This document provides an overview of infrared (IR) spectroscopy. It discusses how IR spectroscopy works by absorbing IR radiation that causes bonds to stretch and bend. It describes the different IR regions and the types of molecular vibrations that occur in each region. Factors that influence vibrational frequencies like hydrogen bonding, electronic effects, and coupling are also summarized. The document concludes by discussing considerations for sampling solids, liquids, and gases for IR spectroscopy analysis.

1. Presented by
Bhabuk Paudel
M.Pharm (Industrial)

2. Introduction:
• IR is a type of absorption spectroscopy which involves chemical
identification of compounds.
• Absorption of IR radiation causes various bond present in the
compound to stretch and bend with respect to each other.
• When an IR radiation is passed to a sample, the bond absorbs the
radiation, resulting in vibration of molecule and thus causes
excitation of molecule from lower to higher vibration level.

3. Cont….
• The compound absorbs IR radiation only when its fundamental
frequency is equal to applied IR frequency
• Eg: N-H group containing compound has fundamental
frequency of 3300cm-1, hence absorbs the IR radiation of
3300cm-1; other remaining are transmitted

4. Major requirement for IR
• Compound should have Dipole moment
• Two compound will never show similar IR spectra, if they are
not enantiomers.

5. Range:
• IR region is a part of electromagnetic spectrum which lies
between visible and microwave region of electromagnetic
radiation.

6. 6
REGION WAVE
LENGTH
λ (μ)
WAVE NUMBER
υ (cm-1)
NEAR 0.8 - 2.5 12500 - 4000
MIDDLE 2.5 - 15 4000 - 667
FAR 15 - 200 667-50
MOST USED 2.5 - 15 4000 - 667
IR-REGION: 12,500 - 50 cm-1
1.Near IR---------------------carbohydrates and proteins
2.Middle IR----organic molecules—functional groups
3.Far IR— in-organic –co-ordination bonds& quaternary
ammonium compounds, metal complexes

7. Units & Measurement
.
=
Wavenumber = _______1_________ cm-1
wavelength
IR spectrum may be expressed by wave number whose unit is
cm-1

8. IR spectrum
It is the plot of %transmitted vs increasing or
decreasing frequency.

9. IR spectra consists of two regions:
i. Group frequency region
ii. Functional group region

10. Group Frequency Region
• Region from 4000-1500 cm-1
• This region is due to absorption band of vibration state due to
various bond present in molecule.
• Mainly used to identify functional group present in the
compound.
• Mostly stretching vibrations occurs in this region.

11. Finger Print Region
• Region from 1500-667 cm-1
• Consists of vibrational and rotational spectra of complex
molecule since no two compounds have same IR spectra, this
region is useful for identification of the compounds.
• Also used to identify isomeric compounds.
• The number of bending vibrations are more hence, mostly
bending vibrations occurs in this region.

12. Finger print region can be further divided into:
i. 1500-1350 cm-1
ii. 1350-1000 cm-1
iii. Below 1000 cm-1
1500-1350 cm-1 :
The appearance of double peaks near 1380cm-1 (medium) and 1365
cm-1 (intense) shows presence of tertiary butyl group.
1350-1000 cm-1 :
alcohols, esters, lactones, acid anhydride show characteristic
absorption in this region due to c-o stretching.
Below 1000 cm-1 :
The number of substituent's and their position(0,m,p) in benzene
ring can be determined from the characteristic absorption band in
this region

13. Theory
• Absorption in IR region is due to the change in dipole moment
or vibrational rotational levels.
• When vibrations with frequency range less then 100 cm-1 are
absorbed, molecular rotation takes place.
• When more energetic radiations in the region 10000-100 cm-1
are passed , molecular vibration takes place. This molecular
vibration energy depends upon:
1. Mass of atom
2. Strength of bond
3. The arrangement of atom with in the molecule

14.  Stretching vibrations
1. Change in Bond length
2. Occurs at higher frequency
Modes of vibration
(types of fundamental vibrations)
It involves change in bond length
either increase or decrease without
any change in bond axis or bond
angle.

15. •Symmetric
Two atoms move away or towards
Central atom without changing bond angle
•Asymmetric
Two atoms move with respect to
Central atom in such a way that, one moves
Towards and another away from central atom
Two types:

16.  Bending vibrations
1. Change in Bond angle
2. Occurs at lower frequency
It involves movements of atoms attached to
Central atom in such a way that there is change
In bond axis or bond angle of each atom without
any change in their bind lengths.
2.Rocking
Two atoms move back and form,
with respect to central atom
1.Scissoring
Two atoms move back and forth,
with respect to central atom
1.Twisting
Two atoms move up and down the plane,
with respect to central atom
2.Wagging
Among two atoms, one is above the plane
and other is below the plane.
Two types:
 In-plane:  Out of plane:

17. Scissoring
Rocking
Twisting
Wagging
In plane: Out of plane:

18. Hooke's law:
The stretching vibration frequency of a bond ban be calculate based on
Hooke’s law
Thus the value of vibrational frequency depends upon:
•Bond strength
•Reduced mass
Vibration in the bond/molecule

19. Masses of attached atoms. As reduced masses decrease, wave number
increases.
C-H stretching absorbs at higher frequency than c-c stretching, due to small
value of reduced mass for C-H, compared to c-c bonds.
Strength of chemical bond. As bond strength increases, wave number
increases.
19
C=C stretching is expected to absorb at higher frequency than c-c
stretching, due to higher bond strength of double bond as
compared to single bond.

20. Factor Affecting Vibrational Freuency
• The value of absorption frequency is shifted if the force constant of a bond
changes with its electronic structure.
• Frequency shifts also take place on working with the same substance in
different states (solids, liquids & vapour).
• A substance usually absorbs at higher frequency in a vapour state as
compared to liquid and solid states.
• Factors responsible for shifting the vibrational frequencies from their
normal values
—Coupled vibrations
—Fermi resonance
—Electronic effects
—Hydrogen bonding

21. Coupled Vibration
• An isolated C-H bond has only one
stretching vibrational frequency where
as methylene group shows two stretching vibrations,
symmetrical and asymmetrical.
• Because of mechanical coupling or interaction between C-H
stretching vibrations in the CH2 group.
• Assymetric vibrations occur at higher frequencies or wave
numbers than symmetric stretching vibrations.
• These are known as coupled vibrations because these
vibrations occur at different frequencies than that required for
an isolated C-H stretching

23. • In carboxylic acid anhydride, coupling occur between 2
carbonyl group linked through -0-
• Resonance in carbonyl oxygen keep system coplanar.
• In amides original characteristic of C=0 and N-H are
modified to amide I and amide II
• Coupling between C –N stretching and N-H bending
occur
• Peak of amide I due to C=O stretching
• Peak of amide II due to coupling
• In aldehyde C-H appear as fundamental and overton of C-
H

24. Fermi Resonance
• When EMR or IR radiation will attack to the molecule, after
that molecule will go from ground state to second excited state
called OVERTONE BAND.
• Intensity of band is very low.
• Then, there may be chance of coincidence with same energy
level of fundamental frequency.
• so in that case a resonance will occur and that resonance
known as FERMI RESONANCE.
• Due to this effect wave number will be INCREASED.

25. • It is also found in the vibrational spectra of aldehydes
• Where the C-H bond in the CHO group interacts with
the second harmonic level, derived from the
fundamental frequency of the deformation vibration of
the CHO group (2*1400 cm-1).
• The result is a Fermi doublet with branches around
2830 cm-1 and 2730 cm-1.
• It is important for Fermi resonance that the vibrations
connected with the two interacting levels be localized
in the same part of the molecule.

26. Electronic Effect
• Changes in the absorption frequencies for a particular group
take place when the substituent's in the neighborhood of that
particular group are changed.
• It includes :
1-Inductive effect
2-Mesomeric effect

27. 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
DECRASES.
• Let us compare the wave numbers of v (C=O) absorptions for the following
compounds :
• —Formaldehyde (HCHO) 1750 cm-1.
• —Acetaldehyde (CH3CHO) 1745 cm-1.
• —Acetone (CH3COCH3) 1715 cm-1.
• 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.

28. Mesomeric Effect
• It causes lengthening of a bond
• Strength of bond decreased
• Force constant decreased
• Decreased wave number

29. Hydrogen Bonding
• Due to hydrogen bonding wave number shifts
towards the LOWER ENDS.
• Examples
N-H H-N
Without Hydrogen bonding (3500 cm-1) Hydrogen bonding (3300 cm-1)
O-H H-O
Without Hydrogen bonding (3650 cm-1) Hydrogen bonding (3450 cm-1)

30. • Stronger the hydrogen bonding greater is the absorption shift from
normal value
• O –H and N –H in compound have more effect due to hydrogen
bonding in I.R spectra
• Interaction of functional group of solvent and solute cause H
bonding
• Selection of non associating solvent (CCl4 , CS2 ) or more polar
solvent (acetone, benzene) also influence hydrogen bonding.
• Alcohols in phenols give broad peak due to strong H bonding in
polymeric association; dimers,trimmersdue to intermolecular H
bonding
• If solution diluted free molecule proportion increases as less
intermolecular H bonding decrease and another peak observed

33. • In carbonyl compound basicity of C = O result to
strong H bonding and lowering of vibration
frequency
• Alkenes and 𝜋bonds behave as lewis bases so can
form H bond with acidic hydrogen and decrease
frequency
• N is less electronegative than O so H bond in
amine are weaker than alcohol, thus shift of
frequency is also less than alcohol.

34. Sample Holders & Sampling of Substances
• State of the sample i.e., Solid, Liquid or Gas.
• The intermolecular forces of attraction are most operative in
solid phase and least in case of gases.
• The sample of same substance shows shift in frequencies of
absorption as it passes from solid to the gaseous state.
• In some cases additional bands are also observed with the
change in state of the sample.
• Important to mention the state of the sample & solvent for
scanning in IR region for correct interpretation of spectra.

35. Cont…
• The alkali halides are widely used, particularly NaCl, which
are transparent at wavelengths as long as 625cm-1.
• Cell windows are easily fogged by exposure to moisture and
require frequent repolishing.
• AgCl3 is often used for moist samples or aqueous solutions.
• But it is soft, is easily deformed, and darkens on exposure to
visible light.
• Avoid used of Water or moisture content because they are
highly absorbed under IR radiation
• For frequencies less than 600cm-1, a Polyethylene cell is
useful.

36. Sampling Of Solids
• The solid whose IR spectras are to be recorded can be
sampled in various ways:
A. Direct Sampling
B. Solids run in solution
C. Nujol Mull Technique
D. Pressed Pellet Technique

37. Pressed Pellet Technique:-
• solid sample (1-2% of KBr) is mixed with powdered KBr.
• After proper mixing particle size of mixer should be less than 2
micrometer.
• This mixture is pressed under very high pressure (at least 25,000
psig) to form a small pellet which is very fragile and transparent to
IR radiation.
• The powder (KBr+sample) is introduced as shown, and then the
upper screw is tightened until the powder is compressed into a thin
disc.
• After compressing the sample, one removes the bolts and places the
steel cylinder with the sample disc inside it in the path of the beam
of IR spectrometer & a blank KBr pellet of identical thickness is
kept in the path of reference beam.

40. Sampling of Liquids
• Two layer of NaCl or KBr pellets/thin layer (0.1-0.3 mm) is
used
• Liquid will be sandwich between two NaCl pellets and this
will be used for the analyzing the sample.
NaCl Pellet
NaCl Pellet
sample

41. Sampling of Gases
• The gas sample cell is used and made of KBr, NaCl.
• Usually they are about 10cm long, but they may be up to 1m
long.
• Multiple reflections can be used to make effective path length
as long as 40m, so that constituents of the gas can be
determined.