Infrared Spectrometry
Section 7I, Chapters 16 & 17
Remember the energy diagram of a molecule as previously discussed.
The energy of infrared radiation corresponds with the vibrational (as
opposed to electronic for UV-visible radiation) energy levels of
molecules.
Molecular vibrations are basically dependent on two parameters:
1. The masses of the atoms involved in the vibration
2. The strength of the bond holding the atoms together.
Thus infrared spectrometry provides a lot of structural/functional group
information (as opposed to electronic UV-visible spectrometry) in
molecules.
Even though there is totally different information content between the
two techniques the spectrum, in a fundamental sense, is generated the
same way.
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Historically the instrumentation of UV-Vis and IR spectrometers were
very similar. They were often both single channel, usually double beam
instruments.
Now single channel infrared spectrometers are extinct, and multichannel
infrared spectrometers have not yet been perfected.
Fourier transform infrared spectrometers rule this region of the
electromagnetic spectrum – single beam, single channel, multiplexing.
The UV/Vis instruments discussed thus far work in the frequency
domain. That is, the measurements are made as a function of frequency
(or wavelength). Fourier transform (FT) spectrometers work in the time
domain. That is, measurements are made as a function of time, then a
Fourier transform (an advanced mathematical manipulation to the data)
is done to convert that to a frequency domain spectrum.
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The basics of optical FT spectroscopy first introduced in Section 7I.
(Note that FT spectroscopy also dominates NMR instrumentation, and is
also a major player in MS instrumentation (FT-MS, ICR). The principles
are the same, the details of how the time domain information is
generated differs.)
Time domain
and
frequency
domain
spectra have
the same
information
content.
As you know, for some historical reasons the x-axis in infrared
spectrometry is in units of cm-1. The mid-IR region where fundamental
molecular vibrations occur is from 4000 – 200 cm-1. This corresponds to
frequencies of 1.2 x 1014 – 6.0 x 1012 s-1.
You can conceive of acquiring a time domain spectrum with the
following instrumental configuration.
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Detectors do not respond to power variations at these high frequencies,
they have a finite “rise time”.
The rise time for a PMT or
vacuum phototube is
10-8 – 10-9 s.
The rise time for a typical IR
detector is 10-3 – 10-4 s.
Since the detector rise time is
<< the frequency of radiation,
the time domain output from
such an instrument would
contain no frequency information.
The high frequency signal must be modulated to a low frequency signal
that available detectors can respond to. The most common way to do this
is to use a Michelson interferometer.
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1.
Parallel beam
strikes beamsplitter, half of light transmitted, half reflected.
2. One beam reflects of fixed mirror, 2nd off moving mirror.
3. Beam recombines at beamsplitter, half is reflected to detector.
If you put a line source (1 frequency) through the interferometer, the
output is determined by the path difference between the beams when
traveling to fixed and movable mirrors.
Retardation δ = 2(OM – OF)
1. When OM = OF, δ = 0. Beams in phase when recombined at the
beamsplitter, constructive interference, maximum detector signal.
2. When OM and OF differ by 0.25λ, δ = 0.5. So pathlength
difference is 0.5λ. Beams out of phase when recombined at the
beamsplitter, destructive interference, minimum detector signal.
The detector signal is a cos function with max signal when δ = nλ
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If you know the distance δ between crests in cos function [I(δ)], get λ (ν)
of monochromatic radiation in interferometer. The amplitude of the cos
function is proportional to signal intensity.
If you move the mirror at a constant known velocity, and measure the
time for constructive interference to occur [I(t)], you will also know I(δ),
and can get I(λ) and I(ν).
I(t) = time domain spectrum
I(ν) = frequency domain spectrum
The time domain spectrum has been modulated to a lower frequency as a
result of passing through the interferometer, the frequency information
in the time domain signal can now be detected.
I(t) = ∫ I(ν) cos2πtdν
This cos Fourier transform finds the frequencies and intensities which fit
the time domain spectrum using a computer algorithm (FFT using
Cooley-Tukey algorithm).
For a continuous source (used in FTIR), all waves constructively
interfere when δ = 0. The signal decays rapidly after that.
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Note that an FTIR has no
monochromator, no slits,
so much more source
intensity reaching
detector. For methods
which are detector noise
limited (IR, because the
detectors aren’t so great),
this is very important for
high S/N. (Throughput or
Jacquinot Advantage).
As with a multichannel
instrument, these
multiplexing FT
instruments afford very
fast spectral acquisition
times compared to single
channel instruments. Thus signal averaging to enhance S/N is practical.
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A small sampling from Chapter 16, 17.
Near-IR region: 13,000 cm-1  4000 cm-1 (800 nm  2500 nm)
Used a lot for quantitative analysis of industrial processes.
Instrumentation more like UV-Vis
Mid-IR region: 4000 cm-1  400 cm-1
Historically used for organic qualitative analysis. Now that and
more.
Energy:
500 nm = 2.4 kJ/mol.
5000 nm (2000 cm-1) = 0.24 kJ/mol
Absorbed energy results in a change in molecule’s dipole moment, so
the amplitude of molecular vibration increases.
What dictates the vibrational frequency? Start with Hooke’s Law.
At the atomic scale
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For single bonds k ~ 500 N/m
For double bonds k ~ 1000 N/m
For triple bonds k ~ 1500 N/m
Calculate the approximate cm-1 of absorption for a C=O stretching
vibration.
μ = 1.1 x 10-26 kg
Calculate the approximate cm-1 of absorption for a C-O stretching
vibration.
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This is the origin of “group frequencies” for functional group qualitative
analysis. C=O, C=C, C-H, O-H etc. all have absorption frequencies
dictated by bond strength and reduced mass.
FTIR is also useful for quantitative analyses. Beer’s Law applies for a
transmission measurement whether it be in the UV-Vis or IR region. The
enhanced selectivity of IR over UV-Vis is the single most important
difference between these instrumental methods.
Transmission techniques for obtaining IR spectra of gases, liquids,
solids.
Window materials require more thought than for UV-Vis.
Gases: easy with the right cell. Allow the gas to expand, long
pathlengths often required because of low concentration.
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Liquids (solutions). Solvent absorption a problem, spectral subtraction
of the solvent can be a successful approach.
For quantitative work a reproducible (and short) pathlength is required.
Determining the exact
pathlength can be done by
obtaining a “spectrum” of the
empty sample holder.
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Reflected radiation from
cell walls interferes with
transmitted radiation
producing interference
pattern. Constructive
interference occurs when
reflected radiation has
traveled distance equal to
an integer multiple of the
wavelength of the nonreflected transmitted
radiation.
λ = 2b/N
Solids. Particle size must be < radiation λ to avoid scattering. Can do
one of 2 things to acquire transmission spectra:
KBr pellet – about 1 mg of sample in 100 mg KBr. Grind, mix, press to
15,000 psi. Yields a transparent disk when done correctly.
Mulls – make dispersion of finely ground sample ~2 mg in a mineral oil
or fluorinated hydrocarbon. Problem – interference with dispersant.
Reflectance IR Spectroscopy
Mirrored solid surface by specular reflectance where
angle of incidence = angle of reflection
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Powdered material instead of mull or KBr pellet
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Spectra of other solids or liquids that can make intimate contact with an
internal reflection element can be acquired by ATR – Attenuated Total
Reflectance. This utilizes the same optical phenomenon used for optical
fibers (Chapter 7 Section G)
Chapter 16 questions/problems:
1, 3, 8, 10, 13
Chapter 17: 10
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