Infrared Absorption Spectros
Infrared Absorption Spectros
Infrared Absorption Spectros
INFRARED ABSORPTION
SPECTROSCOPY
FTIR
7.1 Theory of IR absorption
7.2 IR Instrumentation ; Dispersive and FTIR
7.3 Application : Mid-IR absorption spectroscopy
7.4 Sample handling
7.5 Correlation charts and tables
7.6 Interpretation of IR spectra of simple organic
compounds
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IR SPECTRAL REGION
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Mostly for qualitative analysis.
Absorption spectra is recorded as transmittance spectra.
Absorption in the infrared region arise from molecular
vibrational transitions.
Absorption for every substance are at specific
wavelengths where IR spectra provides more specific
qualitative information.
IR spectra is called “fingerprints” because no other
chemical species will have similar IR spectrum.
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TRANSMITTANCE VS ABSORBANCE
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INFRARED SPECTROSCOPY
Infrared (IR) spectroscopy deals with the interaction of
infrared radiation with matter.
IR spectrum provides:
Important information about its chemical nature and
molecular structure
IR applicability for:
Analysis of organic materials
Polyatomic inorganic molecules
Organometallic compounds
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WHEN IR ABSORPTION OCCUR?
1. IR absorption only occurs when IR radiation interacts
with a molecule undergoing a change in dipole moment
as it vibrates or rotates.
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WHAT HAPPEN WHEN A MOLECULES
ABSORBS IR RADIATION?
Absorption of IR radiation corresponds to energy changes
on the order of 8 to 40 kJ/mole.
Radiation in this energy range corresponds to stretching
and bending vibrational frequencies of the bonds in most
covalent molecules.
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DIPOLE MOMENT
Dipole moment - a measure of the extent to which a
separation exists between the centers of positive and
negative charge within a molecule.
δ-
O
δ+ δ+
H H
1
0
DIPOLE MOMENT
In heteronuclear diatomic molecule, because of the
difference in electronegativities of the two atoms, one
atom acquires a small positive charge (δ+), the other a
negative charge (δ-).
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1
DIPOLE MOMENT
The charge distribution around a molecule is not
symmetric because one of the atom has a higher electron
density.
Example: HCl
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DIPOLE MOMENT
Only bonds which have significant dipole moments will
absorb infrared radiation.
2c m1m2
ν is the wavenumber of the stretching vibration
f is the force constant
m1 and m2 are the masses of the atoms
The exact position of the absorption band depends on
• electron delocalization
• the electronic effect of neighboring substituents
• hydrogen bonding
O O-
O
CH3CCH2CH2CH3
C O C O
at 1720 cm–1 at 1680 cm–1
Putting an atom other than carbon next to the carbonyl group
causes the position of the carbonyl absorption band to shift
~1050 cm–1
CH3CH2 OH
~1050 cm–1
CH3CH2 O CH2CH3
O O-
C C ~1250 cm–1
H3C OH H3C OH
O O-
C C ~1250 cm–1 and 1050 cm–1
H3C O CH3 H3C O CH3
STRONG ABSORBERS
The carbonyl group is one
d-
O
of the strongest absorbers d+
C
infrared beam
C C
+ + O d+
O
- - d-
2
0
MOLECULES
Molecules are composed of atoms held together by
chemical bonds.
The atoms in a molecule are always moving or “vibrating”.
The intensities of vibrations increase when IR radiation is
absorbed
Each chemical bond requires a precise amount of energy
to make it vibrate
Each frequency of IR radiation provides a precise
amount.
c
E h h hc
POLYSTYRENE SPECTRUM
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ROTATIONAL TRANSITIONS
Small energy is required to cause a change in rotational
level
100 cm-1 or > 100m
Spectrum of a gas
A series of closely spaced lines
There are several rotational energy levels for each
vibrational level.
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TYPES OF MOLECULAR VIBRATIONS
Stretching
A continuous change in the interatomic distance along the
axis of the bands between the two atoms
Symmetrical
Asymmetrical
Bending
A change in the angle between two bonds
4 types
Scissoring
Rocking
Wagging
Twisting
Molecular vibration
divided into
back & forth involves change
movement in bond angles
stretching bending
wagging
scissoring
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1
STRETCHING
3
2
STRETCHING
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BENDING
3
4
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ELECTROMAGNETIC SPECTRUM
3
Energy of IR photon insufficient to cause electronic
6 excitation but can cause vibrational excitation
HOOKE’S LAW
VIBRATIONS
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larger K,
higher frequency
m1 m2
μ
1 m1 m2
K
=
2c larger atom masses,
lower frequency
constants
m1 increasing K
=
C=C > C=C > C-C
2150 1650 1200
increasing
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IR INSTRUMENT
Dispersive spectrometers
sequential mode
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1
DISPERSIVE IR INSTRUMENT
Important components in IR dispersive spectrometer
1 2 3 4 5
source sample λ signal processor
detector & readout
lamp holder selector
Detector:
Source:
- Thermocouple
- Nernst glower
- Pyroelectric transducer
- Globar source
- Thermal transducer
- Incandescent wire
4 - Nichrome wire
2
RADIATION SOURCES
Generate a beam with sufficient power in the region of
interest to permit ready detection & measurement.
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3
SCHEMATIC DIAGRAM OF
IR SPECTROPHOTOMETER
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4
FOURIER TRANSFORM INFRARED
(FTIR)
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FTIR
Why FTIR is developed?
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6
FTIR SPECTROMETER
The main optical components are:
The IR source
The interferometer
The beamsplitter
The laser
The IR detector
FTIR
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IR SOURCE
The IR source produces IR radiation beam that
travels through the spectrometer passing through the
sample and to the detector
INTERFEROMETER
Beamsplitter
Two mirrors
fixed
movable
Interferometer
Special instrument which can read IR frequencies
simultaneously.
Faster method than dispersive instrument.
Interferograms are transformed into frequency spectrums
by using mathematical technique called Fourier
Transformation.
FT
Calculations
interferograms IR spectrum
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1
BEAMSPLITTER
Moving
mirror
IR source
He-Ne Laser
Beamsplitter
Sample
Detector
COMPONENTS OF FTIR
Majority of commercially available FTIR instruments are based
upon Michelson interferometer.
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4
5 2
6
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1
The computer reads the interferogram and uses Fourier
transformation to decode the intensity information for
each frequency and presents a spectrum
The spectrometer measures the intensity of a specially
encoded IR beam after it has passed through the sample.
The resulting signal, ”interferogram”
contains information about all frequencies present in the
beam
The IR beam exits the interferometer.
It is deflected by a couple of mirrors before it reaches the
detector
Produces electrical signal in response to the encoded
radiation striking it.
FOURIER TRANSFORMATION
ADVANTAGES OF FTIR
All frequencies are measured simultaneously. Typical scan
times are only a few seconds.
The energy throughput is higher for any resolution, giving a
higher signal to noise ratio.
The laser wavelength is used as a reference for the
calculation of band positions, and is precise.
Stray light can be prevented.
Resolution is constant for the whole spectral range.
Robustness as the FT instruments only have one moving
part.
APPLICATIONS OF IR
SPECTROMETRY
2. Liquid
sodium chloride windows.
“neat” liquid
3. Solid
Pellet (KBr)
Mull
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0
MEASURING GASES
This gas cell provides a fixed
pathlength that is 2 m long
Liquid cells
Holds the liquid between two crystal made from materials
that completely transmit IR radiation.
SOLIDS
There are 2 ways to prepare solid sample for IR
spectroscopy.
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4
SOLIDS
Solid in a liquid or solid matrix
Samples are ground to fine powder
To avoid effects of scattered radiation
KBr PELLET
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6
SOLID- KBr DISK SAMPLING
PELLETING
Most common technique; KBr pelleting
Halide salts have a property of cold flow
Translucent property when pressure is applied to finely
powdered salts
~1 mg (or less) sample is mixed with ~ 100 mg dried KBr
Mix and grind in a mortar and pestle
Apply 10,000 to 15,000 psi in a die
Produce a transparent disk or pellet
Best done in vacuum or store the pellet in a desiccator
before measurement
Bands at 3450 and 1640 cm-1 due to absorbed moisture
MULLS
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9
What is Mull
A thick paste formed by grinding an insoluble solid with
an inert liquid and used for studying spectra of the solid.
What is Nujol
A trade name for a heavy medicinal liquid paraffin.
Extensively used as a mulling agent in spectroscopy.
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0
MULLS
Solids
Not soluble in an IR transparent solvent
KBr pelleting is not convenient
Grind ~ 2 – 5 mg fine powder in a drop of mulling
agents
Heavy hydrocarbon (Nujol)
Fluorolube (halogenated polymer)
Examine as thin film between two salt discs
SOLUTIONS
Cost
Range of transparency
Solubility in solvent
Reactivity with sample or solvent
Identify:
Organic
Inorganic species
Biological
LIMITATIONS TO THE USE OF
CORRELATION CHARTS