Infrared spectroscopy is an analytical method which helps to identify the drug's molecular structure.

1. 4THCOFFEE I N F R A R E D
S P E C T R O S C O P Y
P R I N C I P L E , M O L E C U L A R V I B R A T I O N , S A M P L E H A N D L I N G
GUIDED BY:
DR. DIPTI B. PATEL
ASSOCIATE PROFESSOR
M.PHARM, PH.D
SUBMITTED BY:
RUPALI C. RAVAL
M.SC CHEMISTRY
[PHARMA. ANALYSIS]
SEMESTER 1
CENTRE FOR HEALTH AND APPLIED SCIENCES
GANPAT UNIVERSITY, GANPAT VIDHYANAGAR ,MAHESANA-GOJARIA HIGHWAY, -384012
GUJRAT,INDIA

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 Molecules are made up of atoms linked by chemical bonds. The
movements of atoms and the chemical bonds like spring and balls(vibration).
 This characteristic vibration are called Natural frequency of vibration.
PRINCIPLE

3. 4THCOFFEE  When energy in the form of infrared radiation is applied then it cause the vibration
between the atoms of the molecules and when,
 Applied infrared frequency = Natural frequency of vibration
 Then, absorption of IR radiation takes place and the peak is observed.
 Different functional group absorb characteristic frequencies of IR radiation. Hence
gives the characteristic peak value.
 Therefore, IR spectrum of a chemical substance is a fingerprint of a molecule for its
identification.

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 When radiations with frequency range less than 100 cm−1 are absorbed,
molecular rotations takes place and discrete lines are formed in the spectrum.
 When more energetic radiation in the region 104 to 102 cm−1 are passed
through the sample molecular vibrations are set in.
 Vibrational spectra appears as vibrational-rotational bands ,since a single
vibrational energy is accompanied by a large number of rotational energy
changes.
MOLECULAR VIBRATIONS

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Stretching
vibration
Symmetrical
vibration
Asymmetrical
vibration
TYPES OF MULECULAR VIBRATION
Bending
vibration
In plane
bending vibration
Scissoring
Rocking
Out of plane
bending vibration
Wagging
Twisting

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STRETCHING VIBRATION
In these vibrations distance between the two atoms
increases or decreases. But the atoms are in the same axis.
Bond length is altered.
BENDING VIBRATION
The positions of the atoms changes relative to the original
bond.

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STRETCHING VIBRATION
1. Symmetrical Stretching
It is the one in which two bonds
increase or decrease in length
symmetrically.
2 . Asymmetrical Stretching
It is the one in which one bond
length increases and the other
one is decreases.

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1. In-Plane Bending
There is change in bond angle in these vibrations.
Bending of bonds takes place in the same plane.
 SCISSORING
Two atoms approach each other in which bond angle decreases.
ROCKING
Movement of atom take place in the same direction. Here bond angles are
maintained.
BENDING VIBRATION

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2. Out Plane Bending
Here bending occurs outside the plane of a molecule.
 WAGGING
Two atoms move up and down the plane with respect to the central atom.
 TWISTING
It is the one in which one atom moves up the plane and the other atom
moves down the plane with respect to the central atom.

10. 4THCOFFEE
Symmetrical
Vibration
Asymmetrical
Vibration
Scissoring
Rocking Wagging Twisting

11. 4THCOFFEE SAMPLE HANDLING
 As IR spectroscopy has been used for the characterization of solid,
liquid, gas sample, it is the evident that samples of different phases
have to be handled.
 Sample must be transparent to IR radiations.
 A dilute solution in a non polar solvent give distorted spectra.
 Non polar solvent give same spectra whether they are in
condensed state or in solutions in non polar solvent.
CELLS
Very narrow 0.01nm-1nm.
Teflon spacers to allow variation in path length.

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 Solid may be dissolved in non aqueous solvent.
 Drop of solution is placed on an alkali metal desk and the solvent
allowed to evaporate.
 The absorption due to solvent has to be compensated by keeping the solvent
in a cell of same thickness as the containing the sample, in the path of
reference beam of double beam spectrophotometer.
Demerit :
Can not be used for all solids because suitable solvents are limited in numbers &
there is no single solvents which is transparent through IR region.
1. Solid run in solution
SAMPLING FOR SOLIDS
1. Solid run in solution 2. Solid films
3.Mull technique 4. Pressed pellets technique

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 If appropriate in nature ,the sample is deposited on the surface of
a KBr and NaCl cell by evaporation of a solution of the solid useful
for qualitative analysis.
 If non crystalline, a thin homogeneous film is deposited on the
plate which can be mounted and scanned directly.
Merit and Demerit:
Useful for rapid qualitative but becomes useless for carrying our
quantitative analysis.
2. Solid Films

14. 4THCOFFEE 3. Mull Technique
In this technique the finely ground solid sample is mixed with
Nujol (mineral oil) to make a thick paste which is then made to
spread between IR transmitting windows. This is then mounted
in a path of infrared beam and the spectrum is run. This method
is good for quantitative analysis.

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 Although Nujol is transparent throughout IR region, yet it has
a disadvantage that it has absorption maxima at 2915, 1462,
1376 & 719 cm-1.
 So when IR spectrum of solid sample is taken in Nujol mull,
absorption bands of solid sample that happen to coincide
with the absorption bands of the Nujol mull will be hidden
(but others will be clearly seen in IR spectrum) and then
interferes with the absorption of the sample.
Demerit :

16. 4THCOFFEE 4. Pressed pellets technique
 In this technique a small amount of finely ground solid sample is
intimately mixed with about 100 times its weight of powdered
Potassium bromide, in a vibrating ball mill.
 This finely ground mixture is then pressed under very high
pressure (25000 p sig) in evacuable die or minipress to form a small
pellet (about 1-2 mm thick and 1cm in diameter).
The resulting pellet is transparent to IR radiation and is run as such.

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Preparing Of KBr Disk

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 Liquids sample taken.
 Put it into rectangular cells of KBr, NaCl etc.
 IR spectra obtained.
Sample thickness such that transmittance lies between
15- 20% i.e. 0.015-0.05 mm in thickness.
 For double beam, matched cells are generally employed.
One cell contains sample while other has solvent used in
used.
 Protect from moisture.
SAMPLING OF LIQUIDS

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Gas samples are examined in the lR spectrometer after removal of
water vapor. The simplest gas cell consists of metal/gas cylinder of
10cm long and closed with a appropriate window. The gaseous
sample is passed through a stopcock via a suitable gas handing
apparatus and partial pressure of 5 to 15mmHG gives a reasonable
level of absorption in most cases .The end wall of gas cell is made of
NaCl and for low concentrated gases long path lengths are required.
Multi reflection can be used to make effective path length as long as
40cm so that constituent of gas can be determined.
SAMPLING FOR GAS

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https://www.slideshare.net/DrUMESHSHARMA3/ir-spectroscopy-theory-modes-of-vibration-and-sample-
handling-by-dr-umesh-kumar-sharma-and-anu-aathew
https://www.slideshare.net/razaram1/introduction-and-principle-of-ir-spectroscopy-25872192
https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental
_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Vibrational_Spectroscopy/Vibrational_Modes
/Number_of_Vibrational_Modes_in_a_Molecule
Instrumental methods of chemical analysis, 5th edition,
Gurdeep.R.Chatwal & Sham.K.Anand, Pg. No. 2.29
Instrumental methods of chemical analysis, 26th edition,
B.K.Sharma, Pg. No. 262-264.
REFERANCE