Introduction and applications of FT- IR spectroscopy

INTRODUCTION AND APPLICATIONS OF FT - INFRA – RED
SPECTROSCOPY
By
Mr. K.KANDASAMY,
Assistant Professor ,
Department of Chemistry,
KSR College of Arts and Science for women,
Tiruchengode – 215.

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InfraredSpectroscopy

Spectroscopy
“seeing the unseeable”
Using electromagnetic radiation as a Investigation to obtain
information about atoms and molecules that are too small to see.
Electromagnetic radiation is propagated at the speed of light
through a vacuum as an oscillating wave.

Spectroscopy
Absorption
spectroscopy
UV
spectroscopy
IR
spectroscopy
NMR
spectroscopy
Emission
spectroscopy
Fluorimetry
Flame
photometery

• Infrared spectroscopy
• (IR spectroscopy) is
the spectroscopy that deals with
the infrared region of
the electromagnetic spectrum, that
is light with a
longer wavelength and
lower frequency than visible light
• Infrared Spectroscopy is the
analysis of infrared light interacting
with a molecule.
• It is based on absorption
spectroscopy

IR Spectroscopy
I. Introduction
The entire electromagnetic spectrum is used by chemists:
UVX-rays IRg-rays RadioMicrowave
Energy (kcal/mol)
300-30 300-30 ~10-4> 300 ~10-6
Visible
Frequency, n in Hz
~1015 ~1013 ~1010 ~105~1017~1019
Wavelength, l
10 nm 1000 nm 0.01 cm 100 m~0.01 nm~.0001 nm
nuclear
excitation
(PET)
core
electron
excitation
(X-ray
cryst.)
electronic
excitation
(p to p*)
molecular
vibration
molecular
rotation
Nuclear Magnetic
Resonance NMR
(MRI)

INFRARED REGIONS RANGE
Near infrared region 0.8-2.5 µ(12,500-4000 cm-1)
Main infrared region 2.5-15 µ(4000-667cm-1)
Far infrared region 15-200 m µ(667-100 cm-1)

• When infrared 'light' or radiation hits a molecule, the
bonds in the molecule absorb the energy of the
infrared and respond by vibrating.
IR
radiation
vanllin
Molecular vibrations

Theory of Infrared Absorption
Spectroscopy
 For a molecule to absorb IR, the vibrations or rotations within a molecule must
cause a net change in the dipole moment of the molecule. The alternating
electrical field of the radiation (remember that electromagnetic radiation consists
of an oscillating electrical field and an oscillating magnetic field, perpendicular to
each other) interacts with fluctuations in the dipole moment of the molecule.
 If the frequency of the radiation matches the vibrational frequency of the molecule
then radiation will be absorbed, causing a change in the amplitude of molecular
vibration.

Selection Rules ( Active and Forbidden Vibrations)
(i) Infra-red light is absorbed only when a change in dipole movement character in the molecule takes
place.
(ii) If a molecule has a center of symmetry, then the vibrations are centrosymmetric and inactive in the
Infra-red but are active in the Raman.
(iii) The vibration which are not centrosymmetric are active in Infra-red but inactive in Raman.
DM = 0 IR Inactive
Raman Active
What is Center of symmetry?

MOLECULAR
VIBRATIONS
Fundamental
Vibrations
Stretching
Vibration
Symmetric Asymmetric
Bending
Vibratio
n
In-plane
Bending
Scissorin
g
Rocking
Out Of
Plane
Bending
Waggin
g
Twisting
Non-
fundamental
Vibrations
Over Tones,
Combination
Tones,
Fermi
Resonance
“Molecular Vibrations”

What is a vibration in a molecule?
“Any change in shape of the molecule- stretching of
bonds, bending of bonds, or internal rotation around
single bonds”.
Why we study the molecular vibration?
Because whenever the interaction b/w
electromagnetic waves & matter occur so change
appears in these vibrations.

IR Spectroscopy
I. Introduction
C. The IR Spectroscopic Process
 The quantum mechanical energy levels observed in IR spectroscopy are those of
molecular vibration
 We perceive this vibration as heat
 When we say a covalent bond between two atoms is of a certain length, we are
citing an average because the bond behaves as if it were a vibrating spring
connecting the two atoms
 For a simple diatomic molecule, this model is easy to visualize:

FUNDAMENTAL
VIBRATIONS
• Vibrations which appear as
band in the spectra.
NON-
FUNDAMENTAL
VIBRATIONS
• Vibrations which appears as a
result of fundamental
vibration.
Mol. vibration divided into 2 main
types:

1.Streching
vibration Involves a
continuous change
in the inter atomic
distance along the
axis of the bond
b/w 2 atoms.
2.It requires more
energy so appear
at shorter
wavelength.
STRETCHING
VIB. 1.Bending
vibrations are
characterized by a
change in the angle
b/w two bonds.
2.It requires less
energy so appear
at longer
wavelength.
BENDING
VIB.
Fundamental vibration is also divided into types:

Now, stretching vibration is further divided into :
SYMMETRIC VIB.
• Inter atomic distance b/w
2 atoms
increases/decreases.
(OR)
The Movement of the
atoms with respect to a
particular atom in a
molecule is in the same
direction.
ASYMMETRIC VIB.
• Inter atomic distance b/w 2
atoms is alternate/opposite.
(OR)
One of the atom
approaches the central
atom while the other
departs from it.
Symmetric Stretch Asymmetric Stretch
H
H
C
H
H
C
asymmetric
symmetric

Bending vibration is divided into:
• If all the atoms are
on same plane.
IN PLANE
BENDING
• If 2 atoms are on
same plane while
the 1 atom is on
opposite plane.
OUT OF
PLANE
BENDING

In-Plane bending and Out plane bending is further divided into:
Scissoring
Rocking
Wagging
Twisting
scissor
H
H
CC
H
H
CCH
H
CC
rock
twist
wag
in plane
out of plane

In- Plane Bending
Scissoring:
In this type, Two – atoms approach each other.
Rocking:
In this type, The movement of the atoms takes place in the same direction.
Out Plane Bending
Wagging:
Two atoms move ‘Up and down’ the plane with respect to the central atom.
Twisting:
One the atom moves up the plane while the other moves down the plane with
respect to the central atoms.

C. The IR Spectroscopic Process
 As a covalent bond oscillates – due to the oscillation of the dipole
of the molecule – a varying electromagnetic field is produced
 The greater the dipole moment change through the vibration,
the more intense the EM field that is generated
Infrared Spectroscopy

NON-FUNDAMENTAL VIBRATIONS
NON-
FUNDAMENTAL
OVER TONES:
These are observed
at twice the
frequency of strong
band.
Ex:
carbonyl group.
COMBINATION TONES:
Weak bands that
appear occasionally at
frequencies that are
sum/difference of 2 or
more fundamental
bands.
FERMI
RESONANCE:
Interaction b/w
fundamental
vibration &
overtones or
combination tones.
Ex:CO2

C.The IR Spectroscopic Process
When a wave of infrared light encounters this oscillating EM field
generated by the oscillating dipole of the same frequency, the two
waves couple, and IR light is absorbed
The coupled wave now vibrates with twice the amplitude
IR beam from spectrometer
EM oscillating wave
from bond vibration
“coupled” wave

Characteristic Vibrational Frequencies of Bonds
Bonds are not rigid but behave like a spring with a mass at either end.
• Obey Hooke’s Law: F = -kx - sign is motion in negative site
• This gives rise to a characteristic frequency for the vibration:
m1 and m2 = Mass of the Atoms in grams in particular bond
k = Force constant of the bond. For Single bond, it is approximately 5x105 gm sec-2
It becomes double bonds and triple bonds respectively.
C= velocity of the radiation = 2.998 x cm sec-1
massreduced
k
_2
1
p
  21
21
_
mm
mm
massreduced



The frequency is affected by
• the masses of the atoms in the bond
• the strength of the bond
The lower the mass, the higher the vibrational frequency.
• Stretching frequencies for bonds to carbon: C-H > C-C > C-N > C-O
• The stronger the bond, the higher the vibrational frequency.
• Stretching frequencies
• C≡C > C=C > C-C
• C≡N > C=N > C-N
• C≡O > C=O > C-O
• spC-H > sp2C-H > sp3C-H

DEGREES OF FREEDOM
 Fundamental vibration of molecule depend on degree of freedom
 Each atom has 3 degree of freedom depend on x , y ,z
 For a molecule containing n number of atoms has 3n degree of freedom
n = The Number of atom in a molecule.
3n Degree of freedom = Translational + Rotational + Vibrational.
 Molecule has always three translational degree of freedom.
 Rotational of a molecule about an axis (x,y,z) through the Center of gravity.
 So we calculate only number of vibrational degrees of freedom.
 For Linear Molecule, There are two degree of rotation (x,y Axis only)
 For linear (3n-5)degree of freedom represent fundamental vibrations
Total degree of freedom = 3n
Translational degree of freedom = 3
Rotational degree of freedom = 2
So Vibrational degree of freedom = 3n-3-2= 3n-5
 For non linear molecule 3 degree of freedom represent rotational & translational motion
 For non linear (3n-6)degree of freedom represent fundamental vibrations
Total degree of freedom = 3n
Translational degree of freedom = 3
Rotational degree of freedom = 3 (x,y,z)
So Vibrational degree of freedom = 3n-3-3= 3n-6

For Example
In linear molecule of carbon dioxide (CO2 ), The number of degrees of the freedom
Number of atoms (n) = 3
Total degrees of freedom =3n = 3 x 3 = 9
Translational = 3
Rotational = 2
Vibrational degree of freedom = 9-3-2=4
For Linear CO2 Molecule, the theoretical number of fundamental bands should be equal to FOUR.
DM = 0 ѵ = 2350 cm-1 ѵ= 667cm-1
IR inactive IR active

For Non- Linear Molecule
In Non - linear molecule of H2O, The number of degrees of the freedom
Number of atoms (n) = 3
Total degree of freedom = 3 x 3 = 9
Translational = 3
Rotational = 3
Vibrational degrees of freedom = 9-3-3= 3
So, theoretically there should be THREE Fundamental bands in the Infra-red spectrum of Water.
For Benzene molecule?
It is Non-linear Molecule, Degrees of freedom 3n-6
Number of atoms (n) = 12 Total degrees of freedom = 3 x 12 = 36
Translational = 3
Rotational = 3
Vibrational degrees of freedom = ?
30

Only those vibrational changes
that result in change in dipole
movement appear as band
All vibrational changes don’t appear as
band

Factors Influencing Vibrational Frequencies
1. Coupled Vibrations and Fermi Resonance
Stretching Vibration of CH2 Methylene Group is Lower then the Stretching Vibration of –CH3
IR beam from spectrometer
EM oscillating wave
from bond vibration
“coupled” wave

2. Electronic Effect
Inductive Effect, mesomeric Effect, Field Effect etc.
For Example, The Introduction of alkyl group to produces +I Effect.
(i) Formaldehyde (HCHO) = 1750 cm-1
(ii) Acetaldehyde (CH3CHO) = 1745cm-1
(iii) Acetone (CH3COCH3) ) = 1717cm-1
Introduction of electronegative atom or group causes –I Effect
(i) Acetone ( CH3COCH3) = 1715 cm-1
(ii) Chloroacetone ( CH3COCH2Cl) = 1725 cm-1
(iii) Tetrachloroacetone (Cl2CH-CO-CHCl2) = 1750 cm-1

1. Conjugation – by resonance, conjugation lowers the energy of a double or triple
bond. The effect of this is readily observed in the IR spectrum:
• Conjugation will lower the observed IR band for a carbonyl from 20-40 cm-1
provided conjugation gives a strong resonance contributor
• Inductive effects are usually small, unless coupled with a resonance
contributor (note –CH3 and –Cl above)
O
O
1684 cm-1
1715 cm-1
C=O C=O
C
H3C
O
X X = NH2 CH3 Cl NO2
1677 1687 1692 1700 cm-1
H2N C CH3
O
Strong resonance contributor
vs.
N
O
O
C
CH3
O
Poor resonance contributor
(cannot resonate with C=O)

Effects on IR bands
2. Steric effects – usually not important in IR spectroscopy, unless they reduce the
strength of a bond (usually p) by interfering with proper orbital overlap:
• Here the methyl group in the structure at the right causes the carbonyl
group to be slightly out of plane, interfering with resonance
3. Strain effects – changes in bond angle forced by the constraints of a ring will
cause a slight change in hybridization, and therefore, bond strength
• As bond angle decreases, carbon becomes more electronegative, as well
as less sp2 hybridized (bond angle < 120°)
O
C=O: 1686 cm-1
O
C=O: 1693 cm-1
CH3
O O O O O
1815 cm-1
1775 cm-1
1750 cm-1
1715 cm-1
1705 cm-1

Effects on IR bands
4. Hydrogen bonding
Two Types Hydrogen bonds are there
1. Intermolecular Hydrogen bonding ( Two B/w Atoms)
2. Intramolecular Hydrogen Bonding ( With in the Molecule)
Intermolecular Hydrogen bonds give rise to broad bands whereas bands arising
from intramolecular hydrogen are sharp.
H-bonding can interact with other functional groups to lower frequencies
Intra molecular Steric hindrance to H-bonding
in a di-tert-butylphenol
C=O; 1701 cm-1
OO
H

Structural/Functional Components

Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (single bonds)
O—H (alcohols) 3200-3600
O—H (carboxylic acids) 3000-3100
First examine the absorption bands in the vicinity of
4000-3000 cm–1

1. Alkanes – combination of C-C and C-H bonds
• C-C stretches and bends 1360-1470 cm-1
• CH2-CH2 bond 1450-1470 cm-1
• CH2-CH3 bond 1360-1390 cm-1
• sp3 C-H between 2800-3000 cm-1
Octane
(w – s) (m)

2. Alkenes – addition of the C=C and vinyl C-H bonds
• C=C stretch at 1620-1680 cm-1 weaker as
substitution increases
• vinyl C-H stretch occurs at 3000-3100 cm-1
• The difference between alkane, alkene or alkyne C-H
is important! If the band is slightly above 3000 it is
vinyl sp2 C-H or alkynyl sp C-H if it is below it is alkyl
sp3 C-H
1-Octene
(w – m)
(w – m)

3. Alkynes – addition of the C=C and vinyl C-H bonds
• C≡C stretch 2100-2260 cm-1; strength depends on
asymmetry of bond, strongest for terminal alkynes,
weakest for symmetrical internal alkynes
• C-H for terminal alkynes occurs at 3200-3300 cm-1
• Internal alkynes ( R-C≡C-R ) would not have this band!
1-Octyne
(m – s)
(w-m)

4. Aromatics
• Due to the delocalization of e- in the ring, C-C bond
order is 1.5, the stretching frequency for these
bonds is slightly lower in energy than normal C=C
• These show up as a pair of sharp bands, 1500 &
1600 cm-1, (lower frequency band is stronger)
• C-H bonds off the ring show up similar to vinyl C-H
at 3000-3100 cm-1
Ethyl benzene
(w – m) (w – m)

4. Aromatics
• If the region between 1667-2000 cm-1 (w) is free of interference (C=O stretching
frequency is in this region) a weak grouping of peaks is observed for aromatic
systems
• Analysis of this region, called the overtone of bending region, can lead to a
determination of the substitution pattern on the aromatic ring
Monosubstituted
1,2 disubstituted (ortho or o-)
1,2 disubstituted (meta or m-)
1,4 disubstituted (para or p-)
G
G
G
G
G
G
G

5. Unsaturated Systems – substitution patterns
• The substitution of aromatics and alkenes can also be discerned through the out-
of-plane bending vibration region
• However, other peaks often are apparent in this region. These peaks should only
be used for reinforcement of what is known or for hypothesizing as to the
functional pattern.
R
C
H
C
R
C
H
CH2
R
C
H
C
R
C
R
CH2
R
C
R
C
R
H
R
H
R
H
985-997
905-915
cm-1
960-980
665-730
885-895
790-840
R
R
R
R
R
RR
cm-1
730-770
690-710
735-770
860-900
750-810
680-725
800-860

6. Ethers – addition of the C-O-C asymmetric band and
vinyl C-H bonds
• Show a strong band for the antisymmetric C-O-C
stretch at 1050-1150 cm-1
• Otherwise, dominated by the hydrocarbon
component of the rest of the molecule
Diisopropyl ether
(s)

7. Alcohols
• Strong, broad O-H stretch from 3200-3400 cm-1
• Like ethers, C-O stretch from 1050-1260 cm-1
• Band position changes depending on the alcohols
substitution: 1° 1075-1000; 2° 1075-1150; 3° 1100-1200;
phenol 1180-1260
• The shape is due to the presence of hydrogen bonding
1-butanol
(m– s)
br
(s)

8. Amines - Primary
• Shows the –N-H stretch for NH2 as a doublet
between 3200-3500 cm-1 (symmetric and anti-
symmetric modes)
• -NH2 has deformation band from 1590-1650 cm-1
• Additionally there is a “wag” band at 780-820 cm-1
that is not diagnostic
2-aminopentane
(w) (w)

9. Amines – Secondary
• N-H band for R2N-H occurs at 3200-3500 cm-1
as a single sharp peak weaker than –O-H
• Tertiary amines (R3N) have no N-H bond and
will not have a band in this region
pyrrolidine
(w – m)

10. Aldehydes
• C=O (carbonyl) stretch from 1720-1740 cm-1
• Band is sensitive to conjugation, as are all
carbonyls (upcoming slide)
• A highly unique sp2 C-H stretch appears as a
doublet, 2720 & 2820 cm-1 called a “Fermi doublet”
Cyclohexyl carboxaldehyde
(w-m)
(s)

APPLICATIONS OF INFRA - RED SPECTROSCOPY
1. Identifications of an Organic Compounds:
Most Organic Compounds is conformed in Finger print region
2. Structure Determination:
This technique helps to establish the structure of an unknown compounds.
3. Qualitative analysis of functional groups:
The Presence or absence of absorption bands help in predicting the presence of certain functional group in
the compounds.
Presence of Oxygen may be –OH, C=O, COOR, -COOH etc. But an absorption band between 3600-3200 cm-1
4. Distinction between two types of hydrogen bonding:
To find the Inter Or Intra molecular H- Bonding.
5. Quantitative analysis:
It help to make a quantitative estimation of an organic mixture.
For Example
Xylene commercial is mixture of Ortho, Meta, Para Compound. The separate of the mixture can not
be easily done. But percentage composition of the mixture can be determine.
6. Conformational Analysis:
Chair or Boat Form
7. Geometrical Isomerism: Cis or Trans , Syn or Anti
8. Study the Keto – enol tautomerism:

Disadvantages of IR
Sample Constraint:
Infrared spectroscopy is not applicable to the sample that contains water since this solvent
strongly absorb IR light.
Spectrum Complication:
The IR spectrum is very complicated and the interpretation depends on lots of experience.
Sometimes, we cannot definitely clarify the structure of the compound just based on one single IR
spectrum. Other spectroscopy methods, such as ( Mass Spectrometry) MS and ( Nuclear Magnetic
Resonance) NMR, are still needed to further interpret the specific structure.
Quantification:
Infrared spectroscopy works well for the qualitative analysis of a large variety of samples, but
quantitative analysis may be limited under certain conditions such as very high and low concentrations.

Reference books
1. William Kemp, Organic Spectroscopy – ELBS.
2. Sharma.Y.R, Elementary Organic Spectroscopy, Principles and applications- S. Chand & Co.,
3. Banwell, Fundamentals of Molecular spectroscopy Tata McGraw Hill.
4.
E- Source Reference:
1. SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
2. http://www.chemcenter/org http://
3. www.sciencemag.org
4. http://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/irspec/.htm
5. http://www.kerouac.pharm.uky.edu/asrg/wave/wavehp.html
6. http://hiq.linde-gas.com/international/web/lg/spg/likelgspg.nsf/DocByAlias/anal_infra

Introduction and applications of FT- IR spectroscopy

Introduction and applications of FT- IR spectroscopy

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