Other Instrumental Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 326

Chemical Shift (d)

As shown in Figure 12.11, an NMR spectrum is a plot of NMR signal intensity (the
absorption of rf) vs. chemical shift. Chemical shift (d) measures the difference
between the resonance frequency of the nucleus and a reference standard relative to
the operating frequency of an NMR spectrometer. This quantity is calculated by

n signal n reference

n spectrometer 10 6

d ¼ ð12 : 3Þ

In NMR spectroscopy, this standard is often tetramethylsilane, Si(CH 3 ) 4 ,
abbreviated as TMS. Since the dimensions of n for both signal and TMS are Hz
and the dimension of n spectrometer is MHZ, the unit of chemical shift is reported in
parts per million (ppm). This unit should not be confused with the ppm used to
express the chemical concentration (Chapter 2).
In the preceding section, we know that the signal in the 1 H-NMR originates
from the presence of 1 H in a molecule. If all the protons ( 1 H) in an organic molecule
give exactly the same signal, this would tell us nothing about the structure of the
compound except that it contains protons. The same would be true for 13 C-NMR,
and fortunately this is not the case. As we know, the proton of the 1 H nucleus (or
carbon) is embedded in a cloud of electrons that circulate around the center of
nucleus. In a magnetic field (B applied ¼ B 0 ), the electronic circulation will ''induce''
a local magnetic field (B local ) that is in the opposite direction to the applied magnetic
field (Again, recall from elementary physics that magnetism can be induced from
moving charges and vice versa). This is equivalent to saying that the 1 H nucleus will
''sense'' a magnetic field smaller than an isolated nucleus without an electron.
Because of the variations of electron densities, various 1 H protons in the molecule
will be more or less shielded (called diamagnetic shielding) and sense different
magnetic fields. This will, in turn, requires various frequencies of rf radiation to
bring into the resonance of each type of 1 H.
The above analysis indicates that, by looking at how many groups of signals are
there in a 1 H-NMR spectrum, one should be able to tell how many types of protons
( 1 H) are there in an organic molecule. Protons of the same type have the same
environment and are called chemically equivalent protons. For example, six 1 H
protons in CH 3 OCH 3 are chemically equivalent, hence only one signal (NMR signal
peak) will appear in 1 H-NMR spectrum. 1-Chloropropane (CH 3 CH 2 CH 2 Br) has
three groups of chemically equivalent protons, therefore, the 1 H-NMR spectrum will
have three groups of signals. The three protons in the methyl group (CH 3 ) are
chemically equivalent because of the rotation about the CC bonds. The two
methylene protons and the two protons closest to the bromine atom are chemically
equivalent. The NMR spectrum of 2,6-dinitrotoluene (Fig. 12.11) shows three
groups of peaks at 8.00, 7.55, and 2.58 ppm. This correlates with three groups of
chemically equivalent protons (labeled as A, B, and C) as can be seen from its
chemical structure.
The next question is then how these signals are positioned in an NMR spectrum
with different values of chemical shift (d). By convention, we refer a signal in the far