Chemistry Reference
In-Depth Information
signal; and it naturally resonates at very high field position. It therefore
appears at the right-hand side of a conventionally displayed spectrum. The
instrument is zeroed on TMS and all measurements are made relative to the
TMS peak.
Deuterium exchange
Deuterated water (deuterium oxide or D
2
O) does not give rise to a signal in
NMR and this can be used as a diagnostic test for certain types of hydrogen
atom. Functional groups in which hydrogen is bonded to an electronegative
atom such as oxygen (
SH) are
termed 'exchangeable' because when D
2
O is added to the spectrum, the
weakly acidic hydrogen can exchange with a deuterium from the water
according to the equation below:
a
OH), nitrogen (
a
NH
2
) or sulfur (
a
R-OH
D
2
O
3
R-OD
DHO
The effect of this is that the signal on the NMR due to the acidic hydrogen
disappears when D
2
O is added. The spectrum is run normally in a non-
polar solvent such as deuterated chloroform; then, if the presence of
exchangeable hydrogen atoms in the sample is suspected, a few drops of
D
2
O are added and the original spectrum is re-run. If the peak in question
disappears in the presence of D
2
O, this is proof of the presence of
exchangeable hydrogen atoms in the molecule. An unfortunate side-effect of
adding D
2
O to the sample is that a large peak due to mono-deuterated
water, DHO, appears in the spectrum. This peak can sometimes hide more
interesting peaks in the spectrum.
Structure elucidation using NMR
When NMR is used to elucidate the structure of an unknown compound,
there are three pieces of information which should be considered:
•
the position of resonance of the peak (or
chemical shift
)
•
the number of hydrogen atoms causing the signal (
integration
)
•
the number of peaks constituting the signal (
multiplicity
).
Chemical shift
The position of a line in NMR spectroscopy is the most important piece of
information when solving an unknown spectrum. The difference in
frequency of resonance of different groups of atoms is a very small fraction
of the total magnetic field and, as stated above, the absolute position of
resonance cannot be determined due to variations in the strength of
