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| Fig. 1: NMR Spectrometer. (Source: Wikimedia Commons) |
Nuclear magnetic resonance, or more commonly referred to as NMR, is a property of the nucleus of an atom. It was discovered by two groups of physicists in 1945, one group being at Stanford, the other at Harvard. [1] The Stanford group found it in liquid water, whereas the Harvard group discovered it in paraffin. The two leaders, Edward Purcell and Felix Bloch, shared the 1952 Nobel Prize in physics for their findings. The sequential discovery of a chemical shift allowed chemists to analyze samples to determine molecular identity without ruining the samples in the process. There was less collateral damage, it was faster, and a simpler process than what researchers had available to them before its introduction. [1] The next large discovery in NMR was that of spin-coupling which measures atomic interactions within a molecule. NMR showed great promise, it was no longer a tool for physicists. [1] Chemists could also use it to recognize the structure of a molecule as they synthesized it. Since its discovery, both fields have taken great steps forward.
NMR focuses on the atom's nuclear spin. When exposed to a magnetic field the nucleus can align itself with or against the direction of the field. [2] A nucleus with higher energy generates a magnetic field opposite to the applied field and a nucleus with lower energy generates a magnetic field concurrent with the applied field. This created field indicates to us if the nucleus is higher or lower energy as the energy transfers between one energy level to a higher when the field is applied. [2] When the field is removed from the environment the nucleus releases the energy of the level it attained as it returns to its original state. During this process the signal equivalent to this transfer is analyzed and used to produce an NMR spectrum for the nucleus under consideration. [2] This can be implemented in a number of different disciplines such as; synthetic chemistry, drug discovery and development, biochemistry, and many more. It is used to understand molecular structure and monitor chemical processes.
In order to exploit and use the magnetic properties perpetuated by the nuclear magnetic resonance of an atom, researchers use a nuclear magnetic resonance spectroscopy. Fig. 1 provides a visual of a NMR spectrometer.Though powerful, the process is a simple 4 step process. [3] 1st you place your sample of atoms in a magnet where the magnetic field can be applied as the field aligns the spins of the nuclei in the sample. You then deliver the radio frequency pulses which induce an additional magnetic field. This new magnetic field can twist the atoms out of alignment or push them in the same direction with the magnetic field depending on which atoms are being sampled. [3] After delivering the radio waves you can measure the free induction decay which is the signal produced by the sample as the nuclear spins return to their original states. Finally, you convert the free induction decay data into a spectrum through Fourier transformation. [3] This gives you a NMR spectrum which provides information about the nuclei in the sample.
© Bryce Marion. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] E. R. Andrew, "A Historical Review of NMR and its Clinical Applications," Brit. Med. Bull. 40, 115 (1984).
[2] P. Hore, Nuclear Magnetic Resonance, (Oxford University Press, 2015).
[3] F. A. Bovey, P. A. Mirau, and H. S. Gutowsky, Nuclear Magnetic Resonance Spectroscopy, 2nd Ed. (Elsevier, 1988).
