Chemistry Reference
In-Depth Information
standard. For carbon (
13
C) and hydrogen (
1
H) NMR experiments, a commonly used
standard is tetramethylsilane (TMS). The chemical shift
δ
is defined as
δ
=
ν
−
ν
TMS
ν
TMS
×
10
6
(1.1)
where
ν
TMS
is the absorption of the
corresponding element (
13
Cor
1
H) in the standard. This chemical shift is dimensionless,
since the reference absorption frequency appears in the denominator; this ensures that
experiments on spectrometers with different probe frequencies give the same chemical
shift values for a given sample. The difference in frequencies observed between samples
and the standard is typically only a few kilohertz, whereas the probe frequency will be in
the 200-1000 MHz range. Hence, the factor of 10
6
is introduced to give chemical shifts
that can be quoted using simple numbers, typically 0-12 for
1
H NMR, and the shifts are
quoted as parts per million (ppm).
The chemical shifts from a
1
H NMR spectrum are used as an indication of the chemical
environment of each proton in the molecule of the sample. If two hydrogen atoms are
linked by a symmetry operation, then they will have the same environment and are referred
to as chemically equivalent. The line in the
1
H NMR spectrum for each will occur at exactly
the same position, and so the intensity of the peak at this chemical shift will be twice that
of a hydrogen atom in a unique environment, i.e. not linked to any other hydrogen atoms
by symmetry operations. This allows us to use the intensity of the peaks as an indication
of the number of equivalent hydrogen atoms in a molecule and so may help to determine
the sample's molecular structure.
As an example, Figure 1.21 shows a computer-generated
1
H NMR spectrum for a com-
pound with the chemical formula C
8
H
14
. There are several possibilities for the molecular
structure of this sample, two of which are given in Figure 1.22. The spectrum shows two
peaks with the lower chemical shift having a height six times that of the other signal.
ν
is the absorption frequency of the sample atoms and
3
2
Chemical shift (ppm)
1
0
Figure 1.21
A
1
H NMR spectrum for a compound with the chemical formula C
8
H
14
generated using the ChemDraw package.
