Nuclear Magnetic Resonance
Nuclear magnetic resonance (NMR) is the effect produced when a radiofrequency field is imposed at right angles to a (usually much larger) static magnetic field to perturb the orientation of nuclear magnetic moments generated by spinning electrically charged atomic nuclei. When the perturbed spinning nuclei interact with the very large (10,000-50,000 gauss) static magnetic field, characteristic spectral shifts and fine structure are produced that reflect the molecular or chemical environment seen by the nucleus. Hydrogen nuclei, fluorine, carbon-13, and oxygen-17 all have distinctive magnetic properties that make them suitable for NMR studies.
The sensitivity of nuclear magnetic resonance to molecular structure has made it a valuable research tool in organic chemistry, enabling chemists to determine hydrogen locations in crystals, something that cannot be done using x-ray diffraction. Nuclear magnetic resonance has also been used to study electron densities, chemical bonding, the compositions of mixtures, and to make purity determinations.
The basic requirements for NMR spectroscopy are that the magnetic field be homogeneous over the volume of the sample, that there be a radiofrequency field rotating in a plane perpendicular to the static field, and that there be a means of detecting the interaction of the radiofrequency field with the sample.
Techniques that have been developed for the observation of NMR signals fall into two categories: pulsed and continuous wave. In the case of pulsed methods, an applied rotating (or alternating) magnetic field with a frequency at or near the Larmor frequency (i.e., frequency of precession) of the nucl eus to be studied is directed at a right angle to the static field. If the rotating field is applied at exact resonance, the nuclei precess about that field as though there was no static field. Continuous wave methods are either high resolution or broadline. Broad line widths are produced by most oriented molecules exhibiting strong mag netic dipolar interactions, so broadline spectroscopy does not permit measurements of chemical shifts and spin-spin coupling. High resolution spectroscopy, on the other hand, has been used to identify molecules, to measure subtle electronic effects, to determine structure, to study reaction intermediates, and to follow the m otion of molecules or groups of atoms within molecules. For high resolution studies, the magnetic field must be uniform to 1 part in 108 for a 100MHz instrument if a resolution of 1 MHz is to be obtained. In the case of broadline studies, 5 parts in 106 may be adequate.
Nuclear magnetic resonance has been used to study the physics and chemistry of solids, including metals, semiconductors, magnetic solids, and organic materials. Physical phenomena studied include conduction-electron paramagnetism; spin waves and magnetic fluctuations in ordered mag netic materials; metal-insulator transitions; charge density wave phenomena; spin-freezing in spin glasses; and frequency shift and spin-lattice relaxation effects. At low temperatures, NMR has been used to make temperature measurements and to st udy the superfluid phases of 3 He.
In the fields of organic che mistry and materials science, NMR has been used to study polymers, amorphous systems, and complex molecular solids. In many of these systems, the NMR line widths of the nuclei are dominated by dipolar fields arising from neighboring magnetic moments. These systems exhibit complex NMR spectra due to shifts in nucl ear magnetic resonance frequencies.
In the case of complex molecules in liquid environments, the molecules undergo a tumbling motion, producing very sharp NMR spectra. The technique used to study these systems is known as Fourier transform NMR spectroscopy.
Nuclear magnetic resonance has been adapted to medical studies in the form of magnetic resonance imaging (MRI), a technique that is capable of producing high quality images of the internal human body without requiring invasive surgery. Begun as a tomographic imaging technique because it produced an image correspond ing to a thin slice of the human body, MRI later evolved into a three-dimensional imaging technique.
Like NMR, magnetic resonance imaging is based on the absorption and emission of radiofrequency energy under the influence of a magnetic field. On a molecular level, the human body consists largely of hydrogen-rich fat and water molecules, and approximately 63% of the human body contains hydrogen atoms. The hydrogen nuclei emit signals that can be observed by magnetic resonance imaging.
When exposed to a magnetic field, the spin of the protons in the hydrogen nuclei of water, which ordinarily assume random orientations, line up. Although short pulses of radio waves briefly disturb this spin alignment, the spins promptly realign in the direction of the magnetic field. In the process of realigning, small signals are produced that can be picked up by sensitive scanners. The signals are then compiled by computer and an image is formed.
Because water is the major component of soft tissue, MRI creates excellent images of soft tissue and organs, but poor images of dense structures like bone. MRI thus complements x-ray imaging, which by contrast is fine for dense structures but useless for soft tissue.
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