external field, so it "shields" the nucleus from the external field, and the field that the nucleus actually feels is reduced. The greater the electron density around a nucleus, the more "shielded" it will be. An NMR spectroscope blasts the sample with radio waves at different frequencies to see which frequencies are absorbed. Higher frequencies have more energy, so the higher the frequency that is absorbed, the less shielded the nuclei that absorbed it.
Graph
- NMR data is usually printed out on a graph that looks like a series of vertical lines or "peaks." The chemical shift is the horizontal position of each of these peaks. Chemical shifts are measured relative to a standard called tetramethylsilane; this way chemists can compensate for differences in the machines used by different labs. Larger chemical shifts mean the nuclei are "deshielded," while smaller chemical shifts mean the nuclei are highly shielded. A chemist can take this information and use it to figure out where different atoms are placed within a molecule.

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