This document provides an overview of infrared (IR) spectroscopy. It discusses the principle behind IR spectroscopy, the different modes of molecular vibration, instrumentation including sources, detectors and monochromators. It also covers sample handling techniques, factors that affect vibrational frequencies and applications of IR spectroscopy such as structure elucidation.

1. IR SPECTROSCOPY
PRESENTED BY: JACOB THON BIOR
REG.NO : ND0120003
M.PHARM 1ST SEM
DEPARTMENT of PHARMACEUTICAL
CHEMISTRY
KLE COLLEGE OF PHARMACY,BELAGAVI

2. Contents:
• Introduction.
• Principle involved in the IR.
• Modes of molecular vibration.
• Instrumentation
• Sample handling.
• Factors affecting vibrational freqenciens.
• Application of IR spectroscopy.

3. Introduction
• IR spectroscopy is the study of absorption of IR radiation
which results in vibrational transitions.
• Mainly used in structural elucidation to determine the
functional groups.
• IR region is present between visible and microwave region in
EM spectrum.
• IR of EM spectrum is divided into 3 regions:
Wave number Wavelength
Near IR 14000 – 4000 cm-1 0.8 – 2.5 μm
Middle IR 4000 – 400 cm-1 2.5 – 25 μm
Far IR 400 -10 cm-1 25 – 1000 μm

4. Principle
• The study of interaction of electromagnetic radiation with
vibrational and rotational references with in a molecular structure is
termed as IR spectroscopy.
• When energy in the form of infrared radiation is applied then it
causes the vibration between the atoms of the molecules.
• When IR frequency is equal to the natural frequency of vibration
absorption of IR radiation takes place, a peak is observed.
• IR spectrum of a chemical substance is a finger print of a molecule
for its identification.

5. Modes of molecular vibration

6. Symmetrical stretching Asymmertical stretching
Scissoring Rocking
Wagging Twisting
1)Streching vibrations:
2)Bending vibrations:
a)In-plane vibrations:
b) Out-plane vibrations:

7. INSTRUMENTATION
There are 2 basic types of infrared spectrophotometer,
characterized by the manner in which the infrared
frequencies are handled.
1. Dispersive instruments : In this the infrared light is
seperated into individual frequencies by dispersion,
using a grating monochromator.
2. Fourier transform IR instrument: In this infrared
frequencies are allowed to interact to produce an
interference pattern and this pattern is then analysed
mathematically using fourier transforms , to determine
the individual frequencies and their intensities.

8. DISPERSIVE SPECTROPHOTOMETER:
Single beam spectrophotometer:

9. Double beam spectrophotometer:

10. Fourier transform IR SPECTROPHOTOMETER:

11. Block diagram of IR spectrophotometer
Source Sample Monochromator Detector Readout
•Nernst Glower
•Globar
•Incandescent wire source
•Mercury arc lamp
Prisms
Grating
Thermocouple
Thermopile
Thermister
Bolometer
Pyroelectric
Recorder
11

12. SOURCES
1. Nernest Glower:
• It is combination of Oxides of Zirconium(ZrO2),
Erbium(Er2O3), Ytterium(Y2O3) usually of diameter 2mm&
length of 30mm.
• It is in Non-Conducting at room temperature so heating is
required to bring to conducting state(1000-1800°C).
2. Globar source:
• It is made up of Silicon carbide rod (SiC) usually diameter
of 4mm& length of 50mm.
• Heating is required to produce IR radiation(1300-1700°C).
• Power consumption is normally higher than nernest glower.
• Water cooling system is needed to cool the metallic rods
electrodes attached to the rod.

13. 3. Incandescent lamp:
• A tightly wound nichrome coil heated to about 1100°C by
electric current.
• A black film oxide is formed on coil give acceptable emissivity.
• It has longer life than nernest & globar source.
4. Mercury arc lamp:
• Used for FAR IR regions (v<200cm¯1).
• It is a high pressure mercury arc which consists of quartz
jacketed tube containing mercury vapour at P>1 atm.
• When current passes through the lamp, mercury
is vapourised, excited & ionized, forming a plasma
discharge at high pressure.

14. Sample handling
• IR Spectrometry deals with the measurement of the IR
spectrum.
• It is used for the characterization of solid, liquid and gas
samples.
• Material containing sample must be transparent to the IR
radiation.
• Hence, the salts like NaCl, KBr are only used
A] Sampling of solids.
• Solid film technique
• Pressed pellet technique
• Mull technique

15. 1)Solid film technique:
• The solid films can deposited on the surface of a KBr/NaCl
plates by allowing a solution in a volatile solvent to evaporate
drop by drop on the surface of the flat.
• IR radiation is then passed through the thin layer deposited.
2)Pressed pellet technique: (or KBr pellet method)
• In this technique, a small amount of finely ground solid sample
is mixed with 0.1-2% weight of dried powdered KBr/Nacl and
compressed. (other alkali metal halides can also been used).
• The mixture is compressed under very high pressure (10,000
to 15,000 pounds per square inch) to yield a transparent disk
about 1 cm in diameter and 1-2 mm thick.
• The disk is prepared in vacuum to eliminate air or moisture.
• These pellets are transparent to IR radiation and it is used for
analysis.

17. 3)Mull technique:
• In this technique, the finely crushed sample is mixed with mineral oil
like Nujol (high Mwt liquid paraffin hydrocarbon) in a mortar,with a
pestle to make a thick paste/mull.
• A thin film is applied onto the salt plates.
• This is the mounted in a path of IR beam and the spectrum is
recorded.
• The absorptions of the Nujol effectively blank out the regions of
C- H str and C- H def.
• But these regions can be studied by preparing a mull with a
complementary agent containing no C-H bonds.
Ex: Hexachlorobutadiene &
chlorofluorocarbon oils are used.

18. B] Sampling of liquids
• Liquids are usually examined as a thin film sandwiched
between two polished salt plates (eg: Nacl)
• Aqueous solvents cannot be used because they cannot dissolve
alkali halides . Organic solvents like chloroform can be used.
• Sample thickness should be 0.01-0.1mm to give transmittance
between 15% and 70%.
• Some salts plates are highly soluble in water, so sample and
washing reagents must be anhydrous (eg:Toulene, Chloroform)

20. C] Sampling of gases
• Sampling of Gases can be obtained by permitting the sample
to expand in to evacuated cylindrical cell equipped with
IR transparent windows. (eg: NaCl).
• The gas must not react with the cell windows or the reflecting
surfaces.
• Gas analyses are performed with IR but the method
is not commonly used because of its lack of sensitivity.
• Moisture must be avoided. In addition, the windows and
other instruments components which
are constructed of soluble, salts may
be damaged.

21. D) Solution sampling technique:
• Solution of sample is prepared in a suitable solvent and then
the sample is analysed in the form of solution.
• When sample in solutions, the absorption due to solvent has
to be compensated by keeping the solvent in a cell of same
thickness as that containing the sample.
• The solvent used in this technique are carbontetrachloride,
carbondisulfide or chloroform

22. MONOCHROMATORS
• To select radiation of any desired frequency from source &
eliminate other frequencies.
• This consists of monochromator which consists of Dispersion
element & slit system.
• Two types of dispersion elements
1. Prisms
2. Gratings

23. Prism Monochromators:
Single pass monochromator

24. Double pass monochromator

25. Grating
(parallel grooves)
Grating Monochromator:

26. DETECTORS
1. Thermocouple:
• The thermocouple detector is based upon the fact that when
two different semiconductor metal wires of high thermoelectric
efficiency, are connected and kept at different temperatures, a
potential difference is developed between them which causes
the flow of current. (Response time 60msec).
• A thermocouple is closed in an evacuated steel casing with
KBr window to avoid losses of energy by convection.
2. To enhance sensitivity, several thermocouples connected in
series to give better results
called Thermopile.

27. 3. Bolometer:
• It is based on wheatstone brige principle, For every 1°C rise in
temperature electric resistance of metal increases.
• A bolometer usually consists of a thin metal conductor made
up of the platinum strips coated with lamp black.
• Amount of current flowing through galvanometer measures
the intensity of radiation falling on the detector.
• Response time – 4msec

28. 4. Pyroelectric detector:
• It consists of thin plate of pyroelectric crystal of 0.25 to 12mm
in size.
• The crystal is placed between two electrodes which is
connected to an external electric circuit.
• When the crystal is exposed to IR radiation, it absorbs the heat
leading to change in its dipole moment.
• Various pyroelectric crystals are barium titanate,lead zirconate,
triglycine sulphate etc.,
• Response time- 10msec.

29. Factors affecting vibrational frequencies:
1. Vibrational coupling
2. Hydrogen bonding
3. Electronic effect
4. Bond angles
5. Resonance

30. 1) VIBRATIONAL COUPLING
• An isolated C-H bond has only one stretching vibrational frequency
where as methylene (-CH2) group shows two stretching vibrations,
symmetrical and asymmetrical.
• Because of coupling(or)interaction between C-H stretching vibrations in
the CH2 group. Assymetric vibrations occur at higher frequencies than
symmetric stretching vibrations.
• These is known as vibration coupling because these vibrations occur at
different frequencies than that required for an isolated CH stretching.
2) HYDROGEN BONDING
• Hydrogen bonding, especially in O-H and N-H compounds, gives rise
to a number of effects in infrared spectra, While most organic work will
involve relatively non-associating solvents (CCl4, CS2, CHCI3),
More polar solvents such as acetone (or) benzene will certainly
influence 0-H and N- H absorptions.

31. • 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 of hydroxyl compounds.
• Intermolecular hydrogen bonds gives rise to broad bands, while
intramolecular hydrogen bonds give sharp and well defined bands.
3) ELECTRONIC EFFECTS
• 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

32. 4) BOND ANGLES
• C-H stretching vibrations move to higher frequency in the sequence
alkane -alkene-alkyne. As hybridization goes from sp3 to sp2 to sp1
,the ‘s’ character of the C-H bond increases; bond lengths become
shorter, and frequencies rise.
• Cyclopropanes have high C-H str frequencies for the same reason
(typical values being 3040-3070 cm"): the C-C-C bond angle is
substantially contracted below the normal 109.5°, leading to increased
‘s’ character in the C-H bonds, and thus to higher frequencies
5) RESONANCE
• Because of resonance carboxylic acids(-COOH) show more absorption
compared to alcohols.
• Due to resonance the bond strength increases and results increase in
absorption of frequency
• In this way resonance can affect the absorption of IR of C-O stretching.

33. Application of IR
1. Determination of Molecular Structure.
2. Studying the Progress of the Reactions.
3. Qualitative Analysis of Functional Groups.
4. Detection of Impurity in a Compound.
5. Shape of Symmetry of a molecule
6. Presence of Water in a Sample.
7. Examination of Old Paintings and Artifacts.
8. Identification of Organic Compound

34. REFERENCES
• Organic Spectroscopy by William Kemp,3RD Edition.
• Elementary organic spectroscopy principles and chemical
applications,by Y.R.SHARMA.
• Pharmaceutical Analysis Instrumental Methods by Nirali
Prakashan.
• Instrumental method of Analysis seventh edition by Willard
Merritt, Dean & Settle

35. THANK YOU