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Encyclopedia of Physical Science and Technology EN010C-493 July 19, 2001 20:30

Nuclear Magnetic Resonance (NMR) 703

intensities and the widths of the observed spectral lines. ber” I, which is proportional to its magnetic moment M,
The study of spectra is called “spectroscopy,” hence the the proportionality factor being the gyromagnetic ratio
term NMR spectroscopy. It is seen, therefore, that at the γ ; M = γ I. The values of γ , and therefore the resonant
very least, an NMR experiment requires (1) a source of frequencies for NMR at fixed field differ for each nuclide.
1
radio-frequency (rf) oscillation at some radial frequency For example, H and 13 C have spin quantum numbers
6
−1
ω (rad sec ) tuned to energy splittings between nuclear I = 1/2, whereas Li and 174 Lu are spin 1. 27 Al is spin
magnetic states, (2) a magnet developing a static field B, 5/2. This spin quantum number imparts a special char-
to produce the split nuclear magnetic states, (3) nuclei acter to the nucleus’ ability to detect its local molecular
with magnetic moments M placed in a resonant radio- architecture, as will be seen in Section II. The NMR ab-
frequency (RF) circuit to absorb the rf energy at frequency sorption spectra, examples of which are shown in Sections
ω, and (4) some means of detecting this energy absorp- III and IV, are generally represented on an intensity (ordi-
tion. An NMR spectrometer is basically a high quality FM nate) vs frequency (abscissa) plot, and appear as a series
radio station and accompanying FM receiver. The carrier of peaks of various widths and shapes that are a reflection
is in the (video) megahertz region, generally between 5 of the local molecular environment of the nuclei under
and 700 MHz. The information content generally comes observation. This is to say that the local environments of
through in the (audio) kilohertz region, but the sounds nuclei in matter supply effective fields, B eff , which may
that are produced by resonating nuclei, when sent over be used to infer that environment.
an audio speaker, are generally fairly monotonic, and are We now inquire in more detail as to why NMR has this
not nearly as pleasant as those designed by a Mozart. The remarkable capability, and why this resonant spectroscopy
magnet is an expensive addendum to make a portion of is such a powerful tool, relative to other spectroscopies,
the experiment possible. such as ultraviolet and infrared spectroscopies.
The fundamental relation between the experimenter-
supplied parameters, B and ω, and the nuclear moment M
is II. THE NUCLEUS AS A PROBE OF
MOLECULAR STRUCTURE;
ω ∝ BM .
INTERNAL INTERACTIONS AND
The resonant frequency of absorption of energy of mag- THE EFFECTS OF MOTION
netic nuclei in a magnetic field is proportional to both the
strength of the field, and to the magnetic moment of the nu- A nucleus residing in a molecule, either in a solid or a liq-
cleus. The resonant condition for NMR may be achieved uid sample, has access to quite an intimate view of its local
by varying either B or the driving frequency. As alluded to molecular architecture. This nucleus senses the locations
previously, the local electronic and nuclear environment and types of its nearest neighbors, and in a diffuse man-
aboutanucleusinamolecule,alongwiththeexternalmag- ner, the bulk matter around it. In addition, this nucleus
netic field created by a magnet, contributes to the effective is sensitive to motion of its environment. The nucleus,
value of B. Thus the resonant NMR frequency is a finger- when properly interrogated with resonant excitations, can
print of the local electronic environment of the nucleus, give detailed information about its local molecular envi-
but depends upon the external magnetic field, which is ronment when that environment is motionless. In addition,
at the control of the experimenter. The magnetic moment the alteration of this information caused by molecular mo-
of a nucleus is a quantity fixed by nature, and is not an tion is used to infer details of such motion. It is this type
experimental variable. Table I lists all of the known mag- of information which, when properly interpreted as indi-
netic nuclides, their resonant frequencies in the absence of cated in the introduction, can lead to the wide variety of
interactions associated with the atomic or molecular envi- applications described there.
1
ronment at a field in which H resonates at 100 MHz, and The sensitivity of the nucleus to its environment and
relavent added material, which will become more mean- to motion are all the result of the arrangements of molec-
ingfulasfurtherinformationisdeveloped.Notethatfroma ular framework electrons and nuclei about the nucleus
quick glance at Table I, it is possible to infer that the physi- in question. The effects of this molecular framework
cian, the materials scientist, the chemist, the physicist, the upon the effective magnetic fields, and thus upon the
polymer chemist, the solid state scientist, the geologist, resonance frequencies of nuclei in matter are generally
and the engineer all have problems that may be attacked separated into four contributions, termed interactions:
with the help of NMR, since workers in all of these spe- these are designated (1) “shielding,” (2) “dipolar cou-
cialties deal with systems containing one or more of the pling,” (3) electric field gradients, or “quadrupolar cou-
nuclei listed. pling,” and (4) “scalar coupling.” These interactions are
Table I indicates that each magnetic nucleus has a num- all anisotropic. This means that they are directionally de-
ber of fingerprints. One is its “nuclear spin quantum num- pendent on the relative orientations of the static magnetic