1. Spectroscopy
Infrared Spectra

2. INTRODUCTION
• Atoms and molecules interact with electromagnetic
radiation (EMR) in a wide variety of ways.
• Atoms and molecules may absorb and/or emit EMR.
• Absorption of EMR stimulates different types of motion in
atoms and/or molecules.
• The patterns of absorption (wavelengths absorbed and
to what extent) and/or emission (wavelengths emitted
and their respective intensities) are called ‘spectra’.
• The field of spectroscopy is concerned with the
interpretation of spectra in terms of atomic and
molecular structure (and environment).

4. THE ELECTROMAGNETIC SPECTRUMTHE ELECTROMAGNETIC SPECTRUM
©Atkins & de Paula 2002

5. © Atkins and de Paula 2002
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THE ENERGIES OF
ELECTROMAGNETIC WAVES
THE ENERGIES OF
ELECTROMAGNETIC WAVES
υ
λ
ν
υ
λ
ν
λ
υλυ
hcN
c
hNhNE
hc
c
hhE
c
AAA ===
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1
(nu-bar) represents wavenumber,
the number of wavelengths in 1 cm
λ

6. Theory of IR
• Absorption of IR is restricted to
compounds with small energy differences
in the possible vibrational and rotational
states.
• Molecular rotations: No use.
• Molecular vibrations:
• Stretching: Change in inter-atomic
distance along bond axis

7. Bending: Change in angle between two bonds. There are four types of bend:
Rocking
Scissoring
Wagging
Twisting
The stretching frequency of a bond can be approximated by Hooke’s Law.

8. Vibrational coupling
interaction between vibrations can occur (coupling ) if the vibrating bonds are
joined to a single central atom. Vibrational coupling is influenced by a number
of factors;
Strong coupling of stretching vibrations occurs when there is a common atom
between the two vibrating bonds .
Coupling of bending vibrations occurs when there is a common bond
between vibrating groups
Coupling between a stretching vibration and a bending vibration occurs if the
stretching bond is one side of an angle varied by bending vibration
Coupling is greatest when the coupled groups have approximately equal
energies
A molecule consisting of N atomshas a total of 3n degrees of freedom,
corresponding to the coordinates of each atom in the molecule. a nonlinear
molecule,3 of these degrees are rotational and 3 are translational and the
remaining correspond to fundamental vibrations; in a linear molecule, 2
degrees are rotational and 3 are translational. The net number of fundamental
vibrations for nonlinear and linear molecules is therefore:

9. molecule degrees of freedom
nonlinear 3n– 6
linear 3n– 5
propane,C3H8, has 27 fundamental vibrations, and therefore 27 bands in an IR
spectrum. Water, which is nonlinear, has three fundamental vibrations.
Factors affecting Vibrational Frequencies:
1. Coupled Vibrations and Fermi Resonane: one stretching absorption
frequency for an isolated C-H bond but in methylene (-CH2) – two
absorptions i.e. symmetric and assymetric (higher wave number).
Fermi Resonance: energy of an overtone level coincide with fundamental
mode of different vibrations i.e. transfer of energy from fundamental to
overtone and back again.
2. Electronic Effects:
Inductive:
+I: lengthening or weakening of bond, absorption at lower wave number.
E.g. alkyl group.
HCHO : 1750cm-1
, CH3CHO : 1745cm-1

10. -I: increases wave number.
E.g. CH3COCH3:1715cm-1
, ClCH2COCH3: 1725cm-1
Mesomeric Effects: Conjugation lowers wave number.
E.g. Methylvenyl ketone: CH3COCH=CH2; 1706 cm-1
Acetohenone: C6H5COCH3: 1693 cm-1
Hydrogen Bonding: Downward frequency shift. Stronger
bonding, greater absorption shift towards lower wave
number.
Intermolecular: broad bands, concentration dependent-
band disappear on dilution.
Intramolecular: sharp bands, concentration independent.

11. Detection Electronics
and Computer
Infrared
Source
Determines Frequencies
of Infrared Absorbed and
plots them on a chart
Sample
Simplified Infrared SpectrophotometerSimplified Infrared Spectrophotometer
NaCl
plates
Absorption
“peaks”
Infrared
Spectrum
frequency
intensity of
absorption
(decreasing)
focusing
mirror

12. IR Radiation Source:
• Incandescent lamp: far IR, low emissivity
• Nernst Glower: Hollow rod: 2mm dia, 30
mm length, earth oxides of Zirconia, yttria
and thoria
• Non-conducting at room temperature.
• Heated between 1000-18000
C
• GLOBAR SOURCE:sintored silicon
carbide rod: 50mm length, 4mm dia.

13. Cont…
• Self-starting
• MERCURY ARC:
2. Monochromators:
Prism
Grating
3. Sampling of Substances:
Solids:Nujol Mull:

14. Potassium Bromide disk
Thin Films
Liquids
Gases

15. DETECTORS
• Bolometers
• Thermocouple
• Thermistors
• Golay cells
• Photoconductivity cell
• Semiconductor Detectors
• Pyroelectric
• Fourier Transform

16. Single Beam

17. Instrumentation for Spectroscopy
Source
Wavelengt
h Selector
Sample
Detector Data
Readout
Absorption Spectroscopy

18. • Double Beam Spectrophotometer (figure
above)
– Analysis of Reference and Sample
Simulataneously
• Single Beam
– Only one detector – reference and
sample cuvettes move to come in line to
the path of light.

19. Capabilities of Infrared
Analysis
 Identification and quantitation of organic solid,
liquid or gas samples.
 Analysis of powders, solids, gels, emulsions,
pastes, pure liquids and solutions, polymers,
pure and mixed gases.
 Infrared used for research, methods
development, quality control and quality
assurance applications.
 Samples range in size from single fibers only
20 microns in length to atmospheric pollution
studies involving large areas.

20. Applications of Infrared
Analysis
 Pharmaceutical research
 Forensic investigations
 Polymer analysis
 Lubricant formulation and fuel additives
 Foods research
 Quality assurance and control
 Environmental and water quality
analysis methods
 Biochemical and biomedical research
 Coatings and surfactants
 Etc.

21. A Functional Group Chart
O-H str
NH str
COO-H
=C-H str
Csp3-H
≡C-H
-(C=O)-H
C≡N
C≡C
C=O
-C=N
-C=C
phenyl
C-O
C-N
F C-X
4000 3600 3200 2800 2400 2000 1600 1200 800 group
IBr
Cl

22. Infrared Spectroscopy
For isopropyl alcohol, CH(CH3)2OH, the infrared
absorption bands identify the various functional groups
of the molecule.

23. Interferogram
is made by an interferometer.
Interferogram
is transformed
into a spectrum using a FT.
BKG
SB
3000 2000 1000
[cm-1]
Sample
SB
Sample
3000 2000 1000
[cm-1]
Sample/BKG
IR spectrum
%T
3000 2000 1000 [cm-1]
The Principles of FTIR Method

24. FTIR seminar
Interferometer
He-Ne gas laser
Fixed mirror
Movable mirror
Sample chamber
Light
source
(ceramic)
Detector
(DLATGS)
Beam splitter
FT Optical System Diagram

25. Fixed mirror
Ｂ
Movable mirror
Fixed mirror
Ａ
Movable mirror
Fixed mirror
Ｃ
Movable mirror
Same-phase interference
wave shape
Opposite-phase
interference
wave shape
Same-phase interference
wave shapeλ0
Movable mirror
Ｄ Interference pattern of light
manifested by the optical-path
difference
Continuous phase shift
Signalstrength
Ｉ
(X)
-2λ -λ 0 λ 2λ
-2λ -λ 0 λ 2λ
FTIR seminar
Interference of two beams of
light

26. 1.Better sensitivity and brightness
- Allows simultaneous measurement over the entire wavenumber range
- Requires no slit device, making good use of the available beam
2.High wavenumber accuracy
- Technique allows high speed sampling with the aid of laser light interference fringes
- Requires no wavenumber correction
- Provides wavenumber to an accuracy of 0.01 cm-1
3. Resolution
- Provides spectra of high resolution
4. Stray light
- Fourier Transform allows only interference signals to contribute to spectrum.
Background light effects greatly lowers.
- Allows selective handling of signals limiting intreference
5. Wavenumber range flexibility
- Simple to alter the instrument wavenumber range
CO2 and H2O sensitive
FT-IR Advantages and
Disadvantages

27. FT-IR Advantages
• Fellgett's (multiplex) Advantage
• FT-IR collects all resolution elements with a
complete scan of the interferometer. Successive
scans of the FT-IR instrument are coadded and
averaged to enhance the signal-to-noise of the
spectrum.
• Theoretically, an infinitely long scan would average
out all the noise in the baseline.
• The dispersive instrument collects data one
wavelength at a time and collects only a single
spectrum. There is no good method for increasing
the signal-to-noise of the dispersive spectrum.

28. FT-IR Advantages
• Connes Advantage
• an FT-IR uses a HeNe laser as an internal
wavelength standard. The infrared wavelengths
are calculated using the laser wavelength, itself a
very precise and repeatable 'standard'.
• Wavelength assignment for the FT-IR spectrum is
very repeatable and reproducible and data can be
compared to digital libraries for identification
purposes.

29. FT-IR Advantages
• Jacquinot Advantage
• FT-IR uses a combination of circular apertures
and interferometer travel to define resolution. To
improve signal-to-noise, one simply collects more
scans.
• More energy is available for the normal infrared
scan and various accessories can be used to
solve various sample handling problems.
• The dispersive instrument uses a rectangular slit
to control resolution and cannot increase the
signal-to-noise for high resolution scans.
Accessory use is limited for a dispersive
instrument.

30. FT-IR Application Advantages
• Opaque or cloudy samples
• Energy limiting accessories such as diffuse reflectance
or FT-IR microscopes
• High resolution experiments (as high as 0.001 cm-1
resolution)
• Trace analysis of raw materials or finished products
• Depth profiling and microscopic mapping of samples
• Kinetics reactions on the microsecond time-scale
• Analysis of chromatographic and thermogravimetric
sample fractions

31. To separate IR light, a grating is used.
Grating
Light source
Detector
Sample
Slit
To select the specified IR light,
A slit is used.
Dispersion
Spectrometer
In order to measure an IR spectrum,
the dispersion Spectrometer takes
several minutes.
Also the detector receives only
a few % of the energy of
original light source.
Fixed CCM
B.S.
Moving CCM
IR Light source
Sample
Detector
An interferogram is first made by
the interferometer using IR light.
The interferogram is calculated and transformed
into a spectrum using a Fourier Transform (FT).
FTIR
In order to measure an IR spectrum,
FTIR takes only a few seconds.
Moreover, the detector receives
up to 50% of the energy of original
light source.
(much larger than the dispersion
spectrometer.)
Comparison Beetween Dispersion
Spectrometer and FTIR