This document provides an overview of infrared (IR) spectroscopy. It discusses the basic principles of IR spectroscopy, including molecular vibrations and instrumentation. The major components of IR spectrometers are described, such as IR radiation sources, wavelength selectors, sample handling techniques, detectors, and recorders. Factors that can affect vibrational frequencies are outlined. Finally, applications of IR spectroscopy in fields like pharmaceutical analysis are mentioned.

1. IR SPECTROSCOPY
By-
Dipesh Adesh Gamare
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2. CONTENT
 Introduction to IR spectroscopy
 Modes of molecular vibration
 Sample handling
 Instrumentation of Dispersive-IR and FTIR
 Factors affecting vibrational frequencies
 Applications of IR spectroscopy
 References
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3. 0.0001-0.01nm 0.01-10nm 10-1000nm 1000nm-0.01cm 0.01-100m
WAVELENGTH
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4. IR SPECTROCOPY:
Infrared (IR) spectroscopy is based on IR absorption by molecules as undergo vibrational and rotational
transitions.
The IR spectrum is mainly used to determine the functional groups present in a compound and elucidate the
structure of compound.
The energy of a molecule in IR = Electronic energy + Vibrational energy + Rotational energy
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5. 1.Near IR : carbohydrates and
proteins
2.Middle IR : organic molecules -
functional groups
3.Far IR : inorganic co-ordination
bonds& quaternary ammonium
compounds
REGION WAVE
LENGTH
λ (μm)
WAVE NUMBER
υ (cm-1)
FREQUENCY
Hz
RANGE
NEAR 0.78 - 2.5 12800 - 4000 3.8x1014 -1.2 x1014
MIDDLE 2.5 - 50 4000 - 200 1.2x1014 – 6 x1012
FAR 50 - 1000 200 -10 6x1012 – 3 x1011
MOST USED 2.5 - 15 4000- 670 1.2x1014 - 2 x1013
 Different compounds when absorb infrared radiation give rise to vibrational transitions characteristic to the
molecular structure and those transitions are recorded as IR Spectra.
 Mid IR region is widely used in pharmaceutical studies.
 A compound can be analyzed by IR radiation only if:
1. There is a change in dipole movement.
2. There should be a resonance in applied and natural IR frequency.
 When we apply an external infrared frequency, and it matches the natural frequency of vibration within a
molecule, the applied energy is then absorbed by the molecule and an infrared peak is observed. These peaks
are characteristic to different functional group and parts of the molecule, and they are unique for every
molecule. Hence the IR spectroscopy is considered as the fingerprint analysis of a molecule.
PRINCIPLE OF IR SPECTROSCOPY
5

6. Fingerprint region (400-1500cm−1): This region of IR spectrum provides characteristic peaks for different part of a
molecule and helps in qualitative identification of compounds.
Group frequency region (4000—1500cm−1): Peaks with respect to different functional groups are observed in this
region.
Characteristic IR frequencies of some known
functional groups
Typical IR spectrum depicting regions of
absorption
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7. Example : The spectrum is for a substance with an empirical formula of C3H5N. What is the compound?
Nitrile or
alkyne
group
No
aromatics
Aliphatic
hydrogens
One or
more alkane
groups
Propane nitrile
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8. Types
of
Molecular
Vibrations
1)Bond Stretching 2)Bond Bending
symmetric
asymmetric
In-plane rocking
In-plane scissoring
Out-of-plane wagging
Out-of-plane twisting
Fundamental
vibrations
Non-fundamental
vibrations
Overtones
Combination tones
Fermi resonance
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9. Fundamental vibrations
1. Bond Stretching vibration: In stretching vibration, the length of a bond is changed i.e. either increased or decreased.
It is of two types:
A] Symmetrical stretching: Here the two bonds increase or decreases in length in a symmetric pattern.
B] Asymmetrical stretching: Here when one bond length increases the other one decreases.
2. Bending vibration:
Bending vibrations are of two sub-types.
1) In plane
A] Rocking : The atoms swing back and forth concerning the central atom of a molecule Two hydrogen atoms of a methylene
molecule move outward simultaneously and inward simultaneously
B] Scissoring : The two atoms approach each other and go away from each other.
2) Out of the plane
A] Wagging : Such type of vibration occurs when both the atoms of a molecule move one side of the plane
B] Twisting : In twisting, one molecule is above the plane and the other molecule is below the plane.
Non-fundamental vibrations:
1. Overtone vibrations occur at twice or thrice the value of fundamental vibrations.
2. Combination bands are the bands that result because of coupling of fundamental frequencies.
3. Fermi resonance bands are produced because of coupling between fundamental vibration and an overtone or because of a
combination band.
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10. SAMPLE HANDLING
The sample cells are usually made up of transparent ionic substances such as sodium chloride (NaCl) or potassium bromide (KBr).
However, KBr is preferred because of its compatibility with the instrumental measurement process. The infrared samples can be
considered of three forms such as:
1) Liquids sampling :
 A drop of liquid is placed within the KBr/ NaCl plates with
the thickness less than 0.01 mm.
 Around 1-10mg of sample is required to prepare the
solution.
 Aqueous solutions are not preferred as they tend to dissolve
the sample cells so, the solvent should be anhydrous and
mostly organic solvents such as chloroform is preferred.
2) Solid sampling : Pelletization technique
 The solid samples can be prepared using pressed pellet
technique where the sample is placed in sample holder and
KBr pellets are prepared in form of thin transparent layer
using a hydraulic press.
 However, as KBr is hygroscopic in nature care should be
taken to minimize its atmospheric exposure.
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11. 3) Solution sampling technique
 To prepare a solid solution 1 to 10 mg volumes of 0.1 to 1.0 ml of 0.05 to 10% solution are required for placement within a
cell of 0.1 to 1mm thickness.
 Solvents used are CHCl₃, CS2 and CCl₄ is used for this purpose which absorbs strongly at 785 cm-1.
 Care should be taken to avoid solution combinations that react instantaneously.
4) Semisolid sample preparation
 Another technique called as Nujol-Mull technique (popularly called as Mull Technique) may be used where the mixing up of
the sample with a mineral oil (Nujol) ( liquid paraffin hydrocarbon) is done and afterwards a thin film of the liquid can be
applied on the liquid sample cell for measurements.
5)Gas sampling
 Gas samples are enclosed in a gas cell with windows, which seal the gas cell and allow the infrared beam to pass into the cell.
These cells are made up of KBr, NaCl and so on. These cells are about 10cm long and can increased its path length upto 40cm
by multiple reflections inside it and gas sample can be analyzed easily.
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12. INSTRUMENTATION
The instrumentation of IR spectrophotometer is as follows
1) IR radiation source
2) Monochromators / wavelength selector
3) Sample holder
4) Detector
5) Recorder
Diagrammatic representation of Dispersive-IR spectrophotometer Diagrammatic representation of Fourier Transorm-IR spectrophotometer
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13. FTIR- IRSpirit FTIR Shimadzu
Dispersive IR- IRTracer-100 Shimadzu
13

14. 1) IR radiation source :
a) Nernst Glower
 Rare earth metal oxides (Zr, Ce, Th) heated electrically.
 Apply current to cylinder, has resistance to current flow generates heat (1200o – 2200o C).
 Causes light production similar to blackbody radiation.
 Range of use ~ 670 – 10,000cm-1.
 Need good current control or overheats and damaged.
b) Globar
 Similar to Nernst Glower but uses silicon carbide rod instead of rare earth oxides
 Similar usable range
V
Zr, Ce, Th
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15. c) Incandescent Wire Source
• Tightly wound nichrome or rhodium wire that is electrically heated.
• Same principal as Nernst Glower.
• Lower intensity then Nernst Glower or Globar, but longer lifetime.
d) CO2 Laser
• CO2 laser gas mixture consists of 70% He, 15% CO2, and 15% N2 .
• A voltage is placed across the gas, exciting N2 to lowest vibrational levels.
• The excited N2 populate the asymmetric vibrational states in the CO2 through collisions.
• Infrared output of the laser is the result of transitions between rotational states of the CO2 molecule .
• Gives off band of ~ 100 cm-1 in range of 900-1100 cm-1 .
• Small range but can choose which band used & many compounds have IR absorbance in this region.
• Much more intense than Blackbody sources.
e) Others
1) Mercury arc (far IR) – when continues radiation source is required.
2) Tungsten lamp (4000 -12,800cm-1) (near IR)
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16. IR radiation sources overview
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17. 2) WAVELENGTH SELECTORS / MONOCHROMATORS
 They help in selecting a continuous IR radiation in the desired wavelength region.
 They generally contain a chopper and a complex system of monochromators.
 The choppers are generally used to enhance the signal to noise ratio of the instrument, and they also moderate
the intensity of radiation that reaches to the detector.
 The monochromator is used to select it desired frequency of radiation which will be then allowed to incident on
the detector.
These monochromators are of two types:
1. Prismatic monochromator: They are made up of
glass/quartz and coated with alkyl halides.
A] Mono Pass (Radiation passes only once through the prism)
B] Double Pass (Radiation passes twice through the prism)
2. Grating monochromator: They are grooves made up of
aluminium and provide better dispersion of radiation than prisms.
A] Reflection gratings
B] Transmittance grating
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18. 3) SAMPLE HOLDER/SAMPLE CELL
• There is rugged window material for cuvette or sample holder which is transparent and also inert over this region.
• AgCl cell are used for aqueous and moist samples, but it is soft and easily gets deformed and darkens on exposure to visible
light.
• The alkali halides are widely used, particularly NaCl, KBr and ThBr which is transparent at wavelength as long as 625 cm-1.
KBr pellet (For solid sample)
NaCl windows ( For liquid sample)
4) DETECTOR
1. Thermocouple
2. Bolometer
3. Photoconducting cell
4. Pyroelectric detector
5. Golay cell
6. Thermistor
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19. 1.Thermocouple
 Two pieces of dissimilar metals fused together at the ends.
 When heated, metals heat at different rates.
 Potential difference is created between two metals that varies with their difference in temperature
 Usually made with blackened surface (to improve heat absorption)
 Placed in evacuated tube with window transparent to IR (not glass or quartz)
 IR “hits” and heats one of the two wires.
 Can use several thermocouples to increase sensitivity.
2. Bolometer
 Strips of metal (Pt, Ni) or semiconductor that has a large change in resistance to current with temperature.
 As light is absorbed by blackened surface, resistance increases and current decreases.
 Very sensitive.
3. Photoconducting cell
 Thin film of semiconductor (ex. PbS) on a nonconducting glass surface and sealed in a vacuum.
 Absorption of light by semiconductor moves from non-conducting to conducting state.
 Decrease in resistance  increase in current.
 Range: 10,000 -333 cm-1 at room temperature.
V
- +
hn metal1 metal2
IR transparent
material (NaCl)
A
hn
glass
semiconductor
hn
vacuum
Transparent
to IR
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20. 4. Pyroelectric Detectors
 Pyroelectric (ceramic, lithium tantalate) material get polarized (separation of (+) and (-) charges) in
presence of electric field.
 Temperature dependent polarization
 Measure degree of polarization related to temperature of crystal
 Fast response, good for FTIR
5. Golay cell
 It contains a small metal cylinder which is closed by black metal plate at end and a flexible metal die
programmed at the other end.
 The cylinder is filled with xenon and sealed.
 When IR radiation is allowed to fall on the black metal plate it heats the gas which expands it.
 The signal detected by the photo tube is then modulated according to the power of radiant beam.
6. Thermistor
 Thermistors are usually made up of fused mixture of metallic oxides.
 As the temperature of the mixture increases its electrical resistance decreases and accordingly the
measurements are performed in the higher spectrophotometer.
5) RECORDER OR READ OUT UNIT
The signals collected from detector are amplify and recorded by device in terms of IR spectra
indicating various peaks of functional group present.
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21. FACTORS AFFECTING VIBRATIONAL FREQUENCIES
A] Coupled vibration
 An isolated C-H bond has only one stretching vibration frequency whereas methylene group shows two stretching vibrations
both symmetrical and asymmetrical.
 Because of mechanical coupling or interaction between CH stretching vibrations in the CH2 group.
 Asymmetric vibrations develop at higher frequency or wave numbers than stretching vibrations.
 Such vibrations are called coupled vibrations because these vibrations occur at different frequency then the required for an
isolated CH stretching
B] Fermi resonance
 Coupling of two fundamental vibration modes produces 2 new modes of vibration.
 With frequencies higher and lower than the observed in absence of interaction.
 This interaction takes place between fundamental vibrations and overtones, or combination tone vibration and they are
collectively called us Fermi resonance.
 In Fermi resonance molecule transfer energy from fundamental vibrations into overtone or combination bands.
 The resonance pushes the two levels apart and mixes their character and each level has some amount of fundamental and some
amount of overtone or combination tone characters.
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22. FACTORS AFFECTING VIBRATIONAL FREQUENCIES
C] Electronic effects
 Changing the absorption frequencies for a particular group takes place when the substituent in the neighborhood of the group
is changed.
They include
 Inductive effect: Inclusion of alkyl group gives +ve Inductive effect producing decreased absorption frequency. Adding an
electronegative atom or group produces –ve inductive effect leading to increased absorption frequency.
 Mesomeric effect: It produces lengthening or weakening of a bond leading to lowering of absorption frequency.
 Field effect: 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. Example: Ortho halo acetophenone. This is called field
effect.
D] Hydrogen bonding
 It occurs in any system having a proton donor group and a proton acceptor. If the S orbital of the proton effectively overlaps
the P or 𝜋 orbital of the acceptor group.
 The stronger the hydrogen bond, the longer the OH bond, the lower the vibration frequency, broader and more intense will be
the absorption.
 Intermolecular bonds produce broad bands whereas intramolecular hydrogen bonds produce sharp and well-defined bands.
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23. IR spectroscopy is applied for qualitative as well as quantitative analysis of drugs in pharmaceutical industry.
 It is used for identification of drug substances
 It identifies the impurities present in a drug sample
 It helps in study of hydrogen bonding both intermolecular and intramolecular type.
 It is widely used in study of polymers.
 It elucidates reaction mechanisms.
 It identifies functional groups present in any sample.
 It estimates stability of formulation.
 It distinguishes between Cis and Trans isomers.
 It can predict the keto-enol tautomerism.
APPLICATIONS OF IR SPECTROSCOPY
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24. 1. Chatwal, G.R. and Anand, S.K.J. (2018) Instrumental Methods of Chemical Analysis. Himalaya Publishing House, Mumbai,
2.3-2.82.
2. Marcelli, A., Cricenti, A., Kwiatek, W. M., & Petibois, C. (2012). Biological applications of synchrotron radiation infrared
spectromicroscopy. Biotechnology Advances, 30(6), 1390–1404. https://doi.org/10.1016/j.biotechadv.2012.02.012.
3. Basa’ar, O., Fatema, S., Alrabie, A., Mohsin, M., & Farooqui, M. (2017). Supercritical carbon dioxide extraction of Triognella
foenum graecum Linn seeds: Determination of bioactive compounds and pharmacological analysis. Asian Pacific Journal of
Tropical Biomedicine, 7(12), 1085–1091. https://doi.org/10.1016/j.apjtb.2017.10.010
4. IRTracer-100. (2022). https://www.shimadzu.com/an/products/molecular-spectroscopy/ftir/ftir-spectroscopy/irtracer-
100/index.html
5. Jasco. (2020, October 6). Principles of infrared spectroscopy (1) Molecular vibrations and infrared absorption | JASCO
Global. JASCO Inc. https://www.jasco-global.com/principle/principles-of-infrared-spectroscopy-1-molecular-vibrations-and-
infrared-absorption/
6. Libretexts. (2020, August 5) Information Obtained from IR Spectra. Chemistry LibreTexts.
https://chem.libretexts.org/Ancillary_Materials/Laboratory_Experiments/Wet_Lab_Experiments/Organic_Chemistry_Labs/La
b_I/10%3A_Infrared_Spectroscopy/10.06%3A_Information_Obtained_from_IR_Spectra
REFERENCES
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25. THANK YOU
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