FTIR Spectroscopy
(Fourier Transform Infrared)
Infrared spectroscopy is an important technique in organic chemistry.
It is an easy way to identify the presence of certain functional groups
in a molecule. Also, one can use the unique collection of absorption
bands to confirm the identity of a pure compound or to detect the
presence of specific impurities.
This page will step you through the principles of operation of the FTIR
Spectrometer, and how it differs from classical continuous-wave (CW)
instruments. To skip directly to spectral interpretation,
Continuous Wave spectrometers
Before delving into FT spectrometry, let's review the principles of a
classical spectrometer. If you have used an optical or UV
spectrometer, the principles are identical:
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A source generates light across the spectrum of interest.
A monochromater (in IR this can be either a salt prism or a
grating with finely spaced etched lines) separates the source
radiation into its different wavelengths.
A slit selects the collection of wavelengths that shine through
the sample at any given time.
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In double beam operation, a beam splitter separates the
incident beam in two; half goes to the sample, and half to a
reference.
The sample absorbs light according to its chemical properties.
A detector collects the radiation that passes through the
sample, and in double-beam operation, compares its energy to
that going through the reference.
The detector puts out an electrical signal, which is normally
sent directly to an analog recorder. A link between the
monochromater and the recorder allows you to record energy
as a function of frequency or wavelength, depending on how
the recorder is calibrated.
Although very accurate instruments can be designed on these
principles, there are several important limitations.
First, the monochromater/slit limits the amount of signal one can get
at a particular resolution. To improve resolution, you must narrow the
slit and decrease sensitivity.
Second, there is no easy way to run multiple scans to build up signalto-noise ratios.
Finally, the instrument must be repetitively calibrated, because the
analog connection between the monochromater position and the
recording device is subject to misalignment and wear.
FT instrumentation
Now let's look at an FT instrument.
We still have a source, a sample and a detector, but everything else
is different. Now, we send all the source energy through an
interferometer and onto the sample. In every scan, all source
radiation gets to the sample! The interferometer is a fundamentally
different piece of equipment than a monochromater. The light passes
through a beamsplitter, which sends the light in two directions at right
angles. One beam goes to a stationary mirror then back to the
beamsplitter. The other goes to a moving mirror. The motion of the
mirror makes the total path length variable versus that taken by the
stationary-mirror beam. When the two meet up again at the
beamsplitter, they recombine, but the difference in path lengths
creates constructive and destructuive interference: an interferogram:
The recombined beam passes through the sample. The sample
absorbs all the different wavelengths characteristic of its spectrum,
and this subtracts specific wavelengths from the interferogram. The
detector now reports variation in energy versus time for all
wavelengths simultaneously. A laser beam is superimposed to
provide a reference for the instrument operation.
Energy versus time is an odd way to record a spectrum, until you
recognize the relationship between time and frequency: they are
reciprocals! A mathematical function called a Fourier tranform allows
us to convert an intensity-vs.-time spectrum into an intensity-vs.frequency spectrum.
The Fourier transform:
A(r) and X(k) are the frequency domain and time domain points,
respectively, for a spectrum of N points.
An interferogram
A spectrum
Now, you have to do the Fourier transform for every point in the
interferogram. You may like to do exponential functions by hand, but
I (and most organic chemists) are far too lazy for that. Fortunately,
even a slow computer can efficiently perform this operation. The
output of the detector is digitized, and a small computer program will
do the transform in a matter of seconds (or less). All modern FT
instruments are computer-interfaced.
There are several advantages to this design:
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All of the source energy gets to the sample, improving the
inherent signal-to-noise ratio.
Resolution is limited by the design of the interferometer. The
longer the path of the moving mirror, the higher the resolution.
Even the least expensive FT instrument provides better
resolution that all but the best CW instruments were capable of.
The digitization and computer interface allows multiple scans to
be collected, also dramatically improving the signal-to-noise
ratio.
Most of the computer programs today allow further
mathematical refinement of the data: you can subtract a
reference spectrum, correct the baseline, edit spurious peaks or
otherwise correct for sample limitations.
The one minor drawback is that the FT instrument is inherently a
single-beam instrument; it cannot use the "channel ratio" trick used in
CW operation. One result is that IR-active atmospheric components
(CO2, H2O) will appear in the spectrum. Usually, a "Background"
spectrum is run, then automatically subtracted from every spectrum.
The spectrum below is such a background scan.
You can see CO2 as the storng doublet at around 2400 cm-1, and
water as the "spiky" peaks in the 3800 and 1600 cm-1. The "bell
curve" shape of the spectrum reflects the output spectrum of the
source: strong in the middle, but falling off at the ends.
FTIR Spectroscopy
(Fourier Transform Infrared) A word about units.
Most spectra using electromagnetic radiation are presented with
wavelength as the X-axis. Originally, IR spectra were presented in
units of micrometers. Unfortunately, a linear axis in micrometers
compresses the region of the spectrum 10-15 mm) that usually has
the largest number of peaks.
One could rectify this by presenting the spectrum on a linear scale vs.
frequency (Hz), but the magnitude is unwieldy (10 mm = 3 x 1013
Hz). A different measure, the wave number, is given the unit cm-1.
Spectral Interpretation
Once you collect a spectrum, the real work begins. Spectra of organic
compounds have two general areas:
4000-1500 cm-1
The Functional
Group Region
1500-400 cm-1
The Fingerprint
Region
Peaks in this
region are
characteristic of
specific kinds of
bonds, and
therefore can be
used to identify
Peaks in this
region arise from
complex
deformations of
the molecule.
They may be
whether a
specific
functional group
is present.
characteristic of
molecular
symmetry, or
combination
bands arising
from multiple
bonds deforming
simultaneously.
The two regions of the spectrum overlap to
a degree. (In fact, one always finds overlap
between different regions of any spectrum;
these designations are "guideposts" to help you
orient yourself.) For example, carbon-chlorine
bonds appear at around 800 cm-1, and C-O
single bonds appear at around 1200-1300 cm1
. Also, benzene rings show "overtones" in the
1500-1700 cm-1 region, even though these
arise from complex ring deformations.
The normal way to approach interpretation
of an IR spectrum is to examine the functional
group region to determine which groups might
be present, then to note any unusually strong
bands or particularly prominent patterns in the
fingerprint region. Finally, if you think you have
identified the compound (usually you need
additional information) you can compare the
spectrum with a reference. Matching the
fingerprint region is a very rigorous test.
Some important IR-active functional groups, and examples of
spectra.
Group
Region
C-H
3000-3100
cm-1 (sp2)
2800-3000
cm-1 (sp3)
C=O
1600-1800
cm-1
Acids: 16501700
Esters: 17401750
Aldehydes:
1720-1750
Ketones:
1720-1750
Amides:16501715
3300-3600
cm-1
Monomeric
O-H
forms: sharp.
(alcohol)
H-bonding
leads to
broadening.
Examples of spectra. (Try to find the characteristic peaks.)
O-H
(acids)
2400-3000
cm-1
Very broad,
medium
intensity
C�C
C�N
2200-2100
cm-1
Usually weak;
maybe not
visible
in internal
alkynes.
Nitriles are
quite strong.
C-O
1200-1300
cm-1
Often difficult
to assign,
depending on
fingerprint
region.
N-H
3400 cm-1
Usually
sharper than
O-H.
A final word about symmetry.
Molecular vibrations give rise to IR bands only if they cause a change
in the dipole moment of the molecule. (This comes out of the
quantum mechanics of molecular absorption of energy, so we aren't
concerned too much with why, yet.) If a stretch does not change the
dipole moment, there won't be any IR band. This is why O2 and N2 in
the atmosphere don't show any IR bands. CO2, however, has a
stretch where one O moves in and the other moves out:
Thus we see this band at 2400 cm-1.
Use of the Nicolet IR Spectrometers
This tutorial will help you with sample preparation, instrument setup
and use of OMNIC software. Please follow the menu below based on
which experiment you are doing. You should have read the
background on instrument design and spectral interpretation before
using this area.
Sample
preparation:
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Thin film on
salt plates
(Viscous
liquids, as in
the Synthesis)
Solution cell
(Volatile
liquids, as in
the
Hacac/Fe(aca
c)3
measurement
)
KBr pellet
(solids, as in
the Fe(acac)3
measurement
)
Mounting the
sample
Use of OMNIC to
collect and
manipulate data