Infrared spectroscopy is technique to identify the functional group of the molecule. In Infrared spectroscopy there are two main region finger print region and functional group region. Most of the molecules identifies In the finger print region due to that it is complex region. Now we will see the principle of IR spectroscopy: IR spectroscopy is vibrational energy level changes when IR radiation passes through the material.

1. Khulna University of Engineering & Technology
(KUET)
Presentation on
Presented by….
Shuvodip Mondal
Dept. of Chemistry, KUET

2. What is Spectroscopy?
Spectroscopy is the Study of interaction between matter and electromagnetic radiation
e.g., IR Spectroscopy is concerned with the study of absorption of infrared radiation, which causes vibrational
transition of bonds in molecules. So it is also called vibrational Spectroscopy .
IR Spectroscopy mainly used to identify the functional group, but also gives information likes
No. of bond, Specific stereo-isomer, Atomic arrangement in the molecule.

3. What is Spectroscopy?
Spectroscopy is the Study of interaction between matter and electromagnetic radiation
e.g., IR Spectroscopy is concerned with the study of absorption of infrared radiation, which causes vibrational
transition of bonds in molecules. So it is also called vibrational Spectroscopy .
Why NIR & FIR are not used in IR Spectroscopy???

4. IR Region
Near IR Mid IR Far IR
Wave Length: 0.8 – 2.5 μm
Wave number: 12,500 – 4,000 cm-1
Wave Length: 2.5 – 5.0 μm
Wave number: 4,000 – 200 cm-1
Wave Length: 50 – 1000 μm
Wave number: 200 – 10 cm-1
Interfered with UV
radiation and gives extra
IVCT band on IR spectra
due to delocalization of
electron
FIR is also closer to the radio-wave
region and sometimes bandly interact
& formed extra unwanted band spectra
on the IR spectrum.
MIR is appropriate for the IR spectroscopy. Because it gives fully IR peak on IR spectrum.
So, We used 2.5 – 5.0 μm wavelength in IR Spectroscopy

5. Principle of IR Spectroscopy
When IR radiation interact with sample molecule, the sample molecule will absorb the specific
applied quantized IR light which is matched with the internal vibrational frequency of the
molecule and other residual frequencies are transmitted via the sample. Due to absorption of IR
radiation, the net change of dipole moment in molecule is occurred and causes vibration of bonds in
the molecule like Stretching and bending vibration. The transmitted light is detected by the
detector and IR spectrum interpreted on the computer screen by analysis of the transmitted
light.
IR Radiation
(Mid IR)
Range: 4000 – 400 cm-1
Sample
(absorbs Specific frequency)
Transmitted Light
% 𝑜𝑓 𝑇
Wave number, cm-1
Position Change of Atoms in a molecule
due to net change of dipole Moment
Change in amplitude of molecular Vibration
Stretching & Bending Vibration

6. FIGURE: FOURIER TRANSFORM SPECTROPHOTOMETER
Radiation source: NERST glower or Globar Source which produce Mid IR
Working Function: The source energy strikes the beamsplitter and produces two beams of roughly the same
intensity. One beam strikes the fixed mirror and returns to the beamsplitter. The other beam goes to the moving
mirror. When these two beams meet up again at the beamsplitter, they recombine. The recombined beam
passes through the sample……..

7. Stretching Vibration
Higher Frequency
Bending vibration
Lower Frequency
Molecular Vibration
Symmetric
Stretching
Asymmetric
Stretching
In-Plane
Bending
Out of Plane
Bending
Scissoring Bending Rocking Bending
Twisting Bending Wagging Bending
NOTE: No. of bending vibration is usually more than the no. of stretching vibration in IR spectrum.

8. a) Stretching Vibration involves a continuous change in interatomic
distance along the axis of bond between 2 atoms.
In Symmetric Stretching, the bond length increase & decrease
symmetrically.
In Asymmetric Stretching, length of one bond increase & the other
one decrease at the same time without changing bond angle.
b) Bending vibration involves the change of angle between two atoms
Scissoring is the movement of two atoms toward and away from each
other. So bond angle decrease and increase with time.
Rocking in which the bond angle is maintained but bonds moves within
the same plane. Changing angle b/w a group of atoms.
Wagging in which both atoms move to one side of the plane. Changing
angle b/w the plane a group of atoms
Twisting in which one atom is above the plane and the other is below
the plane. Changing angle b/w the plane 2 groups of atoms

9. What are the conditions or criteria to absorb IR radiation??
Those are,
 When applied infrared frequency in equal to natural frequency of vibration, then the absorption take place
and a peak is observed.
 Only molecule gives IR absorption.
 Molecules must have a dipole moment.
 The mode of vibration of the molecule should not be center of symmetry.
 The molecule should be polar.
Note: It is important to remember that it is not necessary for a compound to have a
permanent dipole moment to be IR active.

10. Modes of molecule with respect to electromagnetic radiation ?
Modes are,
I. Translation
II. Rotational
III. Vibrational
Now, The total no. of degrees of freedom = ( Translation + Rotational + Vibrational ) degrees of freedom
𝑜𝑟, Vibrational degree of freedom = 3N - (Translation + Rotational ) degrees of freedom
∴ Vibrational degree of freedom = 3N - ( 3 + Rotational ) degrees of freedom
For Linear molecule,
Vibrational degrees of freedom = 3N - ( 3 + 2 ) = 3N - 5
For Non-Linear molecule,
Vibrational degree of freedom = 3N - ( 3 + 3 ) = 3N - 6
No. of atoms in the molecule
E.g. Vibrational degrees of freedom of CO = 3 × 3 − 5 = 4 𝑎𝑛𝑑 𝑎𝑙𝑠𝑜
Vibrational degrees of freedom of H2O = 3 × 3 − 6 = 3
For explain this, We should know about degrees of freedom ??

11. Calculation of
Vibrational frequency
For Single bond, K= 5× 105
dyne / cm
Double bond, K = 10 × 105
dyne / cm
Triple bond, K = 15× 105
dyne / cm

12. What is IR Spectrum?
IR Spectrum is the graphical plot of % transmittance verses vibrational frequency in
wavenumbers.
% T
Wave Number, cm-1
T =
𝐼
𝐼∙
=
𝐼𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑇𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑 𝑙𝑖𝑔𝑕𝑡
Intensity of incident light
IR Spectrum
Functional Group Region
4,000 – 1500 cm-1
Finger-print Region
1500 – 500 cm-1
Identifies the functional group with the
consequence of changing stretching vibration
Identifies the exact molecule with the
consequence of changing bending vibration
Only stretching mode of vibration, so this
region contains few peaks.
Stretching and bending both are occurred,
but bending is more than stretching
The pattern of peaks is easy to analyze
because stretching vary within a narrow
range.
The pattern of peaks is more complicated
due to complex vibration which create a
unique IR band to distinguished compounds.
Sharp, Broad, Strong, Medium Peaks Sharp & Weak Peaks

13. Peaks
of Functional Group
Shape
(Width of peak)
Weak
Broad Sharp
Intensity
(height of peak)
Strong
Medium
Due to H-bonding or
Presence of water
molecules
A medium band falls to about half of the y-axis
A strong band covers most of the y-axis
A weak band falls to about one third or less of the y-axis
Factors affecting the Peak intensity :
Electronegativity difference α Polarity α Dipole moment α More Intense Peak

14. Factors affecting for Vibrational frequencies of the functional group
3. More Vibration frequency = More Force-Contact (K)
1. Effect of different mode of vibration: V. frequency of stretching is higher than the bending mode of bond
4. Electronic effect:
Conjugation / Resonance α vibrational frequency
5. H-bonding effect α 1
𝑉𝑖𝑏𝑟𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦
6. Multiplicity of bond: No. of bond α Vibrational frequency
7. Nature of Hybridization: % of S- character α Bond Strength α 1
𝐵𝑜𝑛𝑑 𝑙𝑒𝑛𝑔𝑡𝑕
α V. frequency
2. Reduced Mass of atoms α
1
𝑉𝑖𝑏𝑟𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦

15. How to interpret / analyze IR spectra??
IR spectra isn’t generally used to determine the whole structure of an unknown molecule. But IR
spectra is great for identifying certain specific groups.
Here an overview of the IR window from 4000 to 500 cm-1 with various region of interest highlighted.
Basically 80% of the most useful information for our purpose can be obtained by looking at 3600 to 1600 cm-1 specific
area of spectrum. Reaming forest of peak from 500 – 1400 cm-1 (fingerprint region) comparatively less useful.

16. NO. Peak take place
between (cm-1)
Common Symbols of
bond
Examples of bond of
functional group
ZONE 1 3600 – 2700 X –H O-H bond, C–H bond, N-H bond
ZONE 2 2700 – 1900 X ≡ X C≡N or C≡C bond
ZONE 3 1900 – 1500 X=O, X=X C=O bond, C=C bond
ZONE 4 1500 -- 1000 X –O , X –X C-O bond, C – C Bond
Here, X symbol will be Oxygen [O] or Carbon [C] or Nitrogen [N]
An even more compressed overview looks like this…
For Zone 1:
a) O-H bond stretching at 3650 – 3200 cm-1 with strong intensity & broad ‘U’ elongated Shape
b) Terminal alkynes (≡C–H bond) stretching at 3340 – 3250 cm-1 with stronger & sharper peak

17. c) N-H bond stretching mainly occurred for Primary & Secondary amine:
For primary amine, stretching at 3500 – 3200 cm-1 with two signal. These are
medium in intensity & broad shape.
For 2ndary amine, stretching at 3500- 3200 cm-1 with one signal . These are weak
intensity & sharp shape
d) =C–H Stretching at 3100 cm-1 with weak intense & sharp peak
e) - C–H Stretching at 2900 – 2800 cm-1 with medium intense & sharp peak
NOTE: for aromatic compound, out of plane =C–H bending peak at 900 – 690 cm-1

18. For Zone 2:
a) C≡C bond stretching at 2200 – 2000 cm-1 with variable intense & shape peak
b) C≡N bond stretching at 2260 – 2220 cm-1 with variable intense & shape peak
For Zone 3:
a) C=O bond stretching peak
for aldehyde at 2200 – 2000 cm-1 , Ketone at 1700- 1725 cm-1, Carboxylic acid at 1715 cm-1, Ester at 1750 – 1735 cm-1

19. b) C=C bond stretching peak at 1660 – 1600 cm-1
c) C=C bond stretching for aromatic compound peak at 1510 – 1450 cm-1
For Zone 4:
a) C–C bond stretching: peak at 1200 – 1000 cm-1
b) C–O bond stretching: peak at 1150 – 1000 cm-1

20. Think for 1 min; How can you detect compound from IR Spectra ??

21. Application of IR Spectroscopy
i. Identification of an organic compound
ii. Structure determinations
iii. Identification of functional group
iv. Studying the progress of reaction
v. Distinction between two types of hydrogen bonding
vi. Study of keto-enol tautomerism
vii. Study of Ciss-trans Isomerization
viii. Conformation analysis
ix. Determination of impurities in compound
x. Presence of water in sample

22. Limitations of IR Spectroscopy
a. Can’t determine the molecular weight of the compound.
b. Doesn’t give information about the relative position of different functional
groups in a molecule.
c. From the single IR spectra of an unknown substance, it is not possible to know
whether it is pure compound or a mixture of compound.
d. Sample cells are made of halogen salts which are susceptible to moisture
e. Gas samples can’t be analyzed as the lack of sensitivity.

23. Thanks For having me…