Physical Chemistry , 1st ed.

What equation 16.27 implies is that different nuclei of the same element
experience different total magnetic fields due to their chemical environment,
and so will absorb at slightly different frequencies of radiation. This is best
illustrated by an example such as that in Figure 16.14. The four hydrogen
atoms in methane, CH 4 , are chemically equivalent. Therefore, they will ab-
sorb the same frequency of radiation in an NMR experiment. The six hydro-
gens in ethane, CH 3 CH 3 , are also chemically equivalent and so will also absorb
the same frequency of radiation. However, the protons in propane,
CH 3 CH 2 CH 3 , are not all chemically equivalent. The hydrogens on the end car-
bons are equivalent to each other, but the hydrogens in the middle methylene
carbon are in a different chemical environment than the methyl carbons. They
will experience a slightly different total magnetic field, and so will absorb a dif-
ferent frequency of radiation. This will lead to two separate absorptions in an
NMR spectrum, one for each type of hydrogen (at least in a low-resolution
NMR spectrum). In this manner, structural information about a molecule be-
gins to be obtained.

Example 16.11

How many chemically different hydrogens in each of the following, and how

many NMR absorptions would be expected?

a.Benzene, C 6 H 6

b.Toluene, C 6 H 5 CH 3

16.5 Nuclear Magnetic Resonance 575

8
ppm

6 4 2 0

8
ppm

6 4 2 0

8
ppm

6 4 2 0

Figure 16.14 The NMR spectra of the first three alkanes shows that the protons on “differ-
ent” carbons absorb at different points. This is part of the value of NMR spectroscopy.