INFRARED ABSORPTION SPECTROSCOPY

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INFRARED ABSORPTION SPECTROSCOPY
Semester Dec Apr 2010
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Learning Outcomes

• By the end of this topic, students should be able
  to
• Explain the principles and the working mechanism
  of infrared (IR) absorption spectroscopy
• Identify the molecular species that absorb IR
  radiation
• Interpret IR spectrum
• Explain stretching and bending vibrations in
  relation to IR absorption
• Determine unknown qualitatively using IR
  absorption
• Draw a schematic diagram of a conventional IR
  instrument and a fourier transform IR instrument
  and explain the function of each component of the
  instrument
• Differentiate between a dispersive IR instrument
  and a FTIR spectrometer

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Infrared spectroscopy

• Mostly for qualitative analysis
• Absorption spectra is recorded as transmittance
  spectra
• Absorption in the infrared region arise from
  molecular vibrational transitions
• Absorption at specific wavelengths
• Thus, IR spectra provides more specific
  qualitative information
• IR spectra is called fingerprints
• - because no other chemical species will have
  identical IR spectrum

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Comparison between transmittance (upper) vs
absorbance (lower) plot
The transmittance spectra provide better contrast
btw intensities of strong and weak bands compared
to absorbance spectra
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Electromagnetic Spectrum

• Energy of IR photon insufficient to cause
  electronic excitation but can cause vibrational
  excitation

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INTRODUCTION

• Comparison between UV-vis and IR
• Energy UV gt vis gt IR
• Frequency UV gt vis gt IR
• Wavelength UV lt vis lt IR

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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
• Analysis of organic materials
• Polyatomic inorganic molecules
• Organometallic compounds

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• IR region of EM spectrum
• ? 780 nm 1000 µm
• Wavenumber 12,800 10cm-1
• IR region subdivided into 3 subregions
• 1. Near IR region (Nearest to the visible)
• - 780 nm to 2.5 µm (12,800 to 4000 cm-1)
• 2. Mid IR region
• - 2.5 to 50 µm (4000 200 cm-1)
• 3. Far IR region
• - 50 to 1000 µm (200 10cm-1)

visible
NEAR
MID
infrared
FAR
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microwave
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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.
• 2. Infrared absorption only occurs when the
  incoming IR photon has sufficient energy for the
  transition to the next allowed vibrational state
• Note If the 2 rules above are not met, no
  absorption can occur

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What happen when a molecule absorbs infrared
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.
• In the absorption process, those frequencies of
  IR radiation which match the natural vibrational
  frequencies of the molecule are absorbed.
• The energy absorbed will increase the amplitude
  of the vibrational motions of the bonds in the
  molecule.

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• NOT ALL bonds in a molecule are capable of
  absorbing IR energy. Only those bonds that have
  change in dipole moment are capable to absorb IR
  radiation.
• The larger the dipole change, the stronger the
  intensity of the band in an IR spectrum.

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• What is a dipole moment?
• is a measure of the extent to which a separation
  exists between the centers of positive and
  negative charge within a molecule.

d-
d
d
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• In heteronuclear diatomic molecule, because of
  the difference in electronegativities of the two
  atoms, one atom acquires a small positive charge
  (q), the other a negative charge (q-).
• This molecule is then said to have a dipole
  moment whose magnitude, µ qd

distance of separation of the charge
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Molecular Species That Absorb Infrared Radiation

• Compound absorb in IR region
• Organic compounds, carbon monoxide
• Compounds DO NOT absorb in IR region
• O2, H2, N2, Cl2

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IR Vibrational Modes
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Molecular vibration
divided into
back forth movement
involves change in bond angles
stretching
bending
wagging
scissoring
symmetrical
asymmetrical
rocking
twisting
in-plane vibration
out of plane vibration
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STRETCHING
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BENDING
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Sample Handling Techniques

• Gases
• evacuated cylindrical cells equipped with
  suitable windows
• Liquid
• sodium chloride windows
• neat liquid
• Solid
• Pellet (KBr)
• Mull

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LIQUID

• a drop of the pure (neat) liquid is squeezed
  between two rock-salt plates to give a layer that
  has thickness 0.01mm or less
• 2 plates held together by capillary mounted in
  the beam path
• What is meant by neat liquid?
• Neat liquid is a pure liquid that do not contain
  any solvent or water.
• This method is applied when the amount of liquid
  is small or when a suitable solvent is
  unavailable

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Solid sample preparation

• There are three ways to prepare solid sample for
  IR spectroscopy.
• Solid that is soluble in solvent can be dissolved
  in a solvent, most commonly carbon tetrachloride
  CCl4.
• Solid that is insoluble in CCl4 or any other IR
  solvents can be prepared either by KBr pellet or
  mulls.

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PELLETING (KBr PELLET)

• Mixing the finely ground solid sample with
  potassium bromide (KBr) and pressing the mixture
  under high pressure (10,000 15,000 psi) in
  special dye.
• KBr pellet can be inserted into a holder in the
  spectrometer.

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MULLS

• Formed by grinding 2-5 mg finely powdered sample,
  presence 1 or 2 drops of a heavy hydrocarbon oil
  (Nujol)
• Mull examined as a film between flat salt plates
• This method applied when solid not soluble in an
  IR transparent solvent, also not convenient
  pelleted in KBr

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• What is a 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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Instrumentation
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IR Instrument

• Dispersive spectrometers
• sequential mode
• Fourier Transform spectrometers
• simultaneous analysis of the full spectra range
  using inferometry

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IR Instrument (Dispersive)

• Important components in IR dispersive spectrometer

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1
2
3
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signal processor readout
source lamp
sample holder
? selector
detector
Detector - Thermocouple - Pyroelectric
transducer - Thermal transducer
Source - Nernst glower - Globar
source - Incandescent wire
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Radiation Sources

• generate a beam with sufficient power in the ?
  region of interest to permit ready detection
  measurement
• provide continuous radiation made up of all ?s
  with the region (continuum source)
• stable output for the period needed to measure
  both P0 and P

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Schematic Diagram of a Double Beam Infrared
Spectrophotometer
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FTIRFourier Transform Infrared
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FTIR

• Why is it developed?
• to overcome limitations encountered with the
  dispersive instruments
• especially slow scanning speed due to individual
  measurement of molecules/atom
• utilize an interferometer

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• 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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Components of Fourier Transform Instrument
- majority of commercially available Fourier
transform infrared instruments are based upon
Michelson interferometer
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1
2
5
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• Advantages (over dispersive instrument)
• high sensitivity
• high resolution
• speed of data acquisition ( data for an entire
  spectrum can be obtained in 1 s or less)

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Interpretation Infrared Spectra
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Infrared Spectra

• IR spectrum is due to specific structural
  features, a specific bond, within the molecule,
  since the vibrational states of individual bonds
  represent 1 vibrational transition.
• e.g. IR spectrum can tell the molecule has an O-H
  bond or a CO or an aromatic ring

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Infrared Spectra
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How to Interpret Infrared Spectra?
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How to analyze IR spectra

• Begin by looking in the region from 4000-1300.
• Look at the CH stretching bands around 3000

Indicates
Are any or all to the right of 3000? alkyl groups (present in most organic molecules)
Are any or all to the left of 3000? a CC bond or aromatic group in the molecule
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2. Look for a carbonyl in the region 1760-1690.
If there is such a band
Indicates
Is an OH band also present? a carboxylic acid group
Is a CO band also present? an ester
Is an aldehyde CH band also present? an aldehyde
Is an NH band also present? an amide
Are none of the above present? a ketone
(also check the exact position of the carbonyl
band for clues as to the type of carbonyl
compound it is)
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3. Look for a broad OH band in the region
3500-3200 cm-1. If there is such a band
Indicates
Is an OH band present? an alcohol or phenol
4. Look for a single or double sharp NH band in
the region 3400-3250 cm-1. If there is such
a band
Indicates
Are there two bands? a primary amine
Is there only one band? a secondary amine
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5. Other structural features to check for
Indicates
Are there CO stretches? an ether (or an ester if there is a carbonyl band too)
Is there a CC stretching band? an alkene
Are there aromatic stretching bands? an aromatic
Is there a CC band? an alkyne
Are there -NO2 bands? a nitro compound
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How to analyze IR spectra

• If there is an absence of major functional group
  bands in the region 4000-1300 cm-1 (other than
  CH stretches), the compound is probably a strict
  hydrocarbon.
• Also check the region from 900-650 cm-1.
  Aromatics, alkyl halides, carboxylic acids,
  amines, and amides show moderate or strong
  absorption bands (bending vibrations) in this
  region.
• As a beginning student, you should not try to
  assign or interpret every peak in the spectrum.
  Concentrate on learning the major bands and
  recognizing their presence and absence in any
  given spectrum.

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ALKANE
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C-H Stretch for sp3 C-H around 3000 2840
cm-1. CH2 Methylene groups have a characteristic
bending absorption at approx 1465
cm-1 CH3 Methyl groups have a characteristic
bending absorption at approx 1375 cm-1 CH2 The
bending (rocking) motion associated with four or
more CH2 groups in an open chain occurs at
about 720 cm-1
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ALKENE
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ALKENE
C-H Stretch for sp2 C-H occurs at values greater
than 3000 cm-1. C-H out-of-plane (oop) bending
occurs in the range 1000 650 cm-1 CC stretch
occurs at 1660 1600 cm-1 often conjugation
moves CC stretch to lower frequencies and
increases the intensity
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ALKYNE
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ALKYNE
Stretch for sp C - H occurs near 3300 cm-1.
Stretch occurs near 2150 cm-1 conjugation moves
stretch to lower frequency.
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AROMATIC RINGS
Stretch for sp2 C-H occurs at values greater than
3000 cm-1.
Ring stretch absorptions occur in pairs at 1600
cm-1 and 1475 cm-1.
Bending occurs at 900 - 690cm-1.
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AROMATIC RINGS
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• C-H Bending ( for Aromatic Ring)
• The out-of-plane (oop) C-H bending is useful in
  order to assign the positions of substituents on
  the aromatic ring.
• Monosubstituted rings
• this substitution pattern always gives a strong
  absorption near 690 cm-1. If this band is absent,
  no monosubstituted ring is present. A second
  strong band usually appears near 750 cm-1.
• Ortho-Disubstituted rings
• one strong band near 750 cm-1.
• Meta- Disubstituted rings
• gives one absorption band near 690 cm-1 plus one
  near 780 cm-1. A third band of medium intensity
  is often found near 880 cm-1.
• Para- Disubstituted rings
• - one strong band appears in the region from 800
  to 850 cm-1.

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Ortho-Disubstituted rings
Bending observed as one strong band near 750 cm-1.
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Meta- Disubstituted rings
- gives one absorption band near 690 cm-1 plus
one near 780 cm-1. A third band of medium
intensity is often found near 880 cm-1.
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Para- Disubstituted rings
- one strong band appears in the region from 800
to 850 cm-1.
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ALCOHOL
Primary alcohol 10
Secondary alcohol 20
Tertiary alcohol 30
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ALCOHOL
O-H The hydrogen-bonded O-H band is a broad peak
at 3400 3300 cm-1. This band is usually the
only one present in an alcohol that has not
been dissolved in a solvent (neat liquid).
C-O-H Bending appears as a broad and weak peak
at 1440 1220 cm-1 often obscured by the CH3
bendings. C-O Stretching vibration usually occurs
in the range 1260 1000 cm-1. This band can be
used to assign a primary, secondary or tertiary
structure to an alcohol.
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PHENOL
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PHENOL
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ETHER
C-O The most prominent band is that due to C-O
stretch, 1300 1000 cm-1. Absence of CO
and O-H is required to ensure that C-O stretch
is not due to an ester or an alcohol.
Phenyl alkyl ethers give two strong bands at
about 1250 1040 cm-1, while aliphatic ethers
give one strong band at about 1120 cm-1.
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CARBONYL COMPOUNDS
cm-1 1810 1800 1760 1735
1725 1715 1710
1690 Anhydride Acid Anhydride Ester
Aldehyde Ketone Carboxylic
Amide (band 1) Chloride (band 2)
acid
Normal base values for the CO stretching
vibrations for carbonyl groups
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A. ALDEHYDE
CO stretch appear in range 1740-1725 cm-1 for
normal aliphatic aldehydes
Conjugation of CO with phenyl 1700 1660 cm-1
for CO and 1600 1450 cm-1 for ring (CC)
C-H Stretch, aldehyde hydrogen (-CHO),
consists of weak bands, one at 2860 - 2800
cm-1 and the other at 2760 2700 cm-1.
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B. KETONE
CO stretch appear in range 1720-1708 cm-1 for
normal aliphatic ketones
Conjugation of CO with phenyl 1700 1680 cm-1
for CO and 1600 1450 cm-1 for ring (CC)
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C. CARBOXYLIC ACID
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D. ESTER
CO stretch appear in range 1750-1735 cm-1 for
normal aliphatic esters
Conjugation of CO with phenyl 1740 1715 cm-1
for CO and 1600 1450 cm-1 for ring (CC)
C O Stretch in two or more bands, one stronger
and broader than the other, occurs in the range
1300 1000 cm-1
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E. AMIDE
10
20
30
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AMIDE
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F. ACID CHLORIDE
Stretch appear in range 1810 -1775 cm-1 in
conjugated chlorides. Conjugation lowers the
frequency to 1780 1760 cm-1
Stretch occurs in the range 730 -550 cm-1
Acid chloride show a very strong band for the CO
group.
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F. ANHYDRIDE
Stretch always has two bands, 1830 -1800 cm-1 and
1775 1740 cm-1, with variable relative
intensity. Conjugation moves the absorption to a
lower frequency. Ring strain (cyclic anhydride)
moves absorptions to a higher frequency.
Stretch (multiple bands) occurs in the range 1300
-900 cm-1
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AMINE
20
10
30
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AMINE
Stretching occurs in the range 3500 3300 cm-1.
Primary amines have two bands. Secondary amines
have one band a vanishingly weak one for
aliphatic compounds and a stronger one for
aromatic secondary amines. Tertiary amines have
no N H stretch.
N H
Bending in primary amines results in a broad band
in the range 1640 1560 cm-1. Secondary amines
absorb near 1500 cm-1
N H
Out-of-plane bending absorption can sometimes be
observed near 800 cm-1
N H
Stretch occurs in the range 1350 1000 cm-1
C N
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Primary Amine
Secondary Amine
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Tertiary Amine
Aromatic Amine