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Infrared Spectrometry
Section 7I, Chapters 16 & 17 - a very brief overview.
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.
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.
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.
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. 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.
Detectors do not respond to power variations at these high frequencies,
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.
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
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.
For a continuous source (used in FTIR), all waves constructively
interfere when δ = 0. The signal decays rapidly after that.
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.
A small sampling from Chapter 16.
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
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.
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.
Chapter 16 questions/problems:
1, 3, 8, 13