IR SPECTROSCOPY THEORY

1. IR SPECTROSCOPY
PRINCIPLE:- 1. Absorption of IR radiation by molecules produce excitation in them & this
leads to production of vibrational frequencies in molecules
2. Molecular vibrations are of 2 types:-
A. Molecular Rotations:- This shows when radiation frequency less than 100𝑐𝑚−1
was
absorbed.
B. Molecular Vibrations:- When absorbed radiations of frequency ranges between 100𝑐𝑚−1
to
10,000𝑐𝑚−1
.
Theory:- 1. It is also known as Infrared Spectroscopy.
2. Ranges lies between wavelengths (λ) between 2.5 μm to 25 μm,(1µ𝑚 = 10−6
𝑚).
Near IR Range:- 0.8 μm to 2.5 μm.
Far IR Range:- 15 μm to 200 μm.
3. Wavelength (λ) is inversely proportional to the frequency (ν) and is governed by the
relationship ν = c/λ, where c = speed of light.
4. We know that, E=hν, where h= Plank’s Constant. So, E=hc/λ.
5. Most of the Scientists refers radiation in the IR region of the EM spectrum in terms of unit
called Wavenumber(ν͞ ).

2. 6. Wavenumbers (ν͞ ) are reciprocal of wavelength (λ) & it’s S.I. unit is 𝑐𝑚−1
Wavenumber (ν͞ ) = 1/Wavelength (λ)
7. Higher the wavenumber then higher energy is required to break the bonds. It ranges
between 400 to 4000 𝑐𝑚−1
.
IR Absorption Process:- 1. After absorbing IR radiations molecule reaches to the higher
energy state.
2. Energy change is between 8 to 20 kJ/mole.
3. After absorbing IR radiations covalent bonds in the molecules shows stretching or bending in
the respective bonds.
4. Bonds having Dipole moment are capable of absorbing IR radiations, while Symmetric bonds
does not like 𝐻2 or 𝐶𝑙2.
Uses of the IR Spectrum:- 1. For Fingerprinting of molecules because no two molecules
have similar structure & IR absorption pattern, but if IR peaks will coincide with each other then
both the molecules are identical.
2. It gives structural information about a molecule, because absorption of each type of bond
like (C-H, N-H, O-H, C-X, C=O, C-C, C=C, C≡C, C≡N & so on) are different.

3. 3. Molecules absorption can be defined for each type of bond, but some bonds like C-H have
different range which is 3000±150 𝑐𝑚−1
& C=O have range between 1715±100 𝑐𝑚−1
.
MODES OF MOLECULAR VIBRATIONS:- These are also known as Fundamental
Absorptions. There are mainly 2 types of molecular vibrations which are follows:-
1. STRETCHING
2. BENDING
1. STRETCHING- When the molecules absorbs IR radiations & it leads to increase in their
bond length then it is known as stretching. For ex. -𝐶𝐻3, -𝐶𝐻2-, -𝑁𝐻2, -𝑁𝑂2 etc. It is of
a. Symmetric- Both the groups moves in same direction towards central metal atom.
For ex. Methyl group (-𝐶𝐻3) at 2872 𝑐𝑚−1
b. Asymmetric- Both the groups moves in opposite direction away from central metal
atom. For ex. Methyl group (-𝐶𝐻3) at 2962 𝑐𝑚−1

5. 2. BENDING:- When the molecules absorbs IR radiations & it leads to increase or decarese
in their bond angle w.r.t. central metal atom. It is of further 4 types:
a. Scissoring:- Bond axis gets decreased because both atoms moving towards each
other.
b. Rocking:- Bond axis gets increased because both the atoms moves away from
each other.
c. Wagging:- Both the atoms moves in the same plane either up or down the plane.
d. Twisting:- Both the atoms moves in different plane one moves up the plane &
other moves down the plane.

6. FACTORS AFFECTING MOLECULAR VIBRATIONS:-
There are mainly three factors which affects the molecular vibrations are as follows:-
1. COUPLING VIBRATIONS
2. FERMI RESONANCE
3. ELECTRONIC EFFECTS
1. COUPLING VIBRATIONS:- a. It generally occurs in 𝐶𝐻2 moeity.
c. Hydrogen atoms joined with 𝐶𝐻2 moeity either show symmetric vibrations or
asymmetric vibrations or both together.
2.
3. FERMI RESONANCE:- Molecules absorb IR radiations & collides with each other like
pendulum to transfer energy.
4. ELECTRONIC EFFECTS:- There are mainly 3 types of Electronic Effects which are as
follows:-
A. Inductive Effect:- Ther are 2 types of Inductive Effect are as follows:-

7. a. +I Effect:- In this bond length increases with increase in frequency of Molecular
Vibrations. For ex. 𝐶𝐻3 𝐶𝑂𝑂𝐻 ℎ𝑎𝑣𝑒 𝑣𝑖𝑏𝑟𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 1730𝑐𝑚−1
&
𝐶𝐻3 𝐶𝑂𝐶𝐻3ℎ𝑎𝑣𝑒 𝑣𝑖𝑏𝑟𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 1600𝑐𝑚−1
.
b. –I Effect:- In this bond length decreases with increase in frequency of Molecular
Vibrations. . For ex.𝐶𝐻3 𝐶𝑂𝐶𝐻3ℎ𝑎𝑣𝑒 𝑣𝑖𝑏𝑟𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 1600𝑐𝑚−1
&
𝐶𝐻3 𝐶𝑂𝑂𝐻 ℎ𝑎𝑣𝑒 𝑣𝑖𝑏𝑟𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 1730𝑐𝑚−1
.
B. Mesomeric Effect:- In this bond length increases with decrease in frequency.
For ex. 𝐶𝐻3 𝐶𝑂𝑂𝐻 ℎ𝑎𝑣𝑒 𝑣𝑖𝑏𝑟𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 1730𝑐𝑚−1
&
𝐶𝐻3 𝐶𝑂𝐶𝐻3ℎ𝑎𝑣𝑒 𝑣𝑖𝑏𝑟𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 1600𝑐𝑚−1
.
C. Field Effect :- Combination of both inductive & mesomeric effect due to electric field
applied by molecules on each other. For Ex.
OH
+ 𝐻𝑁𝑂3 =
OH
N
+
O
-
O
Absorptionfrequency Absorptionfrequency
1700𝑐𝑚−1 1650𝑐𝑚−1
INSTRUMENTATIONOF INFRARED SPECTROSCOPY:-
General Instrumentation of IR are as follows:-
A. RADIATION SOURCE:- There are mainly 4 types of radiation sources which are as
follows:- 1. Incandenscent Lamp- For ex. CFL Tube
2.Globar Source- Silicon Carbide Rod
3. Nernst Glower- Made up of Zirconia & Thoria
4. Mercury Arc Lamp- In this mercury vapours are stored at high pressure &
their excitation produce IR Radiation.
B. MONOCHROMATORS:- There are mainly 2 types of monochromators which are used
are as follows:- 1. Prism Monochromator
2.Grating Monochromator

8. C. SAMPLING CELLS:- There are mainly 3 types of Sampling are as follows:-
1. Solid Sampling:- This can be done by 2 ways:-
a. Solid Runin Solution- In this first of all solution of the
solid have been made along with the volatile solvent
which then poured on to the metallic disc & then the
solid sample film can be made on it.
b. Solid Film- In this solution containing solid sample
along with the volatile solvent have been prepared
which then poured on to the NaCl or KBr disc & then
the thin filmof the solid sample have been obtain on
the disc.
2. Liquid Sampling:- In this Liquid Sample solution have been
kept in the NaCl or KBr Chamber.
3. Gas Sampling:- In this gas sample have been kept in the NaCl
or KBr Chamber.
D. DETECTORS:- There are mainly 3 types of detectors used in IR Spectroscopy:-

9. 1. Bolometer- In this Galvanometer show no deflection in case of standard solution
& show deflection in case of Unknown Sample.
2. Thermocouple- Variation in temp. between hot junction & cold junction give us
information about the type of element in unknown sample.
3. Golay Cells – In this Xenon gas is filled in the drum. IR radiation produces
excitation in xenon gas molecules & they collides each other & Xenon gas
expands. Flexible Film also expands which show element in unknown sample.

10. SPECTROMETERS:- There are mainly 2 types of spectrometers are used are as
follows:- a. Dispersive IR Spectrometer
b.Fourier Transform Spectrometer
A. Dispersive IR Spectrometer:- 1. Radiation is produced by hot wire.
2. Then divides into 2 parallel beams of equal
intensity by mirrors
3. One beam is passed through sample & one from reference
4. The beams are then passed through monochromators which disperse into
continuous spectrum of frequency.
5. Monochromators consists of beam chopper. So that the 2 beams pass through
reference & sample at same period of time.
6. Then both the beams reaches to thermocouple detector
7. Then the detector senses ratio between the intensities & detect that which
frequencies are absorbed by sample & reference.
8. The recordings are amplified by the amplifier & record by the recorders in the
form of spectrum on a chart.
9. This instrument is used to record spectrum in the frequency domain.
10. Detector record %transmittance by plotting frequency (wavenumber
𝑐𝑚−1
)vs light transmitted graph.
Percent Transmittance=
𝐼𝑠
𝐼𝑟
x100
Where, Is=Intensity of sample beam
Ir= Intensity of reference beam
Maxima is represented by minima on the chart. So, the absorption traditionally
called a peak.
11. This instrument cancel the effect of IR active atmospheric gases like 𝐶𝑂2 &
water vapours.

11. B. FOURIER TRANSFORM SPECTROMETERS- 1. It is the most modern one.
2. It produces a pattern called interferogram which is the plot between Intensity
& Time.
3. Fourier Transform can separate individual absorption frequencies from the
interferogram.
4. Instrument known as FT-IR (Fourier Transform Infrared Spectrometer) whose
advantage is that FTIR acquires Interferom is less than a second.
5. FTIR uses an interferometer to process the energy sent to the sample. In this
sources of energy passes through the beam splitter & a mirror placed at 45⁰c.
6. Then it separates into 2 parallel beams oriented at 90⁰c.
7. Then these 2 beams meet at beam splitter & then they recombine cause both
constructive & destructive interferences.
8. Combined beam interference pattern called interferogram.
9. Interferogram produced by combining 2 beams by beam splitter.
10. Interferogram contains the information about energy absorbed at every
wavelength.
11. The Final Interferogram contains information about standard & sample.
12. The whole mathematical process called FTIR.

12. APPLICATIONS OF IR SPECTROSCOPY:-
1. IR Spectroscopy used to detect the purity of Pharmaceutical Samples.
2. It is used to detect the purity of Water.
3. It is used to detect the purity of Paints & Varnishes.
4. It is used to detect Old Paintings & Artifacts.
5. It is used to detect the Pheromones (sex hormones released by Ants).
6. It is used to detect the Cinnamaldehyde ( a main constituent of Cinnamic
Acid).
7. It is used to know about the structural information of Polymers.
8. It is used to detect the ortho & para position of different groups.