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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