NMR Introduction

Nuclear Magnetic Resonance Spectroscopy is the most widely used method of structure determination present in modern chemistry. When used in conjunction with Mass Spectrometry and Infrared Spectroscopy, the three techniques make it possible to determine the complete structures of even the most complex molecules. Mass Spectrometry is used to determine the size of a molecule and its molecular formula and Infrared Spectroscopy helps identify the functional groups present in a molecule. NMR Spectroscopy is used to determine the carbon-hydrogen framework of a molecule and works with even the most complex molecules.

NMR is a spectroscopic technique which uses electromagnetic radiation and magnetic fields to determine the structure of organic compounds. Radio-frequency radiation is used to stimulate nuclei present within the molecule and from the information we obtain from doing this we can very accurately determine where the carbon atoms are located and where hydrogen atoms are located. The effect was first noticed in 1902 by P. Zeeman, a physicist, who won a Nobel Prize for noticing that nuclei of certain atoms behave strangely in a magnetic field. Fifty years later F. Bloch and E. Purcell, both physicists put this idea to good use by constructing the first NMR spectrometer. They too received a Nobel Prize for this work.

NMR Spectroscopy

The principle of NMR is based upon the spin of atomic nuclei in an external magnetic field. Many nuclei do not have the ability to spin in a magnetic field, but proton (1H) and an isotope of carbon (13C) can spin in a magnetic field, so they are the most widely used elements in NMR. When no magnetic field is present, the nuclear spins of magnetic nuclei are oriented randomly.

Once a strong magnetic field is introduced to the nuclei, they reorient their spins so their magnetic fields are either parallel or antiparallel to the alignment of the applied force. The orientation parallel to the alignment of the applied force is lower in energy and therefore favored. When the nuclei are irradiated with RF radiation the lower energy nuclei spin-flip to the higher state. When this occurs the nuclei are said to be in resonance, hence the name NMR. The nuclei reach the higher energy state in a negligible amount of time since they spin-flip at an infinitely high rate. Once the strength of the magnetic field which caused them to spin-flip returns to zero, the collection of nuclei become unstable and return to their original lower energy state. This process is called spin relaxation. The data from the spin relaxation is collected in the form of a modulated free induction decay (FID) signal

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A series of mathematical manipulations of the FID data are carried out by the spectrometer, and the resulting data is used to construct a spectrum.