analytical chemistry

1. PRESENTED BY,
PRATIKSHA C CHANDRAGIRIVAR
M PHARMA 1ST SEM
DEPT, OF PHARMACEUTICS
HSK COP BAGALKOT
FACILITATED BY,
Dr. B.S.KITTUR
HOD AND PROFESSOR
DEPT. OF PHARMACHEMISTRY
HSK COP BAGALKOT
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2. IR SPECTROSCOPY 2

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CONTENTS:
1. INTRODCTION
2. PRINCIPLE
3. THEORY
4. MODES OF MOLECULAR VIBRATIONS

4. INTRODUCTION:
DEFINITION:
Infrared spectroscopy is a spectroscopy which
is concerned with the study of absorption of
infrared radiation, its properties, characteristics
etc,. and also its interaction with matter.
◊ It is also known as vibrational spectroscopy.
◊ Infrared (IR) radiation lies in the part of the
electromagnetic spectrum i.e. between the visible
and microwave regions.
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5. IR SPECTROSCOPY 5

6. ◊ Any compound, either organic or inorganic, if it have
covalent bond then it have a ability to absorb some
frequencies of electromagnetic radiation in the IR
region of the electromagnetic spectrum.
◊ This absorption of infrared radiation (quantised)
causes the various bands in a molecule to stretch
and bend with respect to one another.
◊ Because of this reason it is main tool for the
identification of compounds.
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7. In any molecule, there are group of atoms or
a atom is connected by bonds. These bonds are
analogous to springs and not rigid in nature. Because
of the continuous motion of the molecule, they
maintain some vibrations with some frequency,
characteristics to every portion of molecule. This is
called natural frequency of vibration.
When energy in the form of infrared
radiation is applied and if that
Applied infrared frequency = Natural frequency of
vibration
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8. Then absorption of IR radiation takes place and peak is
observed.
Example : Infrared vibrations of ethanol
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9. Every bond or portion of a molecule or functional
group requires different frequency of absorption.
Hence characteristic peak is observed for every
functional group or part of the molecule.
In other words, IR spectra is nothing but a finger
print region of a molecule.
Example: IR spectra of Ibuprofen
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10. The different IR regions are:
1. Near IR region = 0.8µ to 2.5µ
2. Mid IR region = 2.5 to 15µ
3. Far IR region = 15µ to 200µ
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11. Example:
The natural frequency of vibration HCL molecule is
about 8.7 × 1013 sec-1(2890 cm-1).
When IR radiations permitted to pass through a sample
of HCL and transmitted radiation is analysed by the
IR spectrometer.
It is observed that the part of radiation which has a
frequency of 8.7 × 1013 sec-1 has been observed by
HCL molecule where as the remaining frequencies of
the radiation are transmitted.
Thus, the frequency 8.7 × 1013 sec-1 is characteristics
of HCL molecule.
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12. In pharmaceutical analysis, we use mid IR region
of wavelength 2.5 to 15µ ( 2.5 to 25µ) or in terms of
wavenumbers we can say it as 400cm-1 to 4000cm-1.
In IR spectra we use wavenumbers instead of
wavelengths for mentioning the characetristic peak,
Because wave numbers are larger values and easy to
handle than wavelengths which will show only small
differences between functional groups.
o Wave number is nothing but the number of waves
present per cm. which can be calculated from the
wavelength.
1
----------------------- × 104 = wavelength per cm or cm-1
wavelength in µ
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13. For a molecule to absorb IR radiation, it has to fulfill
certain requirements,
Those are,
a) Correct wavelength of radiation
b) Electric dipole
a) Correct wavelength of radiation:
 A molecule absorbs radiation only when the natural
frequency of vibration of some part of a molecule
( i.e. atoms or group of atoms comprising it) is the
same as the frequency of radiation.
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14. b) Electric dipole:
 This is another condition for a molecule to absorb IR
radiation.
 A molecule can only absorb IR radiation when its
absorption causes a change in its electric dipole
(dipole moment).
 A molecule is said to have electric dipole when there
is a slight positive and a slight negative electric
charge on its component atoms.
 As the symmetrical diatomic molecules like O2 and
N2, do not posses electric dipole , they can not be
excited by infrared radiation and for this they do
not give rise to IR spectra.
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15.  When the molecule having electric dipole is kept in
the electric field , it is the same as that when the
molecule is kept in a beam of IR spectroscopy.
 Electric field exerts forces on the electric charges
in the molecules. Opposite charges will experience
forces in opposite directions. This tends to
decrease separation.
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16.  But in the case of electric field of IR radiation
changes its polarity periodically, it means that the
spacing between the charged atoms (electric dipole)
of the molecule also changes periodically.
 When these charged atoms vibrate , they absorb IR
radiation from the radiation source.
 If the rate of vibration at charged atom in a
molecule is fast, the absorption of radiation is
intense and thus, the IR spectrum will have intense
absorption band.
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17.  There is no change in the dipole moment is produced
by the carbon-carbon double bond stretching of the
symmetrical molecule like ethylene. Since there is
no change in dipole moment ,the bond does not
absorb infrared radiation.
 On the other hand, substituition of a bromine for a
hydrogen atom to form bromoethylene which
destroys the symmetry around the double bond.
The stretching of the double bond now generates
a significant change in the dipole moment and strong
absorbance in the infrared is observed.
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18. The closer atoms in a molecule are to each other,
the greater will be the strength of the dipole ,
faster will be the rate of change of the dipole , the
higher will be the frequency of vibration , and the
more intense will be the absorption of radiation.
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19. In order to understand the theory of
absorption spectroscopy, one has to understand the
phenomena of vibration rotational spectra.
 Let us consider a diatomic molecule associated with
a dipole moment.
 The vibratory motion of the nuclei of diatomic
molecule may be similar to the vibration of linear
harmonic oscillator.
 In such oscillator, the force tending to restore an
atom to its original state is proportional to the
displacement of the vibratory atom from the
original position (Hooke’s law).
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21. Suppose the bond between the two nuclei of a diatomic
molecule is distorted from its equilibrium length re
to a new length r.
Then the restoring forces on each atom of the
diatomic molecule will be given by
m1 d2 r1 = -K(r - re) …………… (1)
d t2
m2 d2 r2 = -K(r - re) ……………. (2)
d t2
Where, K = proportionality constant and known as force
constant, it is regarded as a measure of the
stiffness of the bond.
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22. r1 and r2 are the positions of atom 1 and 2 relative to
the centre of gravity of the molecule.
We know,
m2
r1 = ------------ r -------(3)
m1 + m2
m1
r2 = ------------ r --------(4)
m1 + m2
Where, m1 and m2 are the masses of two atoms of a
vibrating dia-atomic molecule.
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23. On substituting equation(3) and equation(1) we get,
m1 m2 d2 r
---------- ----- = - K( r - re) ………….(5)
m1 + m2 dt2
As re is a constant, its differentiation with respect to time
t will be zero, i.e.,
d2 r d2 (r - re)
------ = ------------ …………..(6)
dt2 dt2
Substituition of the above equation in equation(5) yields
m1 m2 d2(r - re)
--------- ------------ = - K( r - re) ………….(7)
m1 + m2 dt2
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24. m2
Put r – re = x and --------- = µ ..........(8)
m1 + m2
on substituting equation
d2 x
µ ------ = - K x ………….(7)
dt2
Or d2 x
µ ------ + K x = 0 ………….(8)
dt2
Or d2 x
µ ------ + w2x = 0 ………….(9)
dt2
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25. Where, w2 = K/µ or w = √ K/µ …………..(10)
Equation(9) is the expression of simple harmonic
motion with frequency of vibration (v) as follows
v = w/2∏ = 1/2∏ √ K/µ
Where,
K is the force constant expressed in dynes per cm
and
µ is the reduced mass of the system.
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26. According to the character of
vibration, normal vibrations can be divided in to two
principle group they are
1. Stretching vibrations
2. Bending vibrations
1.Stretching vibrations:
In this type of vibrations, the atoms
move essentially along the bond axis so that the
bond length increases or decreases periodically, i.e.,
at regular intervals.
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27.  As this type of vibrations corresponds to one
dimensional motion.
 During stretching vibrations, bond angles change
only if it to do so by the centre of gravity resisting
displacement.
Stretching vibrations are of two types:
a) Symmetric vibration
b) Asymmetric vibration
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28. a) Symmetric vibration:
In this type , the movement of the atoms
with respect to a particular atom in a molecule is in
the same direction.
Fig.
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30. b) Asymmetric vibration:
In these vibrations, one atom approaches the
central atom while the other departs from it.
Fig.
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:

31. IR SPECTROSCOPY 31

32. 2.Bending vibrations:
In this type of vibrations, the positions of
the atoms change with respect to the original bond
axis.
 We know that more energy is required to stretch a
spring than that required to bend it.
 We can safely say that stretching absorptions of a
bond appear at high frequencies (high energy) as
compared to the bending of the same bond.
 This kind of vibration is also know as deformations.
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33. There are mainly two types of bending
vibrations are there those are:
A. In-plane deformation vibrations
B. Out-of plane deformation vibrations
A. In-plane deformation vibrations:
 In these vibrations , there is change in bond angle
takes place.
 Bending of bond angle takes place within the same
plane.
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34. It is further can be divided into two types, viz.,
1. Scissoring
2. Rocking
1.Scissoring:
 In this type, two atoms approach each other.
 Here, bond angle decreases.
Fig.
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36. 2. Rocking:
 In this type, the movement of the atoms takes place
in the same directions.
 Here bond angle is maintained, but both bonds
moves within the plane.
fig,
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38. 2.Out-of plane deformation vibrations:
These vibrations takes place outside the
plane of molecule.
It is further can be divided into two types viz.,
a) Wagging
b) Twisting
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39. a) wagging:
Two atoms move “up and down” the plane with
respect to the central atom(i.e., moves one side of
the plane).
Fig.
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41. b) Twisting:
In this type, one of the atoms moves up
the plane while the other moves down the plane with
respect to the central atoms.
Fig.
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44. 1. Text book of pharmaceutical analysis by Dr.
Ravi Sankar – 4th edition.
2. Instrumental methods of chemical analysis
by Gurdeep R Chatwal, Sham K Anand – 6th
edition.
3. Elementary organic spectroscopy by
Y.R.Sharma – multicolour edition.
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45. THANK YOU
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