1. Infrared Absorption Spectroscopy

2. IR spectrum (%T against Frequency)
IR Spectroscopy
deal with the interaction of infrared radiation with
matter
IR spectrum (%T against Frequency)
chemical nature and molecular structure of cpd
Applications
organic materials
polyatomic inorganic molecules
organometallic compounds

3. IR region of the electromagnetic spectrum
wavelength 770 nm to 1000 mm
(wave number 12,900 to 10 cm-1)
IR region is often further subdivided into three
subregions
Near-infrared region (nearest to the visible)
Mid-infrared region
Far-infrared region

4. Table Infrared Spectral Regions
Wavelength (l)
Range, mm
wavenumber
Range, cm-1
Frequency (v)
Range, Hz
Near
0.78 to 2.5
12800 to 4000
3.8x1014 to 1.2x1014
Middle
2.5 to 50
4000 to 200
1.2x1014 to 6.0x1012
Far
50 to 1000
200 to 10
6.0x1012 to 3.0x1011
Most used
2.5 to 15
4000 to 670
1.2x1014 to 2.0x1013

5. IR Spectrum

6. Mid-infrared region 1. Group-frequency region
wavenumber 4000 to 1300 cm-1 (2.5 to 8 mm)
functional group
2. Finger print region
wavenumber 1300 to 650 cm-1
เกิดจากโครงสร้างของโมเลกุลที่สมบูรณ์

7. Infrared Spectrometry
useful for quantitative analysis, although it is
considerably more difficult to achieve accurate and
precise results with IR spectrometry than with
UV-visible methods
Beer’s Law provides the basis of quantitative IR
method as it does in UV-visible spectrophotometry
Electromagnetic radiation
UV-visible electronic transition
infrared vibration, rotation

8. Basis of Infrared Absorption
The IR spectrum can be obtained with gas-phase
or with condensed-phase molecules.
For gas-phase, molecules vibration-rotation spectra
are observed.
For condensed-phase, the rotaional structure is lost.
‘Vibrational spectroscopy’

9. Requirements for the absorption of IR radation
1. The natural frequency of vibration of the molecules
must equal the frequency of the incident radiation
2. The frequency of the radiation must satisfy, E = hv,
where E is the energy difference between the
vibrational states involved
3. The change in vibration must stimulate changes in
the dipole moment of the molecule
IR active / IR inactive

10. Types of Molecular Vibrations
IR Vibration of bonds
Stretching
Bending
Stretching vibration
เกี่ยวข้องกับการเปลี่ยนแปลงความยาวระหว่างอะตอม
ที่เกิดพันธะกัน
Symmetric stretching
Asymmetric stretching

11. Methylene Symmetric stretching (~2853 cm-1) Asymmetric stretching

12. Bending vibration
การเปลี่ยนแปลงมุมระหว่างสองพันธะ
Scissoring
Rocking
Wagging
Twisting

13. In plane
Out of plane
Bending

14. Vibrational mode
of methylene
group

15. Number of Vibrational Modes
Nonlinear molecule
Fundamental vibrational modes = 3N-6
Linear molecule
Fundamental vibrational modes = 3N-5

16. Nonlinear molecule: ็H2O
Vibrational modes = 3(3) - 6 = 3

17. Linear molecule: CO2
Vibrational modes = 3N-5 = 3(3)-5 = 4

18. Molecular Vibration A molecule is made up ofa number of atoms joined
by chemical bonds. Such atoms vibrate about each
other in the same way as weights held together by springs

19. Hooke’s Law states that two masses joined by a spring
will vibrate such that
(1)
where = the frequency (rad/sec), but since
we have
(2)

20. where = the frequency of vibration, k is the force
constant of the bond (N/cm), and is the reduced mass, or
(3)
where M1 is the mass of one vibrating body, M2 the
mass of the other. But is in cyles per second (cps).
During this time light travels a distance measured in
cm/sec (I.e., the speed of light).

21. Therefore, if one divides by c, the result is the
number of cycle per cm. This is , the wavenumber
of an absorption peak (cm-1) and
(4)
It can be deduced that
(5)
(6)

22. Example
Calculate the approximate wavenumber and wavelength
of the fundamental absorption peak due to the stretching
vibration of a carbonyl group C=O
The mass of the carbon atom in kg is given by

23. Similar, for oxygen
and the reduced mass m is given by
The force constant for the typical double bond is about
1x103 N/cm. Substituting this value and m into eq. (5) gives

24. The carbonyl stretching band is found experimentally
to be in the region of 1600 to 1800 cm-1 (6.3 to 5.6 mm)

25. Frequencies of various group vibrations in the group
frequency region and in fingerprint region

26. Instrumentation Three distinct types of instruments employed for IR
absorption spectrometry
1. Dispersive instruments with a monochromator are
used in the mid-IR region for spectral scanning and
quantitative analysis
2. Fourier transform IR systems are widely applied in
the far-IR region and becoming quite popular for
mid-IR spectrometry

27. Instrumentation 3. Nondispersive instruments that use filters for
wavelength selection or an infrared-absorbing-gas
in the detection system are often used for gas analysis
at specific wavelength

28. Block diagram of IR spectrophotometer
source
detector
readout
sample
monochromator
Nernst Glower
Globar
Incandescent wire source
Hg Arc
Grating
Filter
Thermal D
Thermocouple
Thermopile
Thermister
Bolometer
Pneumatic D
Pyroelectric D
Recorder
XY plotter
Printer

29. IR sources: general an inert solid that is heated electrically to a
temperature between 1500 and 2200 K
(provide continuous radiant)
the maximum radiant intensity at these
temperatures occurs at between 5000 and 5900 cm-1
(2 to 1.7 mm)

30. The Nernst Glower (Continuous source)
IR sources
The Nernst Glower (Continuous source)
useful and inexpensive source
rare earth oxides formed into a cylinder having a
diameter of 1 to 2 mm and a length of perhaps 20 mm
platinum leads are sealed to the end of the cylinder
to permit passage of electricity; temperatures between
1200 and 2200 K result
because of a negative temperature coefficient of
resistance, it must be used with ballast resistor in the
heating circuit to prevent burnout

31. The Nernst Glower (Continuous source)
IR sources
The Nernst Glower (Continuous source)
(cont.)
it is rather fragile, and its lifetime depends on the
operating temperature and the care taken in handling it

32. The Nernst Glower (Continuous source)
IR sources
The Nernst Glower (Continuous source)

33. The globar (continuous source)
IR sources
The globar (continuous source)
a silicon carbide rod, usually about 50 mm in length
and 5 mm in diameter
current through the globar causes the rod to heat and
emit radiation at temperature exceeding 1000 oC
the power consumption is normally higher than that
of the Nernst Glower
water cooling is needed to cool the metallic electrodes
attached to the rod
less convenient to use and more expensive because
of the necessity for water cooling

34. Incandescent wire source
IR sources
Incandescent wire source
somewhat lower intensity but longer life than
the Globar or Nernst glower
a tightly wound spiral of nichrome wire heated to
about 1100 K by an electrical current
a rhodium-wire heater sealed in a ceramic cylinder
has a similar properties as a source

35. IR sources
The Mercury arc
for the far-infrared region of the spectrum (l> 50 mm)
provide sufficient energy for convenient detection
consist of a quartz-jacketed tube containing mercury
vapour at a pressure greater than one atmosphere
passage of electricity through the vapour forms an
internal plasma source that provides continuous
radiation in the far-infrared region

36. IR sources
The Mercury arc

37. The Tungsten filament lamp
IR sources
The Tungsten filament lamp
the near-infrared region of
4000 to 12,800 cm-1
(2.5 to 0.78 mm)

38. Infrared Detectors General types of infrared detectors:
1. Thermal Detectors
Dispersive
spectrophotometer
2. Pyroelectric Detectors
3. Photoconducting Detectors
Fourier Transform multiplex
instrument

39. widely used in the IR region of the spectrum
Infrared Detectors
Thermal Detectors
widely used in the IR region of the spectrum
responses depends upon the heating
effect of radiation
Problem:
The problem of measuring infrared radiation by thermal
means is compounded by thermal noise from surrounding

40. Thermal detectors are usually encapsulated and
Infrared Detectors
Solution:
Thermal detectors are usually encapsulated and
carefully shielded from thermal radiation emitted
by other nearby objects

41. Thermal detectors: Thermocouples
Infrared Detectors
Thermal detectors: Thermocouples
a thermocouple is made by welding together at
each end two wires made from different metals.
If one welded joint (called the hot junction) becomes
hotter than the other joint (the cold junction), a small
electrical potential develops between the joints
Metal A
Metal B
welded junction
(cold)
(hot)

42. Thermal detectors: Thermocouples
Infrared Detectors
Thermal detectors: Thermocouples
In IR spectroscopy, the cold junction is carefully
screened in a protective box and kept at a constant
temperature. The hot junction is exposed to the IR
radiation, which increases the temperature of the
junction. The potential difference generated in the
wires is a function of the temperature difference
between the junctions and, therefore, of the intensity
of IR radiation falling on the hot junction.

43. Thermal detectors: Thermocouples
Infrared Detectors
Thermal detectors: Thermocouples
A well-designed thermocouple detector is capable
of responding to temperature difference of 10-6 K.
This figure corresponds to a potential difference of
about 6 to 8 mV/mW
To enhanced sensitivity, several thermocouples
may be connected in series to give what a called a
‘thermopile’

44. Thermal detectors: Thermistor/Bolometer
Infrared Detectors
Thermal detectors: Thermistor/Bolometer
A bolometer is a type of resistance thermometer
constructed of strips of metals such as platinum or nickel,
or from a mixture of metal oxide; the latter devices are
sometimes called thermistors. These materials exhibit a
relatively large change in resistance as a function of
Temperature.
The thermistor is normally placed in a bridge circuit
with a reference thermistor that is not irradiated. The
resistance can be measured by a null-comparison method