Introductory Theory and Terminology
H171804E_14_001 19 / 86
The splitting of NMR signals in the figure Ethylbenzene results from a magnetic interaction
between neighboring protons. The two H
f
protons are magnetically equivalent and do not
interact with each other. Similarly, the three H
e
protons are magnetically equivalent and do
not affect each other. However, the two H
f
protons and the three He protons are in different
local environments and are “coupled” to each other via their bonding electrons. The net result
of this coupling is that the two groups of protons interact with each other and cause the
splitting of the NMR signals.
The two H
f
protons can combine to exist in three possible magnetic states (this is a result of
spin orientation and hence the term spin-spin coupling). As a result of coupling, the NMR
signals emitted by the H
e
protons resonate at three possible frequencies and a triplet is
observed.
Similarly, the effect of the He protons is to split the H
f
signals. The three He protons can
combine to exist in four possible magnetic states. Consequently the H
f
protons resonate at
four possible frequencies, so the signal is split into a quartet.
The signals from the benzene protons have also been split as a result of magnetic non-
equivalence and resulting spin-spin coupling. The question arises why the CH
2
and CH
3
protons of ethylbenzene interact with each other whereas the two comparable groups of
protons in benzylacetate do not. The answer lies in the number of bonds separating the two
groups. In ethylbenzene the two proton groups are attached to adjacent carbon atoms and
may be expected to interact sufficiently with each other. In benzylacetate however, the two
carbon atoms C
c
and C
b
are connected across two extra bonds between oxygen and another
carbon atom. As a result the proton groups are too far away from each other to display
significant spin-spin coupling.
3.7 Decoupling
The effect of spin-spin coupling can be removed by a technique called "decoupling“. The
effect of decoupling is to mask the presence of a particular proton group, e.g. the H
e
protons
in the Ethylbenzene figure. A spectrum is acquired as if the H
e
protons were absent! This is
achieved by transmitting a decoupling pulse sequence at the H
e
resonance frequency f
e
and
thereby permanently changing the spin orientation of these protons. For the spectrum
illustrated in the Ethylbenzene Spectrum figure the decoupling frequency would be 1.25 ppm
above the TMS peak.
Decoupling pulses tend to be longer and of lower power than excitation pulses. The
Decoupling Experiment figure below is a representation of a decoupling experiment, while the
Ethylbenzene Spectrum with Homodecoupling figure shows the decoupled spectrum. The
CH
2
quartet has now become a singlet. Spectroscopists speak of the quartet collapsing to a
singlet. Furthermore, the area under the singlet should be equal to that of the original quartet
(compare the relative heights of the CH
2
and benzene ring peaks in the two figures). The
signal from the CH
3
group at 1.25 ppm is missing from the decoupled spectrum, because the
decoupling pulses effectively removes the effects of the presence of the CH
3
protons.
To Next Page
Loading...
