Theory of I.R. spectroscopy and FT-IR

1. ANJALI TERESA
FIRST YEAR M.PHARM
THEORY OF I.R.
SPECTROSCOPY
& FT-IR
Presented by: ANJALI TERESA
FIRST YEAR M.PHARM
1

2. INTRODUCTION
 Spectroscopy is the branch of science dealing with the
study of interaction of electromagnetic radiation with
matter.
 IR spectroscopy is Absorption spectroscopy in which
molecular vibrations observed due to absorption of IR
radiation.
 Infrared radiation was discovered
in 1800 by William Herschel.
2THEORY OF I.R. & FT-IR.

3. The range of EMR between the visible and microwaves
region is called INFRARED region(14000-40 cm-1 ).
3THEORY OF I.R. & FT-IR.

4.  I.R. spectroscopy is also known as
vibrational spectroscopy since it causes
vibrational transitions.
 The vibrations in the I.R. spectroscopy is
known as Fundamental vibrations.
 I.R. spectrum is mainly used in structural
elucidation to determine the functional
groups.
 The infrared region of the spectrum
encompasses radiation with
wavenumbers ranging from about 12800
to 10cm-1.
4
THEORY OF I.R. & FT-IR.

5. IR region
Near IR Middle IR Far IR
REGION WAVELENGTH
(µm)
WAVE
NUMBER
(cm-1)
NEAR 0.78-25 12800-4000
MIDDLE 2.5-50 4000-200
FAR 50-1000 200-10
5THEORY OF I.R. & FT-IR.

6.  Most of the analytical applications are
confined to the middle IR region because
absorption of organic molecules are high
in this region.
THEORY
• In any molecule, atoms or group of
atoms are connected by bonds which
are similar to springs & not rigid in
nature.
6THEORY OF I.R. & FT-IR.

7. • This characteristic vibration are called Natural
frequency of vibration.
 When energy in the form of infrared radiation is
applied then it causes the vibration between the
atoms of the molecules and when,
Applied infrared frequency = Natural frequency of
vibration
7THEORY OF I.R. & FT-IR.

8. Then, Absorption of IR radiation takes place and a peak
is observed.
Different functional groups absorb characteristic
frequencies of IR radiation. Hence gives the
characteristic peak value.
Therefore, IR spectrum of a chemical substance is a
finger print of a molecule for its identification.
CRITERIA FOR A COMPOUND TO ABSORB I.R
RADIATION
1. Correct wavelength of radiation
2. Change in dipole moment
8THEORY OF I.R. & FT-IR.

9. 1. Correct wavelength of radiation:
A molecule to absorb IR radiation, the natural
frequency of vibrations of some part of a molecule is
the same as the frequency of incident radiation.
2. Change in dipole moment:
• A molecule can only absorb IR radiation when
its absorption cause a change in its electric
dipole
• A molecule is said to have an electric dipole
when there is a slight positive and a slight
negative charge on its component of atoms.
9THEORY OF I.R. & FT-IR.

10.  For isopropyl alcohol, CH(CH3)2OH, the infrared
absorption bands identify the various functional
groups of the molecule.
10THEORY OF I.R. & FT-IR.

11. MODE OF VIBRATION
Degree of freedom is the number of variables
required to describe the motion of a particle
completely. For an atom moving in 3-
dimensional space, three coordinates are
adequate so its degree of freedom is three. Its
motion is purely translational.
If we have a molecule made of N atoms (or
ions), the degree of freedom becomes 3N,
because each atom has 3 degrees of freedom.
11THEORY OF I.R. & FT-IR.

12. In defining the motion of the molecule, we need to
consider,
 Translational motion:The motion of the entire
molecule through space.
 Rotational motion:The motion of the entire
molecule around the centre of gravity.
 Vibrational motion:The motion of each atom relative
to the other atom.
12THEORY OF I.R. & FT-IR.

13. There are two cases,
 Linear molecule
 Non linear molecule
 LINEAR MOLECULE:It is a special case since by definition all of
the atoms lie on a single straight line.Rotation about the bond
axis is not possible and two degrees of freedom suffice to
describe rotational motion.Thus the number of vibrations for a
linear molecule is given by (3N-5).
 NON LINEAR MOLECULE :For non-linear molecule all rotational
degrees of freedom is three and the remaining(3N-6) degree of
freedom constitute vibrational motion.
13THEORY OF I.R. & FT-IR.

14. Non linear
molecule
Linear molecule
TRANSLTIONAL 3 Degrees of
freedom
3 Degrees of
freedom
ROTATIONAL 3 degrees of
freedom
2 Degrees of
freedom
FUNDAMENTAL 3N-6 3N-5
BENDING 2N-5 2N-4
14THEORY OF I.R. & FT-IR.

15. Types of molecular vibrations
There are 2 types of vibrations:
1. Stretching vibrations
2. Bending vibrations
1. Stretching vibrations:
 Vibration or oscillation along the line of bond
 Change in bond length
 Occurs at higher energy: 4000-1250 cm-1
 2 types:
a) Symmetrical stretching
b) Asymmetrical stretching
15THEORY OF I.R. & FT-IR.

16. Symmetric stretching
2 bonds increase or decrease in length simultaneously.
H
H
C
In this, one bond length is increased and other is
decreased.
Asymmetrical stretching
16THEORY OF I.R. & FT-IR.

17. 2.BENDING VIBRATIONS
H
H
C
•Vibration or oscillation not along the line of
bond
•These are also called as deformations
•In this, bond angle is altered
•Occurs at low energy: 1400-666 cm-1
17THEORY OF I.R. & FT-IR.

18. • 2 types:
a) In plane bending: scissoring, rocking
b) Out plane bending: wagging, twisting
In plane bending
Scissoring:
• This is an in plane blending
• 2 atoms approach each
other
• Bond angles decrease
18THEORY OF I.R. & FT-IR.

19. H
H
CC
H
H
CC
19THEORY OF I.R. & FT-IR.

20.  b) Out plane bending
i. Wagging:
• 2 atoms move to one side of the plane. They move
up and down the plane.
ii. Twisting:
• One atom moves above the plane and another
atom moves below the plane.
H
H
CC
H
H
CC
20THEORY OF I.R. & FT-IR.

21. MECHANICAL MODEL OF A STRETCHINGVIBRATION IN
A DIATOMIC MOLECULE
 The characteristics of an atomic stretching vibration can be
approximated by a mechanical model consisting of two
masses connected by a spring.A disturbance of one of these
masses along the axis of the spring results in a vibration
called simple harmonic motion.
 Let us first consider the vibration of a single mass attached
to a spring that is hung from an immovable object.
 If the mass is displaced a distance ‘y’ from its equilibrium
position by the application of a force along the axis of the
spring, the restoring force F is proportional to the
displacement.
21THEORY OF I.R. & FT-IR.

22.  That is,
F = -ky -----------1
 Where, k is the force constant which depends on the
stiffness of the spring
 -ve sign indicate that F is a restoring force.ie, the direction of
the force is opposite the direction of the displacement.
Harmonic Oscillator
-A
y 0
P.E. A
-
A +
A
Displacement y
22THEORY OF I.R. & FT-IR.

23. Potential energy of a harmonic oscillator
 The potential energy E of mass and spring can be
arbitrarily assigned a value of zero, when the mass is in
its rest or equilibrium position.As the spring is
compressed or stretched, however P.E. of this system
increases by an amount equal to the work required to
displace the mass.
 If for example the mass is moved from some position y
to y+dy, the work & hence the change in Potential
energy dE is equal to the force F times the distance dy.
Thus, dE = -Fdy -----------------2
 Substituting 1 in 2 we get, dE=kydy
23THEORY OF I.R. & FT-IR.

24.  Integrating between the equilibrium position y=o &y gives
0
E∫ dE = 0
y∫ydy
E=1/2 ky2 --------------------------3
 The potential energy curve for a simple harmonic
oscillation is parabola. P.E. is maximum when the spring is
stretched or compressed to the maximum amplitude A &it
decreases to zero at the equilibrium position.
Vibrational Frequency
 The motion of the mass as a function of time t can be
deduced from classical mechanics as follows
 Newtons second law states that,
F = ma
Where, ‘m’ is the mass & ‘a’is the acceleration
24THEORY OF I.R. & FT-IR.

25. But acceleration is the second derivative of distance with respect to time.
Thus, a = d2
y/dt2
Substituting a in 1 we get, m d2
y/dt2
=-
ky --------------4
A solution to this equation must be a periodic function such that its second
derivative is equal to the original function times –
k/m. A suitable cosine
relationship meets the requirement. Thus the displacement of mass at time t can be
written as, y=Acos 2∏Vmt --------------------5
Where Vm is the natural vibrational frequency
A is the maximum amplitude of motion
25THEORY OF I.R. & FT-IR.

26. The second derivative of equation 5 is, d2
y/dt2
= -4∏2
Vm
2
Acos2∏Vmt-----------6
Substituting 5 & 6 in 4 we get
A cos 2∏Vmt = 4∏2
Vm
2
m A cos 2∏Vmt
K
The natural frequency of oscillation is then
Vm = 1/2√k/m
Where Vm is the natural frequency of mechanical oscillation.
The equation just developed may be modified to describe the behavior of a system
consisting of two masses m1 & m2 connected by spring.Here we have to substitute
mass µ for single mass m where,
µ =m1m2
m1+m2
26
THEORY OF I.R. & FT-IR.

27. The equation just developed may be modified to describe the behavior of a system
consisting of two masses m1 & m2 connected by spring.Here we have to substitute
mass µ for single mass m where,
µ =m1m2
m1+m2
Thus the vibrational frequency for such a system is given by,
Vm=1/2∏ √k/µ
Vm=1/2∏ √k(m1+m2)/m1+m2
27THEORY OF I.R. & FT-IR.

28. 28THEORY OF I.R. & FT-IR.

29. FTIR
29THEORY OF I.R. & FT-IR.

30.  FT-IR stands for Fourier Transform Infrared,
the preferred method of infrared spectroscopy.
In infrared spectroscopy, IR radiation is passed
through a sample. Some of the infrared radiation
is absorbed by the sample and some of it is
passed through (transmitted).
 The resulting spectrum represents the molecular
absorption and transmission, creating a
molecular fingerprint of the sample.
30THEORY OF I.R. & FT-IR.

31. THEORY
. Fourier Transform Infrared (FT-IR) spectrometry
was developed in order to overcome the limitations
encountered with dispersive instruments.
 The main difficulty was the slow scanning
process. A method for measuring all of the infrared
frequencies simultaneously, rather than individually,
was needed.
 A solution was developed which employed a
very simple optical device called an interferometer.
The interferometer produces a unique type of signal
which has all of the infrared frequencies “encoded”
into it.
31THEORY OF I.R. & FT-IR.

32. The signal can be measured very quickly,
usually on the order of one second or so. Thus, the
time element per sample is reduced to a matter of
a few seconds rather than several minutes.
Most interferometers employ a beamsplitter
which takes the incoming infrared beam and
divides it into two optical beams.
 One beam reflects off of a flat mirror
which is fixed in place. The other beam reflects
off of a flat mirror which is on a mechanism
which allows this mirror to move a very short
distance (typically a few millimeters) away from
the beamsplitter.
32THEORY OF I.R. & FT-IR.

33.  The two beams reflect off of their respective
mirrors and are recombined when they meet back
at the beamsplitter. Because the path that one
beam travels is a fixed length and the other is
constantly changing as its mirror moves, the signal
which exits the interferometer is the result of these
two beams “interfering” with each other.
 The resulting signal is called an interferogram
which has the unique property that every data
point (a function of the moving mirror position)
which makes up the signal has information about
every infrared frequency which comes from the
source.
33THEORY OF I.R. & FT-IR.

34. 34THEORY OF I.R. & FT-IR.

35. 35THEORY OF I.R. & FT-IR.

36. This means that as the interferogram is measured, all
frequencies are being measured simultaneously.
Thus, the use of the interferometer results in
extremely fast measurements.
 Because the analyst requires a frequency
spectrum (a plot of the intensity at each individual
frequency) in order to make an identification, the
measured interferogram signal can not be interpreted
directly.
 A means of “decoding” the individual
frequencies is required. This can be accomplished via
a well-known mathematical technique called the
Fourier transformation. This transformation is
performed by the computer which then presents the
user with the desired spectral information for
analysis.
36THEORY OF I.R. & FT-IR.

37. Fourier transform infrared spectroscopy is
preferred over dispersive or filter methods of
infrared spectral analysis for several reasons:
• It is a non-destructive technique.
• It provides a precise measurement method
which requires no external calibration.
• It can increase speed, collecting a scan every
second.
• It can increase sensitivity.
• It has greater optical throughput.
• It is mechanically simple with only one moving
part.
37THEORY OF I.R. & FT-IR.

38. REFERENCE
1)Instrumental Analysis
By Skoog,Holler,Crouch,
Indian edition(2009),Pg.no:477-490
2)Instrumental analysis of chemical analysis
By Gurdeep.R.Chatwal & Sham.K.Anand,
Himalaya Publishing house, 5thedition,
Pg.no:2.29-2.40
3)Pharmaceutical Drug Analysis
By Ashuthosh Kar
Page No: 315 – 318
4) Introduction to FT-IR Spectrometry
By Thermo Nicolet corporation.
38THEORY OF I.R. & FT-IR.

39. 39THEORY OF I.R. & FT-IR.