Infrared Spectroscopy

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Infrared Spectroscopy
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Theory of Infrared Absorption Spectroscopy
IR photons have low energy. The only
transitions that have comparable energy
differences are molecular vibrations and
rotations.
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Theory of Infrared Absorption Spectroscopy
In order for IR absorbance to occur two
conditions must be met
1. There must be a change in the dipole moment of
the molecule as a result of a molecular vibration
(or rotation). The change (or oscillation) in the
dipole moment allows interaction with the
alternating electrical component of the IR
radiation wave. Symmetric molecules (or bonds) do
not absorb IR radiation since there is no dipole
moment.
2. If the frequency of the radiation matches the
natural frequency of the vibration (or rotation),
the IR photon is absorbed and the amplitude of
the vibration increases.
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Theory of Infrared Absorption Spectroscopy
In order for IR absorbance to occur two
conditions must be met
1. There must be a change in the dipole moment of
the molecule as a result of a molecular vibration
(or rotation). The change (or oscillation) in the
dipole moment allows interaction with the
alternating electrical component of the IR
radiation wave. Symmetric molecules (or bonds) do
not absorb IR radiation since there is no dipole
moment.
2. If the frequency of the radiation matches the
natural frequency of the vibration (or rotation),
the IR photon is absorbed and the amplitude of
the vibration increases.
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DE hn
There are three types of molecular transitions
that occur in IR
a) Rotational transitions
When an asymmetric molecule rotates about its
center of mass, the dipole moment seems to
fluctuate.
Quite low energy, show up as sharp lines that
subdivide vibrational peaks in gas phase spectra.
b) Vibrational-rotational transitions
complex transitions that arise from changes in
the molecular dipole moment due to the
combination of a bond vibration and molecular
rotation.
c) Vibrational transitions
The most important transitions observed in
qualitative mid-IR spectroscopy.
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Vibrational Modes
1. Stretching - the rhythmic movement along a
bond axis wit a subsequent increase and decrease
in bond length.
2. Bending - a change in bond angle or movement
of a group of atoms with respect to the rest of
the molecule.
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The Vibrational Modes of Water
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Mechanical Model of Stretching Vibrations
1. Simple harmonic oscillator.
Hookes Law (restoring force of a spring is
proportional to the displacement)
F -ky
Where F Force k Force Constant (stiffness
of spring) y Displacement
Natural oscillation frequency of a mechanical
oscillator depends on a) mass of the object
b) force constant of the spring (bond)
The oscillation frequency is independent of the
amount of energy imparted to the spring.
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Frequency of absorption of radiation can be
predicted with a modified Hookes Law.
Where n wavenumber of the abs. peak (cm-1) c
speed of light (3 x 1010 cm/s) k force
constant m reduced mass of the atoms
Where Mx mass of atom x in kg My mass of
atom y in kg
Force constants are expressed in N/m (N
kgm/s2)
- Range from 3 x 102 to 8 x 102 N/m for single
bonds - 500 N/m is a good average force constant
for single bonds when predicting k. - k n(500
N/m) for multiple bonds where n is the bond order
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Example 1 Calculate the force constant of the
carbonyl bond in the following spectrum.
Example 2 Predict the wavenumber of a peak
arising from a nitrile stretch.
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Anharmonic oscillators
In reality, bonds act as anharmonic oscillators
because as atoms get close, they repel one
another, and at some point a stretched bond will
break.
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IR Sources and Detectors
Sources - inert solids that heat electrically to
1500 2200 K.
Emit blackbody radiation produced by atomic and
molecular oscillations excited in the solid by
thermal energy.
The inert solid glows when heated.
Common sources
1. Nernst glower - constructed of a rod of a rare
earth oxide (lanthanide) with platinum leads.
2. Globar - Silicon carbide rod with water cooled
contacts to prevent arcing.
3. Incandescent wire - tightly wound wire heated
electrically. Longer life but lower intensity.
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Detectors measure minute changes in
temperature.
1. Thermal transducer
Constructed of a bimetal junction, which has a
temperature dependant potential (V). (similar to
a thermocouple)
Have a slow response time, so they are not well
suited to FT-IR.
2. Pyroelectric transducer
Constructed of crystalline wafers of triglycine
sulfate (TGS) that have a strong temperature
dependent polarization.
Have a fast response time and are well suited
for FT-IR.
3. Photoconducting transducer
Constructed of a semiconducting material (lead
sulfide, mercury/cadmium telluride, or indium
antimonide) deposited on a glass surface and
sealed in an evacuated envelope to protect the
semiconducting material from the environment.
Absorption of radiation promotes nonconducting
valence electrons to a conducting state, thus
decreasing the resistance (W) of the
semiconductor.
Fast response time, but require cooling by
liquid N2.
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Multiplexing (FT) Spectrometers
Collect data in the time domain and convert to
the frequency domain by Fourier Transform.
Detectors are not fast enough to respond to
power variations at high frequency (1012 to 1015
Hz) so the signal is modulated by a Michelson
interferometer to a lower frequency that is
directly proportional to the high frequency.
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B. Multiplexing (FT) Spectrometers
1. Michelson Interferometer
The source beam is split into two beams.
One beam goes to a stationary mirror and the
other goes to a moveable mirror.
Movement of the mirror at a constant rate and
recombination of the two beams results in a
signal that is modulated by constructive and
destructive interference (Interferogram).
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Multiplexing (FT) Spectrometers
The frequency of the radiation (n) is directly
related to the frequency of the interferogram
(f).
n frequency of radiation f frequency of
inteferogram nm velocity of the mirror c
speed of light (3.00 x 1010 cm/s)
FT-IR spectrometers use a polychromatic source
and collect the entire spectrum simultaneously
and decode the spectrum by Fourier Transform.
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Multiplexing (FT) Spectrometers
2. FT-IR instrument
Mirror length of travel ranges from 1 to 20
cm.
Scan rates from 0.1 to 10 cm/s
Detectors are usually pyroelectric or
photoconducting.
Use multiple scans and signal averaging to
improve S/N.
Cost 10,000 - 20,000
Have virtually replaced dispersive instruments.
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Performance Characteristics
Range 7800 to 350 cm-1 (less expensive)
25,000 to 10 cm-1 (Near to far IR, expensive)
Resolution 8 cm-1 to 0.01 cm-1
Qualitative Very good, functional groups
are identifiable
Quantitative Dispersive poor FTIR -
fair