Presented by
N. Lakshmi PriyaM.Pharm( Pharmaceutics)
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Infrared spectroscopy is an important analytical tool
for the extensive and intensive study of the structure
of the molecule.
It is also known as vibrational frequency as it results
in the vibrational transitions.
This technique can determine the functional groups
present in the structure of a compound.
Infrared spectra is due to change in vibrational
energy accompanied by changes in rotational energy.
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The infrared region may be classified into three zones
based on their respective wave numbers and their
wavelengths.
BASED ON WAVELENGTHS :
Mid IR region:2.5-15µ
Near IR region:0.8-2.5µ
Far IR region:15-200µ
BASED ON WAVENUMBERS:
Mid IR region:4000-667 cm-1
Near IR region:12500-4000 cm-1
Far IR region:667-50 cm-1
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NEAR IR REGION
IR REGION FAR IR REGION
0.8 2.5 15 200µ
Wavelength
12500 4000 667 50cm-1
Wave number
Atom or groups in a molecule are connected by bonds.
These bonds are not rigid in nature. Because of the continuous motions of the molecule, they maintain some vibrations with frequency called NATURAL FREQUENCY.
When IR radiation is passed the absorption of infrared radiations causes an excitation of the molecule.
Then a peak is observed as APPLIED FREQUENCY=NATURAL FREQUENCY
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Each vibrational level is associated with a number of closely spaced rotational levels. So IR spectra is considered as VIBRATIONAL ROTATONAL spectra.
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Every bond or portion of a molecule or functional group requires different frequency for absorption.
Hence charecteristic peak is observed for every functional group of a molecule.It is FINGERPRINT of a molecule.
Generally two regions are seen in IR spectra.
GROUP FREQUENCY REGION
FINGER PRINT REGION
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All the bonds in a molecule are not capable of
absorbing infrared energy. Only those bonds which
are accompanied by a change in dipole moment will
absorb in the IR region.
Such vibrational transitions are called as infrared
active transitions.
The vibrational transitions which are not
accompanied by a change in the dipole moment are
called as infrared inactive transitions.
Ex: Vibrational transitions of C=O,N-H,O-H shows
change in dipole moment.
Vibrational transitions of C=C bonds in symmetrical
alkenes and alkynes does not show any change in
the dipole moment.8
When radiations with frequency range less than 100
cm-1 are absorbed,molecular rotations takes place and
discrete lines are formed in the spectrum.
When more energetic radiation in the region 104 to
102 cm-1 are passed through the sample molecular
vibrations are set in.
Vibrational spectra appears as vibrational –rotational
bands,since a single vibrational energy is
accompanied by a large number of rotational energy
changes.
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Stretching vibration
Symmetrical
stretching
vibration
Asymmetrical
stretching
vibration
Bending vibration
In plane
bending
vibration
Scissoring
Rocking
Out of plane
bending
vibration
Wagging
Twisting
VIBRATIONS
The vibrations for the molecules may be of two types. They are
Stretching Vibrations Bending Vibrations
STRETCHING VIBRATIONSIn these vibrations distance between the two atoms increases or decreases. But the atoms are in the same axis. Bond length is altered.
BENDING VIBRATIONSThe positions of the atoms changes relative to the original bond.
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STRETCHING VIBRATIONS
1.Symmetrical stretching:
It is the one in which two bonds increase or decrease in length symmetrically
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2. Asymmetrical stretching :
It is the one in which one bond length increases and the other one decreases.
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BENDING VIBRATIONS:
1. In-Plane bending
There is change in the bond angle in these vibrations.
Bending of bonds takes place in the same plane.
SCISSORING:
Two atoms approach each other in which bond angle decreases.
Rocking:
Movement of atoms takes place in the same direction. Here bond angles are maintained.
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2. OUT PLANE BENDING
Here bending occurs outside the plane of a molecule.
Wagging:
Two atoms move up and down the plane with respect to the central atom.
Twisting:
It is the one in which one atom moves up the plane and the other atom moves down the plane with respect to the central atom.
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VIBRATIONAL FREQUENCY:
The value of stretching vibrational frequency of a bond can be calculated by using HOOKE’S LAW.
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Value of vibrational frequency or wave number depends upon:
Bond strength
Reduced mass
Ex: C=C has higher vibrational frequency than C-C stretching.
O-H has higher vibrational frequency than C-C bonding.
F-H has higher vibrational frequency than O-H stretching.
FACTORS INFLUENCING VIBRATIONAL
FREQUENCIES:
Coupled Vibrations and Fermi Resonance
Electronic Effects
Hydrogen Bonding
Bond Angles
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Coupled Vibrations and Fermi Resonance:
Vibrations which occurs at different frequencies of
higher wave number are called coupled vibrations.
Ex: Consider a –CH3 group.coupled vibrations for
CH3 group takes place at different frequencies.
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C
HH
H
Asymmetric
C
HH
H
Symmetric
In IR spectrum,absorption bands are spread over
a wide range of frequencies.Then the energy of a
overtone level chances to coincide with the
fundamental mode of different vibrations.This
type of resonance is called Fermi Resonance.
Ex: Carbondioxide is linear and four fundamental
vibrations are expected. In this symmetric
stretching vibration is IR inactive.
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ELECTRONIC EFFECTS
Changes in the absorption frequencies for a
particular group takes place when the substituents
next to that of a particular group are changed. The
frequency shifts includes:
Inductive Effect
Mesomeric Effect
Field Effect
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INDUCTIVE EFFECT :
Introduction of alkyl groups causes +I effect. It
results in lengthening or weakening of the bond
and hence the force constant is lowered and wave
number of absorption decreases.
Formaldehyde 1750 cm-1
Acetaldehyde 1745 cm-1
Acetone 1715 cm-1
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The introduction of electronegative atom causes –I
effect. It results in the increase of bond order. Thus
the force constant increases and hence the wave
number of the bonds also increases.
Acetone 1715 cm-1
Chloroacetone 1725 cm-1
Dichloroacetone 1740 cm-1
Tetachloroacetone 1750 cm-1
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MESOMERIC EFFECT
In some cases –I effect is dominated by mesomeric
effect and the absorption frequency falls.
Ex: Absorption frequency of amides and esters
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Benzamide Methyl benzoate
FIELD EFFECT
Lone pair of electrons present on the atoms
influence each other through space interactions
and changes the vibrational frequencies of both
the groups.This effect is called as Field effect.
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HYDROGEN BONDING
Hydrogen bonding gives rise to downward
frequency shifts.
Stronger is the hydrogen bonding,greater is the
absorption shift towards lower wave numbers.
Hydrogen bonding is of two types
Intermolecular hydrogen bonding
Intramolecular hydrogen bonding
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INTERMOLECULAR HYDROGEN BONDING
Gives rise to broad bands
These are concentration dependent.
INTRAMOLECULAR HYDROGEN BONDING
Gives rise to sharp and well defined bands.
These are concentration independent.
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BOND ANGLES
Highest C=O frequencies arises in the strained
cyclobutanes.This can be explained in terms of bond
angle strains.
If the bond angle is reduced below 1200 then C=O
stretching is reduced and is occurred at higher
frequency.
If the bond angle is pushed outwards of 1200 then it
leads to increase in bond angle thereby stretching of
C=0 occurs at lower frequencies
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ABSORBANCE RANGE OF IR
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Bond Type of compoundFrequency
Range Intensity
C-H Alkanes 2850-2970 Strong
C-H Alkenes3010-3095
675-995Medium Strong
C-H Alkynes 3300 Strong
C-H Aromatic Rings 3010-3100
690-900MediumStrong
O-H
Monomeric alcohols, phenolsHydrogen bonded alcohols, phenols
Monomeric carboxylic acidsHydrogen bonded carboxylic acids
3590-36503200-36003500-36502500-2700
VariableVariable,
sometimes broad
N-H Amines, amides 3300-3500 Medium
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C=C Alkenes 1610-1680 Variable
C=CAromatic Rings
Alkynes1500-16002100-2260
VariableVariable
C-NAmines,Amides
Nitriles1180-13602210-2280
StrongStrong
C-O Alcohols,ethers,Carboxylic acids,Esters 1050-1300 Strong
C=O Aldehydes,Carboxylic acids,KetonesEsters 1690-1760 Strong
ABSORBANCE RANGE OF IR
REFERENCE:
Elementary organic spectroscopy, principles and
chemical applications, Y. R. Sharma, pg.no:69-83
Instrumental methods of pharmaceutical analysis, Dr.
Chatwal
http://www.scielo.br.in
http://www.orientchem.org
http://www.researchgate.net
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