Page 342 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: GNH/FEE P2: GPJ Final Pages
Encyclopedia of Physical Science and Technology EN010C-493 July 19, 2001 20:30

Nuclear Magnetic Resonance (NMR) 707

not widely used by chemists, however, until the discov- a spherical cloud, as it would be, for example, for a nucleus
ery, five years later by Procter and Yu, that a given nu- in an inert gas such as helium. When this cloud is exposed
clear type in different chemical environments (e.g., 14 N to a static magnetic field, it responds in accord with the
in the ammonium ion and in the nitrate ion in an aqueous laws of quantum mechanics by effectively developing a
solution of ammonium nitrate, NH 4 NO 3 ) would absorb field that algebraically adds to the static field. In other
radio-frequency irradiation at a frequency specific to the words, the nucleus is “shielded” from the external field
chemical environment; nitrogen in the ammonium ion ab- by the molecular electronic cloud about it. This shielding
sorbed at a frequency different than nitrogen in the nitrate interaction causes the magnetic field that is seen by the
ion. Subsequent to this discovery, NMR was rapidly as- nuclei in the molecule to be different than the static field
similated into the chemical laboratory for routine deter- due to the magnet in the NMR experiment. As mentioned
mination of structures of molecules in liquids. previously, the basic relation in the NMR experiment is
For many nuclei in liquids, the frequency differences of ω = MB. This relation, taking into account the fact that the
a given nucleus, associated with differences in chemical effective magnetic field at the nucleus is provided both by
functionality of the atom in which the nucleus resides, or the static, external field B 0 supplied by the experimenter
shifts, known as chemical shifts from some standard, are and by the local magnetic field due to the environment of
of order of parts per million (ppm) of the applied field. For the nucleus, now becomes ω ∝ MB eff . Now, however, the
example, a proton in a methyl group and a proton in an OH product MB eff depends upon the direction of the external
group in methyl alcohol, CH 3 OH, will have a frequency magnetic field relative to the nonspherical electronic
difference of roughly 7 ppm. Thus, at a resonant frequency environment producing the shielding shift. We, therefore,
of 300 MHz for the proton in the methyl group, the proton digress a moment to consider a physical picture of an
6
−6
in the OH group will absorb at 300 × 10 (1–7 × 10 ) anisotropic shielding environment that will be useful in
Hz, or the difference in resonance frequencies between our discussion of all of the four effects of the molecular
1 HinCH 3− and Hin OH will be 1,800 Hz; this is the framework upon resonance frequencies of nuclei in NMR
1
desired information in the audio region of frequencies. experiments.
Currently, the differences in resolution of absorption A useful pictorial representation of an anisotropic
lines of nuclei in different environments (to be clearly shielding environment is provided by an ellipsoid with
distinguished from quantitative detection, i.e., amount) three unequal axes, shown in Fig. 1. The relation between
available using the highest field magnets now produced, the frequency observed in an NMR experiment, ω obs , and
which currently are 14 T, and liquid samples, is equivalent this ellipsoid is as follows: If the ellipsoid represents the
to being able to distinguish two meter sticks standing half anisotropic shielding, or chemical shift interaction, the
a meter apart on the moon, when observing from the earth. observed angular resonance frequency when the external
One of the most powerful fingerprints of nuclei available magnetic field B 0 is parallel to the x axis of the ellipsoid
to the practicing chemist, the chemical shift permits both would be given by the simple equation
a quantitative and qualitative analysis of the molecules
ω obs = γB eff = γB 0 (1 − σ xx ).
containing the nucleus under investigation, since individ-
ual chemical functionalities such as hydrogen in CH 3 , Here, σ xx is the magnitude of the x axis of the shielding
and hydrogen in OH can readily be distinguished, and ellipsoid. Clearly, γB 0 represents the NMR frequency in
the intensity of the NMR lines corresponding to hydrogen
in these two different environments is proportional to the
number of hydrogens in that environment. For example,
the NMR spectrum of protons in pure methyl alcohol
would consist of two lines about 7 ppm apart, with an
intensity ratio I(CH 3 )/I(OH) = 3:1. Further, protons
in all methyl groups resonate in a small frequency
range compared to the difference between protons in
CH 3 and protons in OH. A similar statement applies to
protons in all hydroxyl groups, with some understandable
exceptions, so one talks of the “methyl group range of
absorption,” etc.
The origin of this “chemical shift” is that a nucleus in a
molecule (including infinite molecules such as metals) is FIGURE 1 Representation of the anisotropy of an internal inter-
action as an ellipsoid. The principal axes of the ellipsoid represent
surrounded by an electron charge cloud that is a reflection
resonant frequencies for absorption. The three angles orienting
of the chemical bonding about the nucleus, and that is in this ellipsoid with an arbitrary coordinate system represent the
general some complicated shape. This is to say that it is not other three independent pieces of information.