Chemistry Reference
In-Depth Information
hydrogen atoms in the CH
2
groups, and so there are two distinct sets of hydrogen atoms.
This gives the same division of the hydrogen atoms between the two environments that
was implied from the spectral data. The dimethylcyclohexene alternative has only a
C
2
axis and no other symmetry axes or planes. As we shall see below, this means that it has
hydrogen atoms in at least three different environments, and so would be expected to give
a more complex
1
H NMR spectrum.
The absorption event in an NMR experiment takes a short, but finite, time. If the pro-
ton in a
1
H NMR experiment is moving rapidly, then it may experience more than one
environment on the time scale of the experiment and only the average chemical shift will
be observed. For example, methyl groups are usually undergoing rapid rotation at room
temperature, and so the three hydrogen atoms will appear equivalent even though they
may not be linked by symmetry operations. In the spectrum of dimethylcyclohexene we
would see only a single chemical shift for all six of the methyl protons, since the two
methyl groups are linked by symmetry operations in the static structure. Similarly, molec-
ular motion would be expected to cause the hydrogen atoms of each CH
2
group in the ring
to interchange rapidly from axial to equatorial as the ring changes its conformation. The
C
2
axis implies that the two CH
2
groups adjacent to the methylated carbon atoms are equiv-
alent and that the remaining CH
2
groups are also equivalent to one another. This gives at
least three distinct
1
H NMR signals.
This type of analysis can also give some information on the dynamics of molecular
motion. Figure 1.24 shows the structure of Al
2
(CH
3
)
6
. The symmetry axes and mirror
(a)
H
3
C
H
3
C
H
3
C
CH
3
Al
C
2
(
Y
)
Al
CH
3
C
H
3
C
2
(
Z
)
C
2
(
X
)
(b)
σ
(
XZ
)
σ
(
XY
)
H
3
C
H
3
C
H
3
C
CH
3
Al
Al
CH
3
σ
(
YZ
)
C
H
3
Figure 1.24
The structure of Al
2
(CH
3
)
6
showing (a) the rotation axes and (b) mirror planes
assuming the structure of the methyl group can be ignored.
