Transcript Document

Infrared Absorption Spect
IR Spectroscopy
• deal with the interaction of infrared ra
matter
IR spectrum (%T against Frequency)
• chemical nature and molecular structu
Applications
• organic materials
• polyatomic inorganic molecules
• organometallic compounds
IR region of the electromagnetic spec
• wavelength 770 nm to 1000 mm
(wave number 12,900 to 10 cm-1)
IR region is often further subdivided
subregions
1. Near-infrared region (nearest to the
2. Mid-infrared region
3. Far-infrared region
Table Infrared Spectral Regions
Region Wavelength (l) wavenumber Frequency (v)
Range, mm Range, cm-1 Range, Hz
Near
0.78 to 2.5
12800 to 4000
3.8x1014 to 1.2x1
Middle
2.5 to 50
4000 to 2001.2x1014 to 6.0x1
Far
50 to 1000
200 to 10
Most used 2.5 to 15
6.0x1012 to 3.0x1
4000 to 6701.2x1014 to 2.0x1
IR Spectrum
Mid-infrared region
1. Group-frequency region
• wavenumber 4000 to 1300 cm-1 (2.
• functional group
2. Finger print region
• wavenumber 1300 to 650 cm-1
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Infrared Spectrometry
• useful for quantitative analysis, althoug
considerably more difficult to achieve ac
precise results with IR spectrometry than
UV-visible methods
• Beer’s Law provides the basis of quant
method as it does in UV-visible spectrop
Electromagnetic radiation
UV-visible
infrared
electronic tra
vibration, rot
Basis of Infrared Absorption
The IR spectrum can be obtained with
or with condensed-phase molecules.
For gas-phase, molecules vibration-rotat
are observed.
For condensed-phase, the rotaional struc
‘Vibrational spectroscopy’
Requirements for the absorption of IR
1. The natural frequency of vibration of t
must equal the frequency of the incident
2. The frequency of the radiation must sa
where E is the energy difference between
vibrational states involved
E  E vib ,1  E vib ,2  h
3. The change in vibration must stimulate
the dipole moment of the molecule
IR active / IR inactive
Types of Molecular Vibrations
IR
Vibration of bonds
1. Stretching
2. Bending
Stretching vibration
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1. Symmetric stretching
2. Asymmetric stretching
H
C
H
Methylene
Symmetric stretching
(~2853 cm-1)
H
C
H
Asymmetric stretching
(~2926 cm-1)
Bending vibration
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1.
2.
3.
4.
Scissoring
Rocking
Wagging
Twisting
In plane
Out of plane
Bending
Vibrational mo
of methylene
group
Number of Vibrational Modes
Nonlinear molecule
Fundamental vibrational modes = 3N-6
Linear molecule
Fundamental vibrational modes = 3N-5
Nonlinear molecule: ็H2O
Vibrational modes = 3(3) - 6 = 3
Linear molecule: CO2
Vibrational modes = 3N-5 = 3(3)-5 = 4
Molecular Vibration
A molecule is made up ofa number of a
by chemical bonds. Such atoms vibrate ab
other in the same way as weights held tog
Hooke’s Law states that two masses joine
will vibrate such that

k
(1)
m
where
we have
1
 
2
k
m
  2 bu
= the frequency (rad/sec),
(2)

where
= the frequency of vibration, k
m
constant of the bond (N/cm),
and
is th
or
M1M 2
m
M1  M 2
(3)
where M1 is the mass of one vibrating bo
 But
mass of the other.
is in cyles per
During this time light travels a distance m
cm/sec (I.e., the speed of light).
Therefore, if onedivides
by c, the resu
 This is
number of cycle per cm.
, the w
of an absorption peak (cm-1) and
 

(4)
c
It can be deduced that
1
 
2c
k
  5.3x 10
12
(5)
m
k
m
(6)
Example
Calculate the approximate wavenumber a
of the fundamental absorption peak due
vibration of a carbonyl group C=O
  5.3x 10
12
k
m
The mass of the carbon atom in kg is give
M1 
12x 10  3 kg / mol
6.02x 10
23
atoms / mol
 2.0x 10  26 kg
x 1 atom
Similar, for oxygen
M2 
(16 x 10  3 )
(6.02 x 10 23 )
 2.7x 10  26 kg
and the reduced mass m is given by
M1M 2
2.0x 10  26 kg x 2.7x 10  26 kg
m

M1  M 2
(2.0  2.7) x 10  26 kg
 1.1x 10  26 kg
The force constant for the typical double
1x103 N/cm. Substituting this value and m
  5.3x 10
12
 1.6x 10 3
s / cm
1x 10 3 N / cm
1.1x 10  26 kg
cm 1
The carbonyl stretching band is found ex
to be in the region of 1600 to 1800 cm-1 (
Frequencies of various group vibrations
frequency region and in fingerprint regi
Instrumentation
Three distinct types of instruments empl
absorption spectrometry
1. Dispersive instruments with a monoch
used in the mid-IR region for spectral sca
quantitative analysis
2. Fourier transform IR systems are wide
the far-IR region and becoming quite po
mid-IR spectrometry
Instrumentation
3. Nondispersive instruments that use filt
wavelength selection or an infrared-abso
in the detection system are often used fo
at specific wavelength
Block diagram of IR spectrophotometer
source
sample
detector readout
monochromator
Grating
Nernst Glower
Filter
Globar
Incandescent wire source
Hg Arc
Thermal D Recorder
XY plotte
Thermocouple
ThermopilePrinter
Thermister
Bolometer
Pneumatic D
Pyroelectric D
IR sources: general
• an inert solid that is heated electrically
temperature between 1500 and 2200 K
(provide continuous radiant)
• the maximum radiant intensity at thes
temperatures occurs at between 5000 a
(2 to 1.7 mm)
IR sources
The Nernst Glower (Continuous source
• useful and inexpensive source
• rare earth oxides formed into a cylinde
diameter of 1 to 2 mm and a length of pe
• platinum leads are sealed to the end of
to permit passage of electricity; tempera
1200 and 2200 K result
• because of a negative temperature coef
resistance, it must be used with ballast re
heating circuit to prevent burnout
IR sources
The Nernst Glower (Continuous source
(cont.)
• it is rather fragile, and its lifetime depe
operating temperature and the care take
IR sources
The Nernst Glower (Continuous source
IR sources
The globar (continuous source)
• a silicon carbide rod, usually about 50 m
and 5 mm in diameter
• current through the globar causes the r
emit radiation at temperature exceeding
• the power consumption is normally hig
of the Nernst Glower
• water cooling is needed to cool the met
attached to the rod
• less convenient to use and more expens
of the necessity for water cooling
IR sources
Incandescent wire source
• somewhat lower intensity but longer li
the Globar or Nernst glower
• a tightly wound spiral of nichrome wire
about 1100 K by an electrical current
• a rhodium-wire heater sealed in a ceram
has a similar properties as a source
The Mercury arc
IR sources
• for the far-infrared region of the spectr
• provide sufficient energy for convenien
• consist of a quartz-jacketed tube conta
vapour at a pressure greater than one at
• passage of electricity through the vapo
internal plasma source that provides con
radiation in the far-infrared region
The Mercury arc
IR sources
IR sources
The Tungsten filament lamp
• the near-infrared region of
4000 to 12,800 cm-1
(2.5 to 0.78 mm)
Infrared Detectors
General types of infrared detectors:
1. Thermal DetectorsDispersive
spectrophotomet
2. Pyroelectric Detectors
3. Photoconducting Detectors
Fourier Transform multiplex
instrument
Infrared Detecto
Thermal Detectors
• widely used in the IR region of the spe
• responses depends upon the heating
effect of radiation
Problem:
The problem of measuring infrared rad
means is compounded by thermal noise f
Infrared Detecto
Solution:
Thermal detectors are usually encapsu
carefully shielded from thermal radiati
by other nearby objects
Infrared Detecto
Thermal detectors: Thermocouples
• a thermocouple is made by welding to
each end two wires made from different
• If one welded joint (called the hot junc
hotter than the other joint (the cold junc
electrical potential develops between th
Metal A
welded junction
(hot)
Metal B welded junction
(cold)
Infrared Detecto
Thermal detectors: Thermocouples
In IR spectroscopy, the cold junction
screened in a protective box and kept at
temperature. The hot junction is expose
radiation, which increases the temperat
junction. The potential difference gener
wires is a function of the temperature d
between the junctions and, therefore, o
of IR radiation falling on the hot junctio
Infrared Detecto
Thermal detectors: Thermocouples
A well-designed thermocouple detec
of responding to temperature differenc
This figure corresponds to a potential d
about 6 to 8 mV/mW
To enhanced sensitivity, several therm
may be connected in series to give wha
‘thermopile’
Infrared Detecto
Thermal detectors: Thermistor/Bolomete
A bolometer is a type of resistance the
constructed of strips of metals such as pla
or from a mixture of metal oxide; the latt
sometimes called thermistors. These mate
relatively large change in resistance as a f
Temperature.
The thermistor is normally placed in a
with a reference thermistor that is not irr
resistance can be measured by a null-com