1. INFRARED SPECTROSCOPY
BY
Dr. Suman Pattanayak
Associate Professor
Department of Pharma Analysis & QA.
Vijaya Institute of Pharmaceutical Sciences for Women
IV B. Pharm/ I Sem
Pharmaceutical Analysis
3. 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 an useful tool to
identify functional groups in organic molecules
IR spectroscopy is a result of molecular vibrational
transitions that occur when light interacts with matter
Molecules are always vibrating For a molecule to be IR
active, the vibrations should give rise to a net change
in dipole moment Infrared spectroscopy
4. The absorption of IR radiations can be expressed either
in terms of wavelength() or in wave number ( ).
Relationship between wavelength() and wave number
( ).
wave number( )= 1/ wavelength() in cm
suppose wavelength() is 2.5 = 2.5× 10-4 cm, then
wave number( )= 1/ 2.5× 10-4 cm=4000 2.5× 10-4 cm
1m = 10-6 m ;
cm-1 = no. of waves per cm of path
= 1/[ (cm)]
energy of wave
E = h = hc
5. Infrared region
LIMIT OF RED LIGHT: 800 nm, 0.8 m,
12500 cm-1
NEAR INFRARED: 0.8 -2.5 m, 12500 - 4000
cm-1
MID INFRARED: 2.5 - 25 m, 4000 - 400 cm-1
FAR INFRARED: 25 - 1000 m, 400 - 10 cm-1
6.
7. Principle:
IR radiation does not have enough energy to induce
electronic transitions as seen with UV.
Absorption of IR is restricted
to compounds with small energy differences in the
possible vibrational and rotational states. For a molecule
to absorb IR, the vibrations or rotations within a
molecule must cause a net change in the dipole moment
of the molecule.
The alternating electrical field
of the radiation (remember that electromagnetic
radiation consists of an oscillating electrical field and an
oscillating magnetic field, perpendicular to each other)
interacts with fluctuations in the dipole moment of the
molecule. If the frequency of the radiation matches the
vibrational frequency of the molecule then radiation will
be absorbed, causing a change in the amplitude of
molecular vibration.
8. Theory of infra red absorption
IR radiation does not have enough energy to induce electronic
transitions as seen with UV. Absorption of IR is restricted to
compounds with small energy differences in the possible
vibrational and rotational states. For a molecule to absorb IR,
the vibrations or rotations within a molecule must cause a net
change in the dipole moment of the molecule.
The alternating electrical field of the radiation (remember that
electromagnetic radiation consists of an oscillating electrical field
and an oscillating magnetic field, perpendicular to each other)
interacts with fluctuations in the dipole moment of the molecule.
If the frequency of the radiation matches the vibrational
frequency of the molecule then radiation will be absorbed,
causing a change in the amplitude of molecular vibration.
9. Molecular rotations
Rotational transitions are of little use to the spectroscopist. Rotational levels
are quantized, and absorption of IR by gases yields line spectra. However, in
liquids or solids, these lines broaden into a continuum due to molecular
collisions and other interactions.
Molecular vibrations
The positions of atoms in a molecules are not fixed; they are subject to a
number of different vibrations. Vibrations fall into the two main catagories of
stretching and bending.
Stretching: Change in inter-atomic distance along bond axis
10. Bending: Change in angle between two bonds. There are four types of
bend:
•Rocking
•Scissoring
•Wagging
•Twisting
11. Vibrational coupling
In addition to the vibrations mentioned above, interaction between
vibrations can occur (coupling) if the vibrating bonds are joined to a
single, central atom. Vibrational coupling is influenced by a number of
factors;
1.Strong coupling of stretching vibrations occurs when there is a
common atom between the two vibrating bonds
2.Coupling of bending vibrations occurs when there is a common bond
between vibrating groups
3.Coupling between a stretching vibration and a bending vibration occurs
if the stretching bond is one side of an angle varied by bending vibration
4.Coupling is greatest when the coupled groups have approximately
equal energies
5.No coupling is seen between groups separated by two or more bonds
12. Factors Affecting Frequency of Absorption
Bond strength
C=O stretching (1700 cm-1) vs C-O stretching (1200 cm-1)
C=C stretching (1650 cm-1) vs C-C stretching (1200 cm-1)
It takes more IR energy to stretch short strong bonds than it
does to stretch long weak bonds
It also takes more energy to stretch a bond between two heavy
atoms
than it does if the atoms are less massive
Atomic Size
C-H (3000 cm-1)
C-C (1200 cm-1)
C-Cl (800 cm-1)
C-Br (550 cm-1)
Bigger masses vibrate at lower energy
13. ♦ As a bond stretches, the atoms are moved
apart from each other
♦ If the bond elongation changes the net
dipole moment of the molecule, an IR peak
is manifested
Examples of large and small peaks
♦ Large peaks are observed for C=O bonds
♦ Small peaks are observed for C=C bonds
♦ If the atoms that stretch have different
electro negativities, you are likely to see
larger peaks
14.
15. Ranges are broad, not exact
• Peaks are generally broad, not sharp
• Exact frequency depends upon
– conjugation
– proximity effects
16.
17. Detection Electronics
and Computer
Infrared
Source
Determines Frequencies
of Infrared Absorbed and
plots them on a chart
Sample
Simplified Infrared Spectrophotometer
NaCl
plates
Absorption
“peaks”
Infrared
Spectrum
frequency
intensity of
absorption
(decreasing)
focusing
mirror
18. Sources
• Tungsten incandescent lamp – black body source for
measurements in NIR
• Nichrome (or rhodium) wire – Coiled, heated by resistance to
incandescence.
Black oxide layer forms on surface. Temperature 1100°C. Requires little
maintenance and no cooling required. Emits in Mid-IR but less power
than other
sources. Cheaper instruments
• Nernst Glower (rare earth oxides) – More intense emitted
radiation.
Constructed from mixture of fused oxides of Zr, Th and Cs. Non-
conducting at ambient temperatures but at temperatures >800 °C it is
electrically conducting, maintains high temperature by resistive heating.
Good energy output (intensity 2x nichrome wire or globar)
• Globar – A rod of silicon carbide 6-8 mm in diameter.
Characteristics between
nichrome wire and Nernst Glower. Self starting and operates at 1300
°C. Globar
must be water cooled – brass jacket surrounds globar.
• Carbon Dioxide Laser – Useful for narrow radiation bands
19.
20. Instrumentation-Components
♦ Sample Cells and Preparation
• Solids
• Mull - suspend ground solid in oil of similar refractive index
(Nujol, perfluorocarbon)
• KBr Pellet - few mg sample + 0.5 to 1 g dry KBr ground +
compressed at very high pressure
• Disposable polyethylene film strips (dissolve solid in volatile
solvent,
“paint” on the film or on a salt plate)
• Liquids
• Gases
♦Optics - dessicated salts such as NaCl, CsBr, LiF, KBr and front
surface mirrors. Glass and quartz lenses cannot be used because
they absorb IR radiation
♦Chopper (modulation and tuned amplifier)
21. Most flexible system for analyzing all 3 states
of matter (solid, liquid, gas)
“Neat” (analysis of liquids/oils)
Pellet (analysis of solids)
Thin Cell (analysis of dissolved solid samples -
solutions)
Long Cell (analysis of gases)
25. Sample cells
To obtain an IR spectrum, the sample must be placed in a
“container "or cell that is transparent in the IR region of
the spectrum. Sodium chloride or salt plates are a
common means of placing the sample in the light beam
of the instrument.
These plates are made of salt (NaCl or KBr) and must be
stored in a water free environment
26.
27.
28.
29. b) Nernst glower
aux.
heater
2 - 5 cm
Has - temp coefficient.
of resistance.
1 - 3 mm dia.
ceramic holder
Y2O3,
ThO2,
ZrO2
heated up
to 1500oC
Pt leads
cement
30. c) Globar
5 cm
6 - 8 mm dia.
SiC rod
heated
to
1300oC
water-
cooled
brass
tube
with slot
+ temp
coeff. of
resistance
G 15 m
650 cm-1
NG
G
10 m
1000 cm-1
NG 1 m
10000 cm-1
31. e- prom oted from valence band
to unfilled conduction band,
causing e- hole pair form ation.
N o. of pairs depends on light
intensity.
photovoltaic:
pd caused by separation of e-
hole pairs betw een n, p layer.
photoconductive:
R changes w ith radiation pow er,
for sem iconductor.
photoelectrom agnetic:
utilise H all E ffect in
sem iconductor.
http://www.chem.vt.edu/chem-ed/scidex.html
Photon detectors
32. Analyzing IR Spectra
Look for C=O peak (1820-1660 cm-1)
If C=O check for OH (3400-2400 cm-1)
◦ indicates carboxylic acid
If C=O check for NH (3500 cm-1)
◦ indicates amide
If C=O check for C-O (1300-1000 cm-1)
◦ indicates ester
If no OH, NH or C-O then ketone
33. Analyzing IR Spectra
If no C=O check for OH (3600-3300 cm-1)
◦ indicates alcohol
If no C=O check for NH (3500 cm-1)
◦ indicates amine
If no C=O & no OH check C-O (1300 cm-1)
◦ indicates ether
Look for C=C (1650-1450 cm-1) then
aromatic
35. Sample IR Spectrum #1
A - CO-OH stretch (3000)
B - CH stretch (2800)
C - C=O ester (1757)
D - C=O carboxy (1690)
E - C=C aromatic (1608)
F - C=C aromatic (1460)
36. Sample IR Spectrum #2
A
B
C
C CH3
O
Acetophenone
A) C=O (1730) B) C=C aromatic (1590) C) C-H aromatic (3050)
37. Applications
Qualitative “fingerprint” check for
identification of drugs
Used for screening compounds and
rapid identification of C=O groups
Can be used to characterize samples in
solid states (creams and tablets)
Can detect different crystal isoforms
(polymorphs)
Water content measurement
40. Applications of Near IR (NIR)
Quality control of pharmaceutical
formulations
Determination of particle size
Determination of blend uniformity
Determination or identification of
polymorphic drugs
