Detail Description of Spectroscopy

1. Infrared Spectroscopy
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2. Spectroscopy
 “Seeing the unseen able”
 Using electromagnetic radiation as a probe to obtain
information about atoms and molecules that are too small
to see.
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Infrared Spectroscopy (Vibrational Spectroscopy)
 It is the spectroscopy that deals with the infrared region of
the electromagnetic spectrum (light with a longer wavelength
and lower frequency than visible light is involved).
 The infrared portion of the electromagnetic spectrum is
usually divided into three regions; the near-, mid- and far-
infrared, named for their relation to the visible spectrum.
Used to study the structure and functional group of organic
compounds

3. • The higher-energy near-IR: approximately 14000-4000 cm−1 (0.8–
2.5 μm wavelength) give overtone and combination vibrational band
• The mid-infrared: approximately 4000–400 cm−1 (2.5–25 μm), used to
study the fundamental vibrations
• The far-infrared: approximately 400–10 cm−1 (25–1000 μm), lying
adjacent to the microwave region, has low energy.
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4. History of IR spectroscopy
• History of spectroscopy begins with Sir Isaac Newton (1643-1727).
• Although he is better known for his work in mechanics, mathematics
etc. but the optics was among his interests after 1687 and the first course
given by Newton at Cambridge University in 1669 was about optics.
• His last book, Opticks, published in English language in 1704, were
studies about the light.
• Frederick William Herschel’s (1738-1822) knowledge of optics allowed
him to complement the work initiated by Newton and in 1800 he
discovered the infrared radiation.
• Placing a thermometer just below the red, he found that the temperature
was higher than the ambient temperature. As this radiation is invisible
to the human eye, he concluded that there are components in white light
outside the visible region.
• The discoveries of Newton and Herschel, created the basis for infrared
spectroscopy.
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Sir Isaac
Newton
Frederick William
Herschel

5. • The natural frequency of vibration of covalent bond is given by
Hook’s Law:
C = velocity of light
• ‘n’ vary with K and with reduced mass of atoms attached with
vibrating covalent bond
• The value of K(constant) vary from one bond to an other and is
• single for single bond (5 X 105 dynes/cm),
• double for double bond (10 X 105 dynes/cm) and
• triple for triple bond (15 X 105 dynes/cm)
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6.  Infrared absorption process
 The absorption of infrared radiation is, like other absorption
processes, a quantized process.
 Absorption take place as;
frequencies of infrared radiations = natural vibrational
frequencies of the molecules in question
 Important Condition for IR absorption is;
 Only those covalent bonds which have a dipole moment that
changes as a function of time when molecule vibrate, are capable
of absorbing infrared radiations.
After de-excitation of the molecule, IR radiations are dissipated
as heat.
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7.  Molecular vibrations
A molecule can vibrate in many ways, and each way is called
a vibrational mode.
For molecules with N atoms in them, linear molecules have
3N – 5 degrees of vibrational modes, whereas nonlinear molecules
have 3N – 6 degrees of vibrational modes (also called vibrational
degrees of freedom).
The IR active vibrational modes for organic molecules having
CH2X2 group include two basic types (called fundamental
vibrational modes);
 Stretching
 Bending
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8.  Stretching vibrations
In these vibrations, bond length increased or decreased
periodically.
• These are of two types;
a) Symmetrical stretching
2 bonds increased or decreased in length symmetrically.
b) Asymmetrical stretching
In this, 1 bond length is increased and other is decreased.
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9.  Bending vibrations
In these vibrations, bond angle is changed so these are also called
deformations .
These are of two types;
a) In plane bending scissoring and rocking
b) Out of plane bending wagging and twisting
 Scissoring vibration
 This is in plane bending in which bond angles are decreased and
two atoms approach each other
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10.  Rocking vibration
 The bond angle is maintained in it but the bond vibrates with in
the plane.
 Wagging vibration
 This is out of plane vibration in which two atoms move one side
of the plane i.e., up the plane or down the plane
 Twisting vibration
 This is also out of plane bending in which one atom move up the
plane and other down the plane
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11. BRUKE TENSORTM
Series
Perkin ElmerTM
Spectrum One
Instrumentation
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12.  Dispersive instruments: with
a monochromator, the spectrum
is recorded by scanning over the
wavelength range in IR region
Fourier transform IR (FTIR)
systems: with a interferometer,
whole range of IR light is used
to produce a IR spectrum
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Types of IR Spectrometer

13. Sources
An inert solid is electrically heated to a temperature in the
range 1500-2200°C will then emit IR radiation.
 Nernst glower
 A cylindrical rod or tube composed of a mixture of certain
oxides (ZrO2, Y2O3 and Er2O3 ) Heated to a temperature
between 1000-1800°C, and produces maximum radiation at
about 7100cm-1
 Globar source
Globar is made of silicon carbide (SiC), usually about 5cm
long and 7mm in diameter, Electrically heated (1300-1500°C)
and have the advantage of positive coefficient of resistance,
better suited for use in evacuated systems.
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14. Sample preparation techniques
No good solvents exist that are transparent throughout the
region of interest.
As a consequence, sample handling and preparation is
frequently the most difficult and time-consuming part of
an infrared spectrometric analysis.
Infrared spectra may be obtained for gases, liquids or solids
(neat or in solution)
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15. Gases
 The spectrum of a low-boiling liquid or gas can be
obtained by permitting the sample to expand into an
evacuated cylindrical cell, Gas cells are available in
lengths in few cm to 40cm, Long length cells are
obtained by multiple reflection optics. The cell is
closed at both ends with an appropriate window
materials (NaCl/KBr) and equipped with valves or
stopcocks for introduction of the sample.
 Liquids
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16. Solids
 Most organic compounds exhibit numerous absorption peaks
throughout the mid-infrared region, and finding a solvent that
does not have overlapping peaks is often impossible.
Pelleting
 One of the most popular techniques for handling solid
samples has been KBr pelleting.
 A milligram or less of the finely ground sample is mixed
with about 100 mg of dried potassium bromide powder.
 The mixture is then pressed in a die at 10,000 to 15,000
pounds per square inch to yield a transparent disk.
 The disk is then held in the instrument beam for
spectroscopic examination.
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17. Monochromators
The radiation source emits
radiation of various frequencies
but the sample absorbs only
radiation of certain frequency
therefore the use of
monochromator is necessary
Two types of monochromators
are
• Prism monochromator
• Grating monochromator
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18. Detectors
Thermocouples
 Photo conducting
 Bolometers
 Thermistors
 Semiconductor detector
 Pyroelectric detectors
 All detectors are thermal detection based.
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19. Applications in Organic compounds
 Determination of functional group:
From an examination of positions of absorption bands in the
spectrum, it is easy to establish the nature of groups present in
molecule.
 Studying the progress of reactions
 Detection of impurities
 Isomerism in organic chemistry
 Qualitative analysis – simple, fast, nondestructive
• Monitoring trace gases: Rapid, simultaneous analysis of GC,
moisture, N in soil. Analysis of fragments left at the scene of a
crime
 Quantitative determination of hydrocarbons on filters, in air,
or in water
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20. Miscellaneous examples
A) Determination of purity:
If impurity is present in a compound it reduces
sharpness of individual bands causes
appearance of extra bands and a general
blurring of spectra.
B)Shape of symmetry of a molecule
C)Measurement of paints and varnishes
D)Examination of old paintings
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21. Conclusion
 Infrared spectroscopy is a most important analytical
technique.
 it has proved to be one of the most valuable methods for
characterizing, both qualitatively and quantitatively the
multitude of organic compounds and mixtures of
compounds encountered in research and industry.
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