Other Instrumental Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 327

left of the x-axis to be the downfield and the far right to be the upfield. From the
upfield to downfield, the resonance frequency is in an increasing order. Whether a
nucleus appears in an upfield or a downfield depends on the degree of the shielding,
which in turn depends on the electronic environment of the chemical. In general, we
can state that protons in electron-rich molecular environment are more shielded.
These shielded protons sense a smaller magnetic field and thus come into resonance
at a lower frequency. These protons ( 1 H) will appear on the right-hand side of the
NMR spectrum (upfield). Conversely, protons in electron-poor molecular environ-
ment are less shielded. These less shielded or deshielded protons sense a larger
magnetic field and, thus, come into resonance at a higher frequency. These 1 H will
appear on the left-hand side of the NMR spectrum (downfield).
Therefore, a chemical shift provides information about the average effective
magnetic field due to a nuclear shielding, which is directly related to the variations of
the electron density present at various locations within a molecule. For example, the
proton of TMS used in an NMR standard appears at the lowest frequency than most
signals because silicon (Si) is the least electronegative in organic molecules.
Consequently, the methyl protons of TMS are in more electron-dense environmental
than most protons in organic molecules. Thus, the protons in TMS are the most
shielded. On the contrary, the protons in methyl fluoride (CH 3 F) are much less
shielded because the strong electronegativity of fluorine atom will draw electrons
away from the methyl group. At this point, it should become easy to understand, in the
NMR of 2,6-dinitrotoluene, why the signal of two protons (A) closest to the electron-
withdrawing functional group (NO 2 ) will locate in the downfield (d ¼ 8 : 00 ppm),

whereas the least affected methyl protons (C) is in the upfield (d ¼ 2 : 58 ppm).

Integral (Signal Intensity)

Integral is the relative area of absorption peaks in the NMR spectrum. Here an
absorption peak is defined as the family of peaks centered at a particular chemical
shift. For example, if there is a triplet (discussed later) of peaks at a specific chemical
shift, the number is the sum of the area of the three. Just like the peak areas in
chromatographic analysis, integral can be readily obtained by any modern NMR
spectrometer. The rule useful to interpret NMR spectrum is that peak area
is proportional to the number of a given type of spins ( 1 H proton) in the molecule.
We again use Figure 12.11 and a few more examples to illustrate such a relationship.
Consider two chemicals, 1-chloropropane (CH 3 CH 2 CH 2 Br) and 1-bromo-2,2-
dimethylpropane, (CH 3 ) 3 CCH 2 Br. As we discussed, the first chemical should give
three signals due to three types of chemically equivalent protons. When areas are
integrated, the area ratio should give 3:2:2. For (CH 3 ) 3 CCH 2 Br, we have two types
of chemically equivalent protons, namely, nine methyl protons (CH 3 ) and two
methylene proton (CH 2 ). If these two corresponding peak areas are integrated and
normalized, the ratio should be 9:2. Given these examples, now the reader should
become confident in deducing the ratio of three types of chemically equivalent
protons in 2,6-dinitrotoluene in Figure 12.11. The ratio of protons labeled as A, B,
and C should be 2:1:3.