Infrared spectroscopy is a technique used to identify functional groups in molecules by detecting the vibrational and rotational frequencies of chemical bonds. It works based on the absorption of infrared radiation by the molecule, which causes changes in the dipole moment of the bonds. There are two main types of vibrations detected - stretching and bending. Factors like coupled vibrations, hydrogen bonding and electronic effects influence the vibrational frequencies observed. IR spectroscopy has applications in structure elucidation, quantitative analysis, reaction monitoring and more.

1. INFRA-RED SPECTROSCOPY
R.PRASANTH
M.Pharm 1st Semester[Pharmaceutics]

2. DEFINITION:
 It is the study of absorption of infrared radiation which results in vibrational transitions.
 IR spectrum is an important record which gives sufficient information about the structure of a
compound and also determine the functional group.
 IR spectroscopy is a useful tool to identify functional groups in organic molecules.
The absorption of IR radiations can be expressed either in terms of wavelength() or in wave
number ().
THEORY:
 The total energy of a molecule is the sum of its electronic, vibrational and rotational energies
each of which are quantized.
 The energy associated with IR radiation is capable of inducing only changes in vibrational
energy levels of the molecule.
 When energy in the form of IR is applied and if the applied IR frequency is equal to the natural
frequency of vibration, the absorption of IR takes place & a peak is observed.
Eg: In HCl molecules the natural frequency of vibration is 2890cm-¹, if IR radiation of this frequency
falls to HCl molecule IR radiations absorbed, whereas remaining frequencies are transmitted.

3. Dipole moment changes during vibrations and rotations :
 To absorb IR radiation, a molecule must undergo a net change in dipole moment as it
vibrates or rotates.
 Vibration of the molecule cause change in charge distribution which result in
alternating electric field.
 This couples the molecular vibration with oscillating electric field of EM radiation.
Eg: The charge distribution around a molecule such as hydrogen chloride is not
symmetric because chlorine has a higher electron density than hydrogen.
 As the hydrogen chloride vibrate, a regular fluctuation in its dipole moment occurs & a
field is established that can interact with the electric field associated with radiation.
 The symmetrical diatomic molecules like O2 and N₂ do not possess dipole moment,
hence cannot be excited by IR radiation.

4. TYPES OF VIBRATIONS.
1. Stretching
2. Bending
1. Stretching Vibrations
 If the dipole moment of a molecule would not change i.e as in symmetric stretching the absorption spectra of
radiation cannot be obtained. Such spectra is called as forbidden or inactive IR spectra.
 If the molecule vibrates asymmetrically, the change in its dipole moment takes place so absorption spectra of
this molecule can be obtained. This is called active IR spectra.

5. 2. BENDING VIBRATIONS:
 involve movement of atoms which are attached to a common central atom, such that there is change in
bond axis & bond angle of each individual atom without change in their bond lengths.
TYPES OF BLENDING VIBRATIONS
a) In Plane Rocking: in plane bending of atoms occurs wherein they swing back & froth with
respect to the central atom.
b) In Plane Scissoring: 2 atom connected to central atom move towards and away from each
other.
c) Out plane Wagging: two atoms oscillate up and below the plane with respect to the central
atom.
d) Out planeTwisting: one of atom moved up the plane while other down the plane with
respect to ntral atom.

6. Sample Handling.
Solids - Pressed pellet technique.

7. IR spectroscopy is used for the characterization of solid, liquid or gas samples. Material containing sample must be
transparent to the IR radiation. So, the salts like NaCl, KBr are only used.
1. Sampling of solids
Various techniques used for preparing solid samples are as follows
a)Mull technique: In this technique, the finely crushed sample is mixed with Nujol (mulling agent) in n
a marble or agate mortar, with a pestle to make a thick paste. A thin film is applied onto the salt plates. This is
then mounted in a path of IR beam and the spectrum is recorded.
b) Solid run in Solution – In this technique, solid sample may be dissolved in a non-aqueous solvent provided
that there is no chemical interaction with the solvent and the solvent is not absorbed in the range to be studied. A
drop of solution is placed on the surface of alkali metal disc and solvent is evaporated to dryness leaving a thin
film of the solute.
c) Case film technique – If the solid is amorphous in nature then the sample is deposited on the surface of a KBr
or NaCl cell by evaporation of a solution of the solid and ensured that the film is not too thick to pass the
radiation.
d) Pressed pellet technique – In this technique, a small amount of finely ground solid sample is mixed with 100
times its weight of potassium bromide and compressed into a thin transparent pellet using a hydraulic press.
These pellets are transparent to IR radiation and it is used for analysis

8. 2.Sampling of liquids:
Liquid sample cells can be sandwiched using liquid sample cells of highly purified
alkali halides, normally NaCl. Other salts such as KBr and CaF2 can also be used. Aqueous solvents cannot be
used because they cannot dissolve alkali halides. Organic solvents like chloroform can be used. The sample
thickness should be selected so that the transmittance lies between 15-20%. For most liquids, the sample cell
thickness is 0.01-0.05 mm. Some salt plates are highly soluble in water, so the sample and washing reagents
must be anhydrous.
3.Sampling of gases
The sample cell is made up of NaCl, KBr etc. and it is similar to the liquid sample cell. A sample cell with a
long path length (5 – 10 cm) is needed because the gases show relatively weak absorbance.

9. Liquids (Demountable cell).
 Non-volatile liquids (high-boiling-point, high-viscosity oils, etc.)
 Qualitative analysis of samples by Nujol method, film method, etc.
 Thickness adjustable using spacers or tightening the screws.
Nujol Technique:
Liquid paraffin(nujol).
Nujol has absorption at 2915,1462,1376&719cm-1
This interference has been avoided by using hexachlorobutadiene in combination with nujol which absorbs in
Regions 1630-1510cm-1,1010-760cm-1
Gas Cell.
•The use of internal mirrors permits the beam to be reflected several times through the sample to increase the
sensitivity.
•A sample cell with a long path length (5 – 10 cm) is needed because the gases show relatively weak
absorbance.

10. INSTRUMENTATION.
Dispersive infrared spectrophotometer
Fourier Transform infrared spectrophotometer
Dispersive infrared spectrophotometer.
Jean-Baptiste Joseph Fourier (1768 –1830)
Time domain - Frequency domain
Interferogram – Visually complex
Conversion of one to other
Interferogram - FFT - Single beam IR
Spectrum.
.

11. Factors influencing vibrational frequency:
 Coupled vibrations
 Fermi resonance
 Electronic effects
 Hydrogen bonding.
1. Coupled vibrations.
 An isolated C-H bond has only one stretching vibrational frequency whereas methylene group
shows two stretching vibrations, symmetrical and asymmetrical.
 Because of mechanical coupling or interaction between C-H stretching vibrations in the CH2 group.
 Asymmetric vibrations occur at higher frequencies than symmetric vibrations.
 These are known as coupled vibrations because these vibrations occur at different frequencies than
that required for an isolated C-H stretching.
Symmetric Assymetric

12. 2. Fermi resonance.
 In IR spectrum, absorption bands are spread over a wide range of frequencies. Then the energy of an overtone level
chances to coincide with fundamental mode of different vibrations.
 This type of resonance is called “Fermi resonance”.
 CO2 is a linear molecule and four fundamental vibrations observed. In this symmetric stretching vibration is IR
inactive.
3. Electronic effects.
Changes in the absorption frequencies for a particular group take place when the substituents in the neighborhood of
that particular group are changed.
 Inductive effect
 Mesomeric effect
 Field effect.

13. a)Inductive effect.
 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
 Presence of electronegative atom, the force constant increases and the wave number of absorption rises.

14. b)Mesomeric effect.
It causes lengthening or the weakening of a bond leading in the lowering of absorption frequency.
Methyl Benzoate Benzamide
ν, C=O 1730 cm-1 ν, C=O 1693 cm-1
 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.

15. c)Field Effect.
•In ortho substituted compounds, the lone pair of electrons on two atoms influence each other through space interactio
•This effect is called field effect.
4. Hydrogen bonding.
 It occurs in any system containing a proton donor group (X-H) and a proton acceptor. if the s-orbital of the proton can
effectively overlap the P or π orbital of the acceptor group.
 The stronger the hydrogen bond, the longer the O-H bond, the lower the vibration frequency and broader and more intense
will be the absorption band.
 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.
 Nitrogen is less electronegative than oxygen so the hydrogen bonding in amines is weaker than that in hydroxyl compounds.
 Intermolecular hydrogen bonds give rise to broad bands, while intramolecular hydrogen bonds give sharp and well defined
bands.
 The inter and intramolecular hydrogen bonding can be distinguished by dilution.
 Intermolecular hydrogen bonds are temperature dependent.

16. Applications of IR spectroscopy:
 Identification of functional groups & structure elucidation of organic compounds.
 Quantitative analysis of a number of organic compounds. Study of covalent bonds in molecules.
 Studying the progress of reactions. Detection of impurities in a compound.
 Ratio of cis-trans isomers in a mixture of compounds. Shape of symmetry of an inorganic molecule.
 Study the presence of water in a sample. Measurement of paints and varnishes.
 Synthesis
 Impurity studies
 Structural elucidation
 Thermodynamics
 Kinetics
 Food technology
 Pesticide analysis
 Polymer technology