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isotope 13 C has only a 1.1% abundance and its gyromagnetic ratio (g)isalsomuch
smaller than that of 1 H. This implies that the 13 C-NMR is much less sensitive than
1 H-NMR. The weak signal of 13 C-NMR, however, does not preclude its application in
structural elucidation. In fact, the combined information from 1 H-NMR and 13 C-NMR
spectra helps to determine the exact carbon-hydrogen framework of organic molecules.
Since 1 H-NMR and 13 C-NMR have the same principle, we focus on 1 H-NMR in the
following discussions. For 13 C-NMR, only its unique features are briefly described.
12.2.2 Molecular Structures and NMR Spectra
In this section, we use a high-resolution 1 H-NMR spectrum of 2,6-dinitrotoluene
(Fig. 12.11) to illustrate how molecular structures are correlated with the spectral
characteristics of an NMR spectrum. We need to be acquainted with three spectral
parameters that are directly related to the structures of molecules: chemical shift,
coupling constants, and integral. Chemical shifts are related to the number of signals
and the position of signals. In an 1 H-NMR spectrum, chemical shift can be used to
tell how many types of H and what types of H are there in the molecule. (Similarly in
a 13 C-NMR spectrum, chemical shift can be used to tell how many types of C and
what types of C are there). The integral is the relative peak area or the magnitudes of
signals in an NMR spectrum, so it can be used to tell how many H of each type are
there. The third parameter, coupling constant, can be used to tell the connectivity of
the chemical bonds.
Figure 12.11 1 H-NMR spectrum of 2,6-dinitrotoluene (NMR conditions: 89.56 MHz; 0.044 g
2,6-dinitrotoluene in 0.5 ml CDCl 3 ) (SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/ (National
Institute of Advanced Industrial Science and Technology, accessed Aug. 2006)
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