�ݺ�ߣ

FACTORS INFLUENCING
VIBRATIONAL FREQUENCIES IN
I R
Submitted by
P.Anuradha
1st
M. Pharmacy

2
The calculated value of wave number is never exactly equal to the
experimental value, because the vibrations of each group gets influenced
by the structure of the molecule in the neighbourhood of the bond/group.
 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
3
Coupled vibrations
Fermi resonance
Electronic effects
Hydrogen bonding

COUPLED VIBRATIONS
4
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.
Similarly ,coupled vibrations of –CH3 group take place at
different frequiences compared to –CH2- group.

5
• The interaction is very effective probably because of the partial double
bond character in the carbonyl oxygen bonds due to resonance which
also keeps the system planar for effective coupling.
•A strong vibrational coupling is present in carboxylic acid anhydrides in
which symmetrical and asymmetrical stretching vibrations appear in the
region 1720 – 1825 cm-1.

Requirements
6
For interaction to occur, the vibrations must be of same symmetry
species.
There must be a common atom between the groups for strong
coupling between stretching vibrations.
For coupling of bending vibrations , a common bond is necessary.
Interaction is greatest when coupled groups absorb, individually,
near the same frequency.
Coupling is negligible when groups are separated by one or more
carbon atoms and the vibrations are mutually perpendicular.

FERMI RESONANCE
7
Coupling of two fundamental vibration modes produces two new
modes of vibration ,with frequencies higher and lower than that
observed in absence of interaction.
Interaction can also take place between fundamental vibrations
and overtones or combination tone vibrations and such
interactions are known as Fermi Resonance.

8
In this , a molecule transfers its energy from fundamental
vibrational level to overtone or combination tone level and back
again.
Resonance pushes the two levels apart and mixes their character,
consequently each level has partly fundamental and partly overtone
or combination tone character.

9

Carbondioxide molecule(triatomic) is linear and four fundamental
vibrations are expected for it. Out of these ,symmetric vibration is IR
inactive since it produces no change in the D.M of the molecule
For eg. Fermi Resonance is given by aldehydes in which C-Hstretching
absorption usually appears as a doublet(2820cm-1
and 2720cm-1
)due to
interaction between C-Hstretching(fundamental)and the overtone of C-
H deformation(bending)
Asymmetric stretching Symmetric stretching

Hydrogen bonding
10
 Stronger the hydrogen bonding, greater is the absorption shift
towards lower wave number than the normal value.
Intermolecular hydrogen bonds give rise to broad bands and
intramolecular hydrogen bonds are sharp and well defined.
Intermolecular hydrogen bonds are concentration dependent on
dilution,the intensities of such bands decrease and finally
disappear.Intramolecular hydrogen bonds are independent of
concentration.
The N-H stretching frequencies of amines are also affected by
hydrogen bonding as that of the hydroxyl group but frequency shifts
for amines are lesser than that for hydroxyl compounds.
 Because nitrogen is less electronegative than oxygen so the
hydrogen bonding in amines is weaker than that in hydroxy
compounds.

11
Mostly non associating solvents like CS2 ,CHCl3,CCl4 are used because some
solvents like benzene,acetone influence O-H&N-H compounds.
For example, amines show N-H stretching at 3500cm-1
in dilute solutions and in
aliphatic alcohols,a sharp band appears at 3650cm-1
due to free O-H group.
In enols&chelates,H-bonding is exceptionally strong and absorption due to O-
H stretching takes place at very low values.As these bonds are not broken easily
on dilution by an inet solvent, the free O-H stretching,may not be seen at low
concentrations.
It is because of the fact that the bonded structure is stabilised by resonance

12
The strength of hydrogen bonding is also affected by :
Ring strain
Molecular geometry
Relative acidity and basicity of the proton donor and
acceptor groups

Electronic effect
13
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
Mesomeric effect
Field effect

INDUCTIVE EFFECT
14
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.
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.

15
Mesomeric effect :
 It causes lengthening or the weakening of a bond leading in the
lowering of absorption frequency.
 As nitrogen atom is less electronegative than oxygen atom, the electron
pair on nitrogen atom in amide is more labile and participates more in
conjugation.
 Due to this greater degree of conjugation, the C=O absorption
frequency is much less in amides as compared to that in esters.
It is important to note that only inductive effect is considered when the
compound is meta substituted.

Field effect :
16
In ortho substituted compounds, the lone pair of electrons on
two atoms influence each other through space interactions and
change the vibrational frequencies of both the groups.
This effect is called field effect.
Ortho halo acetophenone

REFERENCES
17
Organic spectroscopy,Y.R.Sharma,pg no:77-82
Instrumental methods of chemical analysis , Gurdeep R. Chatwal,
Sham K. Anand, pg no :2.55 – 2.59

�ݺ�ߣ

Ir spectroscopy ppt

More Related Content

Ir spectroscopy ppt

Editor's Notes