IR SPECTROSCOPY & ITS APPLICATIONS
Amit Agnihotri
Defence Reserach Laboratory
Tezpur
 Spectroscopy is a technique used to determine the
structure of a compound by the study of the interaction
between matter and electromagnetic radiation
 Nondestructive (destroys little or no sample).
EMR
ANALYTE
SPECTROPHOTOGRAPH
1.UV-Visible radiations--------excitation of electrons---------UV-visible spectrum
2.IR-radiations-----------------------vibration changes --------------------IR spectrum
3.Radio frequency---------------spin rotational changes-------------N.M.R spectrum
THE ELECTROMAGNETIC
SPECTRUM
IR REGIONS
Near infrared region
RANGE
0.8-2.5 µ (12,500 - 4000 cm-1)
Mid infrared region
2.5-15 µ (4000 - 667cm-1)
Far infrared region
15-200 µ (667 - 50 cm-1)
3
TYPES OF
SPECTROSCOPY

UV- Vis spectroscopy uses electronic transitions to
determine bonding patterns

Infrared (IR) spectroscopy measures the bond vibration
frequencies in a molecule and is used to determine the
functional group.

Nuclear magnetic resonance (NMR) spectroscopy
analyzes the environment of the hydrogens in a
compound. This gives useful clues as to the alkyl and
other functional groups present.

Mass spectrometry (MS) fragments the molecule and
measures their mass. MS can give the molecular weight
of the compound and functional groups
PRINCIPLE OF IR SPECTROSCOPY
Molecules are made up of atoms linked by bonds.
 The movement of atoms and the chemical bonds like spring
and balls (vibration)

This characteristic vibration are called Natural vibration.
 The energy of molecular vibration is quantized.


When EMR (IR) is applied then it causes the vibration
between the atoms of the molecules when,
Applied infrared frequency = Natural frequency of vibration
Then, Absorption of IR radiation takes place and a peak is
observed.

Different functional groups absorb characteristic
frequencies of IR radiation. Hence gives the
characteristic peak value.

Therefore, IR spectrum of a chemical substance is a
finger print of a molecule for its identification.

Like a fingerprint no two unique molecular
structures produce the same infrared spectrum. This
makes infrared spectroscopy useful for several types of
analysis.
CRITERIA FOR A COMPOUND TO ABSORB IR
RADIATION
Change in dipole moment
A molecule can only absorb IR radiation when its
absorption cause a change in its electric dipole
A polar bond is usually IR-active.
A nonpolar bond in a symmetrical molecule will absorb
weakly or not at all.
MOLECULAR VIBRATIONS
There are 2 types of vibrations:
1. Stretching vibrations
2. Bending vibrations
1. Stretching
vibrations:
 Vibration or oscillation along the line of bond
 Change in bond length
 Occurs at higher frequency: 4000-1250 cm-1
 2 types:
a) Symmetrical stretching
b) Asymmetrical stretching
A) SYMMETRICAL STRETCHING:
Both bonds increase or decrease in length simultaneously.
H
C
H
B) ASYMMETRICAL STRETCHING

in this, one bond length is increased and other is
decreased.
H
C
H
2. BENDING VIBRATIONS
Vibration or oscillation not along the line of bond
• Also called as deformations
• In this vibrations bond angle is altered
• Occurs at low frequency : 1400-666 cm -1
• 2 types:
a) In plane bending: E.g. scissoring, rocking
b) Out plane bending: E.g. wagging, twisting
•
A) IN PLANE BENDING
i.



Scissoring:
This is an in plane blending
2 atoms approach each other
Bond angles are decrease
H
C
H
ii.

Rocking:
Movement of atoms take place in the same
direction.
H
C
H
B) OUT PLANE BENDING
i. Wagging:

2 atoms move to one side of the plane. They move up and
down the plane.
H
C
H
ii. Twisting:

One atom moves above the plane and another atom moves
below the plane.
H
C
H
IR STRETCHING FREQUENCIES (ν)
DEPEND ON?
ν = frequency
k = spring strength (bond stiffness)
= reduced mass (~ mass of largest atom)
Directly on the strength of the bonding between the two
atoms (ν ~ k)
Inversely on the reduced mass of the two atoms (v ~ 1/m)
STRETCHING FREQUENCIES
 isolated
C=C
1640-1680 cm-1
 conjugated C=C 1620-1640 cm-1
 aromatic C=C
approx. 1600 cm-1
Conjugation lowers the frequency
 Frequency decreases with increasing atomic weight.
 Frequency increases with increasing bond energy.

SUMMARY OF IR ABSORPTIONS
19
CLASSIFICATION OF IR BANDS
Three types : strong (s), medium (m), or weak (w)
Depending on their relative intensities in the IR spectrum.
O—H AND N—H STRETCHING

Both of these occur around 3300 cm-1, but they look
different:
 Alcohol
O—H is broad with rounded tip.
 Primary amine (RNH2) is broad with two sharp
spikes
 Secondary amine (R2NH) is broad with one sharp
spike.
 No signal for a tertiary amine (R3N) because
there is no hydrogen.
IR SPECTRUM OF ALCOHOLS
IR spectrum of alcohols broad, intense O—H
stretching absorption around 3300 cm-1.
 The broad shape is due to the hydrogen bonding
interactions of alcohol molecules.

IR SPECTRUM OF AMINES

The IR spectrum of amines show a broad N—H
stretching absorption centered around 3300 cm-1.
IR SPECTRUM OF AMIDES
strong absorption for the C═O at 1630–1660 cm-1.
 there will N—H absorptions at around 3300 cm-1.

APPLICATIONS
IDENTIFICATION OF SUBSTANCES

To compare spectrums.

No two samples will have identical IR spectrum.

Criteria: Sample and reference must be tested in
identical conditions, like physical state, temperature,
solvent, etc.
The “Fingerprint” Region (1200 to 600 cm-1) :

Small differences in structure & constitution of molecule 
result in significant changes in the peaks in this region.

Hence this region helps to identify an unknown compound.
STUDYING PROGRESS OF REACTIONS

Observing rate of disappearance of characteristic
absorption band in reactants

Rate of increasing absorption bands in products of a
particular product. E.g. : O—H = 3600-3650 cm -1 ,
C=O = 1680-1760 cm-1

Measure the degree of polymerization in chemical
compounds.
DETERMINATION OF MOLECULAR STRUCTURE

Used along with other spectroscopic techniques.

Identification is done based on position of absorption
bands in the spectrum.Eg.: C=O at 1717 cm-1.

Absence of band of a particular group indicates
absence of that group in the compd.
DETECTION OF IMPURITIES

Determined by comparing sample spectrum with
the spectrum of pure reference compound. Eg.:
ketone impurity in alcohols.

Detection is favored when impurity possess a
strong band in IR region where the main
substance do not possess a band. Eg :Impurity in
bees wax (with petroleum wax)
MONITORING THE STRUCTURAL PLASTICITY OF
PLANT CELL WALLS
carbonyl band near 1700 cm−1indicates a high concentration in the
older plant.
higher protein content in the young plant is suggested by the negative
amide I and amide II bands at 1650 and 1545 cm−1, respectively.
there is a significant difference in the cellulosic nature of the two
samples, with the older plant having a higher relative cellulose content
PROTEIN
QUANTITATION
IR spectroscopy is one of the most well established
techniques for the analysis of protein structure
 In protein amino acids are covalently linked via amide
(peptide) bonds. it absorb in multiple regions of the
mid-IR spectrum,
 By measuring amide bonds in protein chains, we can
accurately quantifies an intrinsic component of every
protein

Amide A (about 3500 cm-1) is with more than 95% due to the N-H stretching
vibration
Amide I (between 1600 and 1700 cm-1) most intense absorption band in
proteins. stretching vibrations of the C=O (70-85%) and C-N groups (10-20%)
In order to determine protein and peptide concentration, the Spectrometer
measures the intensity (peak height) of the Amide I band,
NON-INVASIVE BLOOD GLUCOSE
MONITORING

Near Infrared spectroscopy
is used across the ear lobe to measur
e glucose

Amount of near infrared light passing
through the ear lobe depends on the
amount of blood glucose in that region

The ear lobe was chosen due to the
absence of bone.tissues and also bec
ause of its relatively small thickness
OTHER APPLICATIONS
1.
Determination of unknown contaminants in industry
using FTIR.
2.
Determination of cell walls of mutant & wild type
plant varieties using FTIR.
3.
Biomedical studies of human hair to identify disease
states (recent approach).
4.
Identify odor & taste components of food.
5.
Determine atmospheric pollutants from atmosphere
itself.
6.
Examination of old paintings

It is also used in forensic analysis in both criminal and
civil cases, example in identifying polymer degradation,
in determining the blood alcohol content etc.

Chemical Analysis: Testing Pill Quality. According to
"Medical News Today," scientists at the University of
Maryland have been successful in using the method of
near-infrared spectroscopy (NIR) to make a prediction
regarding quick dissolution of pills inside the body.
STRENGTHS AND LIMITATIONS
IR alone cannot determine a structure.
 Some signals may be ambiguous.
 The functional group is usually indicated.
 The absence of a signal is definite proof that the
functional group is absent.
 Correspondence with a known sample’s IR spectrum
confirms the identity of the compound.
