some basic about IR spectroscopy

1. IR SPECTROSCOPY
• PRESENTED TO DR SHAISTA GILLANI
• PRESENTED BY RIMSHA BAKHTAWAR RASHEED
• FA-19 MSC(CHEM)006
• 12-4-2021
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2. CONTANTS
 INTRODUCTION OF SPECTROSCOPY
 TYPES
 INFRARED SPECTROSCOPY
 PRINCIPLE OF IR
 MODES OF VIBRATION
 HOOKS LAW
 FACTORS
 APPLICATION
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3. WHAT IS SPECTROSCOPY
Method of “Seeing the unseeable”
EMR
So by Using electromagnetic radiation we obtain information about
atoms and molecules that are too small to seen.
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4. 4
Spectroscopy is an instrumentally aided study of the interactions between matter
(sample being analyzed) and energy (any portion of the electromagnetic spectrum)

5. IR SPECTROSCOPY
• Infrared spectroscopy is the measurement of the interaction of infrared
radiation with matter by absorption, emission, or reflection. It is used to study
and identify chemical substances or functional groups in solid, liquid, or
gaseous forms.
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6. IR SPECTROSCOPY
When infrared light radiations between 4000-400 cm-1 are
passed through a sample of an organic compound, some of
these radiations are absorbed by the sample and are
converted into energy of molecular vibrations. So it is also
called vibrational spectroscopy.
Due to vibration there is a
change of bond length
and bond angle in the
molecules.

7. TYPES
• Most of the analytical applications are
confined to the middle IR region
because absorption of organic
molecules are high in this region.
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8. PRINCIPLE OF IR SPECTROSCOPY
• The two atoms joined together by a chemical bond (may be single, double
or triple bond), macroscopically can be composed as two balls joined by a
spring
• These vibrations depend on the
strength of the spring and also the
mode (stretching or bending) in
which the force is being applied.
• This characteristic vibration are
called Natural frequency of
vibration.
• Applied infrared frequency =
Natural frequency of vibration
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9. 9
• Vibration or oscillation along the line of
bond
• Change in bond length
• Occurs at higher energy: 4000-1250 cm-1
•Vibration not along the line of bond
•Bond angle is altered
•Occurs at low energy: 1400-666 cm-1

10. STRETCHING VIBRATION
• A stretching vibration changes the bond length. There are two types of stretching vibrations
•
• In asymmetric stretching, some bonds are getting
shorter at the same time as others are getting longer.
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 In symmetric stretching, two or more
bonds vibrate in and out together.

11. BENDING VIBRATIONS(IN PLANE)
• Scissoring is the movement of two atoms
toward and away from each other
• Rocking is like the motion of a pendulum on a
clock, but an atom is the pendulum and there
are two instead of one
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12. BENDING VIBRATION(OUT THE PLANE)
• Wagging is like the motion in which make a
"V" sign as we make with our fingers and bend
them back and forth from your wrist.
• Twisting is a motion as if the atoms were
walking on a treadmill.
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13. GRAPHICAL REPRESENTATION
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Each type of vibration contributes to an infrared spectrum. For example,
here is the spectrum of formaldehyde, H₂C=O.

14. HOOKE’S LAW
• It says that the amount of stress we apply on any object is
equal to that amount of strain is observed on it, which
means Stress ∝ Strain.
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Hooke’s Law Formula is given as
F = -K x
Where,
F is the amount of force applied in
N,
x is the displacement in the spring
in m,
k is the spring constant or force
constant.

15. HOOKES' LAW
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Consider a bond and the connected
atoms to be a spring with two
masses attached.
Using the force constant k (which
reflects the stiffness of the spring)
and the two masses m1 and m2,
then the equation indicates how the
frequency, u, of the absorption
should change as the properties of
the system change.

16. TRENDS
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absorption bands depends on bond order. Higher the bond order larger is the
band frequency.
. The force constant(K) is a measure of the strength of a bond.
Thus the value of stretching frequency of a bond increases with increasing the bond
strength and decreasing the reduced mass of the bonded atoms.

17. CONTINUE..
As the atom attached to carbon a particular atom say
carbon increases in mass, the reduced mass increases
and hence the frequency of vibration
decreases compare:
C-H (3000 cm-1)
C-0 (1100cm-1)
C-C (1000 cm-1)
C-Cl (800 cm-1)
C-Br (550 cm-1)
C-I (about 500 cm-1)
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Larger mass
lesser
absorption

18. TRENDS THAT CAN BE ACCOUNTED FOR USING
HOOKES' LAW.
• It also gives an approximate outline of where specific types of bond stretches may be found.
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19. FACTORS AFFECTING THE
ABSORPTION FREQUENCIES.
• The vibrational frequency of a bond increase as a strength increase and reduce mass
decrease µ
• Frequency shift also take place when working on same substance at different states (
solid , liquid , vapor)
• A substance usually absorb at higher frequency in a vapor state as compared to liquid
or solid state.
• Factors responsible for shifting the vibrational frequencies to their normal value are
• Electronic effect
• Hydrogen effect
• Ring stain
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20. ELECTRONIC EFFECT
Changes in the absorption frequencies for a particular group take place when the
substituents in the neighbourhood of that particular group are changed.
It includes :
• Inductive effect
• Resonance effect
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21. INDUCTIVE EFFECT
• The introduction of alkyl group causes +I
effect which results in the lengthening or the
weakening of the bond
• Hence the force constant is lowered and
wave number of absorption decreases.
• Introduction of an electronegative atom or
group causes –I effect which results in the
bond order to increase.
• Hence the force constant increases and the
wave number of absorption rises.
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1750cm-1 1745cm-1
1715cm-1
More +I less wavenumber
More –I more wavenumber

22. RESONANCE EFFECT
• Resonance effect has significant influence on the absorption frequency in the infrared
spectra the conjugation lowers absorption frequency or wave number
• This is because conjugation tends to increase the bond length of the group so that its bond
strength decrease. this in turn lowers the force constant of the bond and the absorption
frequency decrease.
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23. HYDROGEN BONDING:
•
The infrared spectra of organic compounds such as alcohol ,phenols, carboxylic acid and amines are
strongly influenced by the presence of hydrogen bond.
• Hydrogen bonding results in the weakening of bond involved so that force constant decrease and
absorption shift toward lower frequency or wave number.
• Hydrogen bonding to a C=O group withdraws electrons from oxygen and lowers the C=O double
bond character. This results in lowering of C=O absorption frequency. More effective is the hydrogen
bonding, higher will be the lowering in C=O absorption frequencies.
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24. CONTINUE ….
• The monomeric carboxylic acids absorb at the frequency of 1760 cm-1 where dimers at
1710cm-1 due to the presence of hydrogen bonding with carbonyl group.
• In methyl salicylate lowers the C=O stretching frequency to 1680 cm-1 than observed
at 1700 cm in case of methyl p-hydroxybenzoate.
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1710cm-1

25. RING STAIN
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As the size of the ring reduce their strength is continuously increasing
And due to size reduction their mass also reduce so frequencies
increase.

26. APPLICATION
1.
Identification of functional group and structure
Each atom of the molecule is connected by bond and each bond requires different IR region so characteristic
peaks are observed. This region of IR spectrum is called as finger print region of the molecule.
2. Detection of impurities
3. Detection of hydrogen bonding & distinguish inter & intramolecular
4. Identification of drugs
• Ibuprofen in IP, BP and USP identified by IR- because it contains COOH group so, chemical tests are not useful
for identification.
5. Identify odor & taste components of food
6. Determine atmospheric pollutants from atmosphere itself.
7. Examination of old paintings & artifacts
8. To determination of raw materials.
9. Quality control of pharmaceutical formulation
10. Determination of particle size
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27. QUIZ
1 . On which factors the vibrational stretching frequency of diatomic molecule depend?
a) Force constant
b) Atomic population
c) Temperature
d) Magnetic field
2 . What is the order of decreasing vibrational frequency for C — Cl, C — Br, C — C, C —
O and C — H?
a) C-H, C-C, C-O, C- Cl, C-Br
b) C- Cl, C-Br, C-C, C -H, C-O
c) C-O, C-H, C-Br, C- Cl, C-C
d) C-Br, C- Cl, C-C, C-O, C-H
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28. 3 . Infrared spectroscopy is based on
a) Absorption spectroscopy
b) Emission spectroscopy
c) Elastic scattering
d) Reflection spectroscopy
4 . What is the relation between restoring force, f to the displacement q in
Hooke’s law?
a) a) f = -kq
b) b) f = kq
c) c) f = kq2
d) d) f = -kq2
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29. 5 . What is the correct increasing order of stretching frequencies for C ≡ C, C = C and
C — C?
a) C — C > C = C > C ≡ C
b) C ≡ C > C = C > C — C
c) C — C > C = C < C ≡ C
d) C ≡ C < C — C > C = C
6 . Which of the following molecules will not show infrared spectrum?
a) H2
b) HCI
c) CH4
d) H20
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30. Q NO 1 The carbonyl stretching frequency in RCOCH3 (~1720 cm-1) is lower than
acid chloride RCOCl (1750-1820 cm-1)?why
Q NO 2 Why asymmetric stretching is higher than symmetric stretching?
Q N0 3 What factors influence IR?
Q NO 4 why C-C not show peaks in IR spectrum ?
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31. REFERANCES
• https://socratic.org/questions/what-are-the-differences-between-stretching-
vibration-and-bending-vibrations.
• https://byjus.com/hookes-law-formula/
• https://www.slideshare.net/VrushaliTambe2/factors-affecting-ir-absorption-frequency
• http://epgp.inflibnet.ac.in/epgpdata/uploads/epgp_content/S000005CH/P000667/M
009109/ET/1459227637CHE_P12_M8_e-Text.pdf
• https://www.slideshare.net/rashmiraveendran92/factors-affecting-vibrational-
frequencies-in-ir
• https://classroom.google.com/u/0/c/Mjk2NDA0NDM2Mzk3
• https://www.youtube.com/watch?v=8SVHLzs35lI
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