Chemical Engineering April 2020 - 26

Facts At Your Fingertips
Nuclear Magnetic Resonance Spectroscopy
Department Editor: Scott Jenkins
N
uclear magnetic resonance
(NMR) spectroscopy is an
analytical chemistry technique
used to elucidate molecular
structure and help identity unknown
compounds. It is a powerful research
tool used for quality control,
reaction monitoring, purity analysis
and other functions.
Nuclear spin
The properties of subatomic particles
give rise to spin, a form of intrinsic angular
momentum that can be represented
by imagining the particles as
tiny spheres spinning on their axes.
Although nuclei don't actually spin,
the concept can help explain the principles
underlying NMR.
In atoms where the total number of
neutrons or protons is an odd number,
the nuclei possess an overall
spin. For example, 12C has a spin of
0, while the isotope 13C (6 protons,
7 neutrons), has nonzero spin. NMR
spectroscopy only detects nuclei with
nonzero spin. Examples include the
following: 13C, 1H, 15N, 19F, and 31P.
Magnetic fields
Behaving as spinning charged particles,
each NMR-active nuclei generates
its own magnetic field, which
is oriented in a particular direction.
Normally, the magnetic moments
generated by a sample are oriented
randomly in all possible directions.
But in an externally applied magnetic
field, the nuclei align their magnetic
fields with the applied field - either
in the same direction of the applied
field (known as the alpha spin state),
or aligned against it (beta spin state).
Alpha state is lower energy (more
stable and more common) than beta,
and the difference between the two
is proportional to the strength of the
external magnetic field. The strength
of an NMR magnet determines spectral
resolution. Modern superconducting
magnets routinely reach magnetic
fields greater than 11 Tesla.
Resonance and relaxation
To generate an NMR spectrum, a
sample is placed between the poles
26
of a superconducting
magnet and is irradiated
with bursts of
radio-frequency energy.
The radio waves
are absorbed by the
nuclei when the frequency
of the radiation
matches the frequency
at which the nuclei
precess around its
axis. When the energy
is absorbed, the protons
flip from the alpha
state into the beta
(higher energy) state.
When this occurs, the
nuclei are said to be in resonance.
When a nuclei returns to its original
Ethyl acetate
H
H
H
Higher chemical
shift for protons
colored red
because of more
deshielding
H
5
4
H
C - C - O - C - C H
O
H
H H
H
Reference signal
for TMS
Spin-spin coupling
H
1,300
1,200
1,100
1,000
900
800
700
600
500
400
300
200
100
3
2
Chemical shift (ppm)
1
-100
FIGURE 1. An example H-NMR spectrum for the compound ethyl acetate
shows different chemical shifts for the three sets of protons in the compound,
each having a different electronic environment
state from an excited energy state, it
is said to undergo " relaxation. " As the
nuclei relax, they emit electromagnetic
signals whose frequencies can be
detected by an NMR spectrometer.
The signals (peaks) are plotted on a
graph of frequency versus intensity.
Chemical shift
The precise resonant frequency at
which the flip from lower to higher
energy occurs depends on the effective
magnetic field at the nucleus.
And the effective magnetic field is influenced
by electrons around the nucleus,
which shield the nucleus from
the applied magnetic field. In general,
the more electronegative a nucleus
is, the less shielding occurs and the
higher the resonant frequency. In this
way, the chemical environment surrounding
the nucleus, and hence, the
chemical structure of a compound,
can be probed.
In 1H-NMR, each group of chemically
equivalent protons (protons with
the same electronic environment) in
a molecule gives rise to a separate
signal, which appears at a slightly different
resonant frequency. These different
frequencies are expressed as
shifts away from a reference signal,
which is set to zero. For 1H-NMR,
tetramethylsilane (TMS) is used as
the reference signal, and the distance
from the reference is called a
chemical shift (Figure 1).
The effective magnetic field experienced
by a nucleus also depends on
the orientation of neighboring nuclei.
This effect, known as spin-spin coupling,
results in the splitting of signals
into two or more lines. This indicates
the number of chemically bonded
nuclei near the observed nuclei. Splitting
patterns allow experimenters to
determine structural information.
Applications
The following are examples of industrial
applications for NMR.
* Pharmaceuticals. NMR is used for
drug discovery, structural confirmation
and identifying pharmaceutical
effects within natural products
* Biofuels. NMR is used to characterize
biodiesel, such as quantifying
its percentage in fuel mixtures
* Polymers. NMR can help observe
polymerization reaction completeness
and uniformity, qualify relative
composition, identify stereoisomers,
and determine modular number,
molecular weight and polydispersity
index
* Process
analytical
technology.
NMRs can act as digital detectors
for process-line reaction monitoring
* Catalysts. NMR spectroscopy can
help characterize catalysts deposited
on substrates
* Petrochemicals. NMR provides
information on the identities and
characteristics of fluids present in
a mixture, and their compositions
and viscosities
n
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