Transcript UV Spectroscopy

Course: B.E & B.Tech
Subject: Applied Chemistry
Unit: IV
SPECTROSCOPY
Syllabus:
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Electromagnetic spectrum
Absorption of radiation
Rotational transition
Vibrational transition
Electronic transition
Intensities of spectral lines
Beer Lambert’s law
Principle,
theory,
instrumentation
applications of:
• Colorimetric analysis
• Visible and UV spectroscopy
• IR spectroscopy
• Flame photometry
and
Dr. K. SIVAKUMAR
Department of Chemistry
SCSVMV University
[email protected]
1

Electromagnetic Waves - Terminologies
Electromagnetic wave parameters:
Wavelength (λ): Wavelength is the distance between the consecutive peaks or crests
Wavelength is expressed in nanometers (nm)
1nm = 10-9 meters = 1/1000000000 meters
1A = 10-10 meters = 1/10000000000 meters
2

Electromagnetic Waves - Terminologies
Electromagnetic wave parameters:
Frequency (): Frequency is the number of waves passing through any point per second.
Frequency is expressed in Hertz (Hz)
3

Electromagnetic Waves - Terminologies
Electromagnetic wave parameters:
Wave number ( ): Wave number is the number of waves per cm.
Wavelength, Wave number and Frequency are interrelated as,
1
Where,

 =

c
 is wave length

is wave number
 is frequency
c is velocity of light in vacuum. i.e., 3 x 108 m/s
4
Electromagnetic Spectral regions
nm
EM
waves
10-4 to 10-2
g-rays
10-2 to 100
X-rays
100 to 102
102 to 103
103 to 105
UV
Visible
IR
105 to 107
107 to 109
Microwave Radio
5
Electromagnetic Spectrum
E = h
h – Planck’s constant
6
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The Electromagnetic wave lengths & Some examples
7
Electromagnetic radiation sources
EM radiation
Gamma rays
X-rays
Ultraviolet
Visible
Infrared
Microwave
Radio wave
Spectral method
Gamma spec.
X-ray spec.
UV spec.
Visible spec.
IR spec.
ESR spec.
NMR spec.
Radiation source
gamma-emitting nuclides
Synchrotron Radiation Source (SRS),
Betatron (cyclotron)
Hydrogen discharge lamp
tungsten filament lamp
rare-earth oxides rod
klystron valve
magnet of stable field strength
8
Electromagnetic Spectrum – Type of radiation and Energy change involved
9
Electromagnetic Spectrum – Type of radiation and Energy change involved
10
Electromagnetic Spectrum – Type of radiation and Energy change involved
11
Effect of electromagnetic radiations on chemical substances
The absorption spectrum of an atom often contains sharp and clear lines.
Absorption spectrum of an atom; Hydrogen
Energy levels in atom; Hydrogen
12
Effect of electromagnetic radiations on chemical substances
But, the absorption spectrum of a molecule is highly complicated with closely
packed lines
This is due to the fact that molecules have large number of energy levels and
certain amount of energy is required for transition between these energy levels.
Energy levels in molecule
Absorption spectrum of a molecule; Eg: H2O
13
Lambert’s law
fraction of the monochromatic light absorbed by a
homogeneous medium is independent of the intensity of the
incident light and each successive unit layer absorbs an equal
fraction of the light incident on it
Lambert
Beer’s law
fraction of the incident light absorbed is proportional to the number of
the absorbing molecules in the light-path and will increase with
increasing concentration or sample thickness.
Beer
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Beer–Lambert law / Beer–Lambert– Bouguer law / Lambert – Beer law
log (I0/I) =  c l = A
Where,
I0 - the intensity of incident light
I - the intensity of transmitted light
 - molar absorptivity / molar extinction coefficient in cm2 mol-1 or L mol-1 cm-1.
c - concentration in mol L-1
l - path length in cm
A - absorbance (unitless)
Molar absorptivity
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Absorption intensity 
max Intensity of absorption is directly proportional to the transition probability
A fully allowed transition will have max > 10000
A low transition probability will have max < 1000
max wavelength of light corresponding to maximum absorption is designated as max and
can be read directly from the horizontal axis of the spectrum
Absorbance (A) is the vertical axis of the spectrum A = log (I0/I)
I0 - intensity of the incident light; I - intensity of transmitted light
max = 20000
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Beer–Lambert law / Beer–Lambert– Bouguer law / Lambert – Beer law
Bouguer
Actually investigated the
range of absorption Vs
thickness of medium
Lambert
Extended the concepts
developed by Bouguer
Beer
Applied
Lambert’s
concept to solutions of
different concentrations
?
Bernard
Beer released the results
of Lambert’s concept just
prior to those of Bernard
17
Effect of electromagnetic radiations on chemical substances
The radiation energies absorbed by molecules may produce Rotational,
Vibrational and Electronic transitions.
18
Effect of electromagnetic radiations on chemical substances
Rotational transition
Microwave and far IR radiations bring about changes in the rotational energies
of the molecule
Example: Rotating HCl molecule
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Effect of electromagnetic radiations on chemical substances
Vibrational transition
Infrared radiations bring about changes in the vibration modes (stretching,
contracting and bending) of covalent bonds in a molecule
Examples:
Example:
Vibrating HCl molecule
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Effect of electromagnetic radiations on chemical substances
Electronic transition
UV and Visible radiations bring about changes in the electronic transition of a molecule
Example: Cl2 in ground and excited states
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Effect of electromagnetic radiations on chemical substances
Cl2 in Ground state
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Effect of electromagnetic radiations on chemical substances
Cl2 in Excited state
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Spectrometer
The instrument used for recording the absorption spectra of a compound is called
spectrometer.
The different components present in various types of spectrometer are shown here.
UV - hydrogen
discharge lamp
Visible - tungsten
filament lamp
Variable slit,
rheostat, etc
Prism,
ilter,
monochromator,
grating
Cuvette,
test-tube,
cell
Photographic plate,
photocell,
photomultiplier,
photoconductivity
device etc
Galvanometer;
pen recorder;
cathode ray
oscillograph
IR - electrically
heated rod of rareearth oxides
Microwave - klystron
valve
NMR - magnet of
stable field strength.
24
Colorimetric analysis
Principle: Colorimetry analysis method is useful in determining the concentration of
coloured solutions using the visible region (400nm–750nm) of electromagnetic
spectrum and Beer Lambert’s law.
If the test solution is colourless then a suitable complexing agent can be added to test
solution to get coloured which will absorb light.
Example: For cuprous ions (Cu2+) estimation NH4OH can be added to get blue colour.
Instrumentation: Tungsten filament lamp is used to generate visible
region (400nm – 750nm) light.
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Colorimetric analysis…
The molecules in the cuvette absorb light and the remaining light is
transmitted to the photocell. In photocell,
Current generated  Amount of light transmitted
But the amount of light transmitted depends on the depth of colour of
test solution. i.e., concentration of test solution.
If high concentration solution is analysed then, more number of
molecules will be in the path of light and more amount of light will be
absorbed.
So, the amount of light transmitted will be very less and generates only
less current.
26
Colorimetric analysis: Applications
useful in estimating the concentration of coloured solutions
Example: Estimation of CuSO4 by colorimetry
Series of CuSO4 solution with known concentration are prepared and
ammonium hydroxide is added to each solution to get blue colour.
Absorbance of each standard CuSO4 solution is measured with same filter
and tabulated.
Concentration (C) of CuSO4
Absorbance (A)
0.0001
A1
0.0002
A2
0.0003
.
0.0004
.
0.0005
.
0.0006
A6
test solution
At
A=  C l
27
UV Visible Spectroscopy
Principle:
Visible and ultraviolet spectroscopy is a study of electronic spectra of organic
molecules which are found in the wavelength region of 100nm-400nm (UV region)
and 400nm-750nm (Visible region).
UV and visible radiations absorbed by the molecules will bring transition of outer shell
electrons(,  and n electrons).
According to molecular orbital theory when a organic molecule absorbs UV or visible
radiations its electrons are promoted from a bonding to an antibonding orbital.
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The Ultraviolet region [10 – 800nm]
The Ultraviolet region may be divided as follows,
1. Far (or Vacuum) Ultraviolet region [10 – 200 nm]
2. Near (or Quartz) Ultraviolet region [200 – 380 nm]
3. Visible region [380 - 800 nm]
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VISIBLE region in Electromagnetic Spectrum
•Violet
•Indigo
•Blue
•Green
•Yellow
•Orange
•Red
: 380 - 420 nm
: 420 - 440 nm
: 440 - 490 nm
: 490 - 570 nm
: 570 - 585 nm
: 585 - 620 nm
: 620 - 800 nm
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UV - VISIBLE SPECTROSCOPY
•
In UV - Visible Spectroscopy, the sample is irradiated with the broad
spectrum of the UV - Visible radiation
•
If a particular electronic transition matches the energy of a certain band
of UV - Visible, it will be absorbed
•
The remaining UV - Visible light passes through the sample and is
observed
•
From this residual radiation a spectrum is obtained with “gaps” at these
discrete energies – this is called an absorption spectrum
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Electronic Energy Levels
•
Absorption of UV - Visible radiation by an organic molecule leads to
electronic excitation among various energy levels within the molecule.
•
Electron transitions generally occur in between a occupied bonding or lone
pair orbital and an unoccupied non-bonding or antibonding orbital.
•
The energy difference between various energy levels, in most organic
molecules, varies from 30 to 150 kcal/mole
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Electronic Energy Levels
* (anti-bonding)
* (anti-bonding)
Energy
n (non-bonding)
 (bonding)
 (bonding)
 - orbitals are the lowest energy occupied molecular orbitals
* - orbitals are the highest energy unoccupied molecular orbitals
 - orbitals are of somewhat higher energy occupied molecular orbitals
* - orbitals are lower in energy (unoccupied molecular orbitals) than *
n - orbitals; Unshared pairs (electrons) lie at the energy of the original atomic orbital.
Most often n - orbitals energy is higher than  and .
since no bond is formed, there is no benefit in energy
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Electronic Energy Levels
Graphically,
*
Unoccupied levels
*
Energy
Atomic orbital
n
Atomic orbital
Occupied levels


Molecular orbitals
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Electronic Transitions
•
The valence electrons in organic molecules are involved in bonding as
 - bonds,  - bonds or present in the non-bonding form (lone pair)
•
Due to the absorption of UV - Visible radiation by an organic molecule different
electronic transitions within the molecule occurs depending upon the nature of
bonding.
•
The wavelength of UV - Visible radiation causing an electronic transition depends
on the energy of bonding and antibonding orbitals.
•
The lowest energy transition is typically that of an electron in the
Highest Occupied Molecular Orbital (HOMO) to the
Lowest Unoccupied Molecular Orbital (LUMO)
*
*
Energy
Atomic orbital
n


Molecular orbitals
Unoccupied levels
Atomic orbital
Occupied levels
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Types of Electronic Transitions
Transition between bonding molecular orbitals and anti-bonding molecular orbitals
They are of three types:
  *
  *
  *
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Types of Electronic Transitions
Transition between bonding molecular orbitals and anti-bonding molecular orbitals
  * (bonding  to anti-bonding )
* (anti-bonding)
•
  * transition requires large energies in
far UV region in 120-200nm range.
* (anti-bonding)
•
Molar absorptivity: Low
max = 1000 - 10000
n (non-bonding)
 (bonding)
 (bonding)
•
Examples: Alkanes - transition @ 150nm
Methane
Cyclohexane
Propane
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Types of Electronic Transitions
Transition between bonding molecular orbitals and anti-bonding molecular orbitals
  * (bonding  to anti-bonding )
* (anti-bonding)
•
  * occur in 200-700nm range.
* (anti-bonding)
•
Molar absorptivity: High
max = 1000 - 10000.
n (non-bonding)
Examples:
•Unsaturated compounds
•double or triple bonds
•aromatic rings
•Carbonyl groups
•azo groups
•Conjugated  electrons
Carbonyl
Azo
 (bonding)
 (bonding)
max is high because the  and * orbitals
are in same plane and consequently the
probability of jump of an electron from  
* orbital is very high.
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Types of Electronic Transitions
Transition between bonding molecular orbitals and anti-bonding molecular orbitals
  * (bonding  to anti-bonding )
•
  * occur only in <150 nm range.
•
Molar absorptivity: Low
Examples: Carbonyl compounds
* (anti-bonding)
* (anti-bonding)
n (non-bonding)
 (bonding)
 (bonding)
•  * and   * transitions: high-energy, accessible in
vacuum UV (max <150 nm). Not usually observed in
molecular UV-Vis.
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Types of Electronic Transitions
Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals
They are of two types:
* (anti-bonding)
n  *
n  *
n  * (non-bonding n to anti-bonding )
•
n  * occur in 200-700nm range.
•
Molar absorptivity: Low
max = 10 - 100
•
* (anti-bonding)
n (non-bonding)
 (bonding)
 (bonding)
Examples:
•
Compounds with double bonds involving unshared pair(s) of electrons
•
Aldehydes, Ketones
•
C=O, C=S, N=O etc.,
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Types of Electronic Transitions
Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals
n  * (non-bonding n to anti-bonding )
•
* (anti-bonding)
* (anti-bonding)
Excitation of an electron in an unshared
pair on Nitrogen, oxygen, sulphur or
halogens to an antibonding  orbital is
called n  * transitions.
•
n  * occur in 150-250nm range.
•
Molar absorptivity: Low
max = 100 - 3000
n (non-bonding)
 (bonding)
 (bonding)
Example:
Methanol
max = 183nm
( = 500)
max = 257nm
( = 486)
Trimethylamine max = 227nm
( = 900)
1-Iodobutane
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Types of Electronic Transitions
  * (bonding  to anti-bonding )
  * (bonding  to anti-bonding )
  * (bonding  to anti-bonding )
* (anti-bonding)
* (anti-bonding)
n  * (non-bonding n to anti-bonding )
n  * (non-bonding n to anti-bonding )
n (non-bonding)
 (bonding)
 (bonding)
Energy required for various transitions obey the order:   * > n  * >   *> n  *
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Chromophores
The coloured substances owe their colour to the presence of one or more unsaturated
groups responsible for electronic absorption. These groups are called chromophores.
Examples: C = C, C=C, C = N, C=N, C = O, N = N, etc..
Chromophores absorb intensely at the short wavelength
But, some of them (e.g, carbonyl) have less intense bands at higher wavelength due to
the participation of n electrons.
Methyl orange
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Auxochromes
An auxochromes is an auxillary group which interact with chromophore and deepens
colour; its presence causes a shift in the UV or visible absorption maximum to a longer
wavelength
Examples: NH2, NHR and NR2, hydroxy and alkoxy groups.
Property of an auxochromic group:
• Provides additional opportunity for charge delocalization and thus provides smaller
energy increments for transition to excited states.
• The auxochromic groups have atleast one pair of non-bonding electrons (lone pair)
that can interact with the  electrons and stabilizes the * state
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Auxochromes: examples
Auxochrome
Unsubstitued
chromophore
max (nm)
Substituted chromophore
max
(nm)
-CH3
H2C=CH-CH = CH2
217
H2C=CH-CH=CHCH3
224
-OR
H3C-CH=CH-COOH
204
H3C-C(OCH3) = CHCOOH
234
-C1
H2C=CH2
175
H2C = CHCl
185
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Instrumentation
I0
I1
detector
monochromator/
beam splitter optics
I0
reference
UV-VIS sources
sample
log(I0/I) = A
200
700
, nm
I
I2
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Instrumentation…
Radiation source, monochromator and detector
Two sources are required to scan the entire UV-VIS band:
Deuterium lamp – covers the UV – 200-330
Tungsten lamp – covers 330-700
The lamps illuminate the entire band of UV or visible light; the
monochromator (grating or prism) gradually changes the small bands of
radiation sent to the beam splitter
The beam splitter sends a separate band to a cell containing the sample
solution and a reference solution
The detector (Photomultiplier, photoelectric cells) measures the difference
between the transmitted light through the sample (I) vs. the incident
light (I0) and sends this information to the recorder
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Sample Handling
Virtually all UV spectra are recorded solution-phase
Only quartz is transparent in the full 200-700 nm range;
plastic and glass are only suitable for visible spectra 380 – 800nm
Concentration: 0.1 to 100mg
10-5 to 10-2 molar concentration may safely be used
Percentage of light transmitted: 20% to 65%
At high concentrations, amount of light transmitted is low, increasing the
possibility of error
A typical sample cell (commonly called a cuvet):
Cells can be made of plastic, glass or quartz
(standard cells are typically 1 cm in path length)
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UV - VISIBLE SPECTROSCOPY - Applications
Structure identification
Useful in the identification of a newly synthesized compound.
The spectrum of unknown compound can be compared with the absorption spectrum of
several known compounds in the literature.
On comparison, If the spectrum of unknown compound correlates with a specifically known
compound then the structures of both will also be similar. This method is called finger
printing technique.
Concentration of impurities
During the purification process of a compound, if the absorption spectrum is recorded at a
particular interval of time the decrease in the concentration of impurities can be
monitored. The purification can be continued till it gives a less value of absorbance.
To study the rate of a reaction
To study the rate of formation of product in a reaction, the absorbance of product can be
measured at definite intervals of time. We know that, the absorbance is directly
proportional to concentration and hence the absorbance value will be increasing with
respect to time.
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UV Spectroscopy (Electronic Spectra) - Terminologies
Beer-Lambert Law
Absorbance
Molar absorptivity
Extinction coefficicent
concentration
Path length
A = .c.l
A, a measure of the amount of radiation that is absorbed
, absorbance of a sample of molar concentration in 1 cm cell.
An alternative term for the molar absorptivity.
c, concentration in moles / litre
l, the length of the sample cell in cm.
max The wavelength at maximum absorbance
max The molar absorbance at max
Band
Term to describe a uv-vis absorption which are typically broad.
HOMO
Highest Occupied Molecular Orbital
LUMO
Lowest Unoccupied Molecular Orbital
Chromophore
Auxochrome
Structural unit responsible for the absorption.
A group which extends the conjugation of a chromophore by
sharing of nonbonding electrons
Bathochromic shift
The shift of absorption to a longer wavelength.
Hypsochromic shift
shift of absorption to a shorter wavelength
Hyperchromic effect
An increase in absorption intensity
Hypochromic effect
A decrease in absorption intensity
Isosbestic point
point common to all curves produced in the spectra of a compound
taken at various pH
50
Infra RED Spectral region
• Infrared refers to that part of the electromagnetic spectrum between the visible and
microwave regions.
• The IR region is divided into three regions as given below,
Few absorptions of organic molecules
‘E’ corresponds to differences
observed between vibrational states
Very few absorptions occur here
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Infra RED Spectroscopy – Wave number
•
•
In IR, frequency is expressed as wave numbers
Wave numbers have units of reciprocal cm (cm-1)
1
1
Wavenumber in cm =
=
x 104
 in cm  in m
1
•
•
Using this scale, the IR region is 4000-400 cm-1
The corresponding wavelength range is 2.5 m to 25 m
2.5 m = 4000 cm-1
25 m = 400 cm-1
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Infra RED Spectroscopy – Wave number…
Therefore,
HIGHER WAVENUMBER
LOWER WAVENUMBER
Higher frequency
Lower frequency
Higher energy
Lower energy
Shorter wavelength
Longer wavelength
Energy, frequency, and wavenumber are directly proportional to each other.
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Infra RED Spectroscopy – An IR spectrum
• In IR spectroscopy, an organic molecule is exposed to IR radiation. When the radiant
energy matches the energy of a specific molecular vibration, absorption occurs.
• Compares intensity of IR striking sample (Iin) with intensity of IR leaving sample (Iout)
• 100%T no light absorbed by sample
• 0% all light absorbed by sample
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Infra RED Spectroscopy – An IR spectrum
•An IR spectrum is a graph between Wavenumber (in cm-1) Vs Transmittance (%).
IR spectrum of
octane
An absorption of IR energy is therefore represented by a “trough” in the curve.
55
Infra RED Spectroscopy – An IR spectrum…
• Band intensities can also be expressed as absorbance; A = log10 (1/T)
IR spectrum of
octane
56
Infra RED Spectroscopy – Principle
• Molecules are flexible.
• Atoms in organic molecules are constantly vibrating around average positions.
• Therefore, A vibrating bond can be thought of as two masses connected by a spring.
• Molecules are vibrating in different modes i.e., different ways.
• Bond lengths and bond angles are continuously changing due to this vibration.
• These vibrational changes depend on the spatial arrangement and masses of the atoms in
that particular molecule.
57
Infra RED Spectroscopy – Principle…
• If infrared radiations are passed through the naturally vibrating molecules then IR waves matching with the
vibrating
frequencies
of
molecule
will
be
absorbed
and
increases the amplitude
of vibration in molecules.
• Because, IR radiations does not have sufficient energy to cause the excitation of electrons, however, it causes
atoms and groups of atoms of organic compounds to vibrate faster about the covalent bonds which connect
them.
• The IR spectrum (vibrational spectra) appears as band and not as line because; the vibrational changes are
also accompanied by several numbers of rotational changes.
58
Infra RED Spectroscopy –Functional group & Finger print region
• Every molecule absorbs IR waves at particular frequencies and produces absorption bands.
• There are two sections in the IR spectra.
Functional group region > 1500cm-1
Fingerprint region < 1500cm-1
Fingerprint region: The absorption band in the finger print region is due to the complex
vibrations involving in entire molecules. It is impossible for any two different compounds (except
enantiomers) to have precisely the same infrared spectrum. The pattern of absorptions in this
regions are unique to any particular compound, just as a person’s fingerprints are unique.
59
Infra RED Spectroscopy – Finger print region can be subdivided in to three regins
(i) 1500 – 1350cm-1 region:
•
The absorption bands near 1380cm-1 and 1365cm-1 show the
presence of tertiary butyl group in the compound.
•
Gem-dimethyl shows a medium band near 1380cm-1
•
Nitro compounds show one strong band at 1350cm-1
(ii) 1350 – 1000cm-1 region:
•
Compounds like alcohol, esters, lactones, acid anhydrides show characteristic absorptions in
this region, due to the C-H stretching.
•
Primary alcohols form two strong bands at 1350-1260cm-1 and near 1050cm-1.
•
Phenols absorb near 1200cm-1
•
Esters show two strong bands between 1380-1050cm-1. Absorption bands in the region
1150-1070cm-1 is due to C-O and C-O-C group in ethers.
(iii) Below 1000cm-1 region:
•
The bands at  700cm-1 and at 970-960cm-1 are useful to distinguish between cis and trans
alkenes. The higher value indicates that the hydrogen atoms in the alkene are trans w.r.t. each
other.
•
The presence of mono substituted and also disubstituted (at ortho, para and meta positions)
benzene shows characteristic absorptions in this region.
•
Example: A band in the region 750-700cm-1 is due to mono substituted benzene.
60
Infra RED Spectroscopy –Functional group region
Functional group region: Like molecules, different functional groups absorb IR waves at
particular frequencies and produces absorption bands in the IR spectrum. For example, the IR
spectrum of all compounds with carbonyl group consists of absorption band in wave number range
1800cm-1 and 1650cm-1.
•
4000-2500 cm-1 N-H, C-H, O-H (stretching)
– 3300-3600 cm-1 N-H, O-H
– 3000 cm-1 C-H
•
2500-2000 cm-1 CC and CN (stretching)
•
2000-1500 cm-1 double bonds (stretching)
– 1680-1750 cm-1 C=O
– 1640-1680 cm-1 C=C
•
Below 1500 cm-1 “fingerprint” region
61
Infra RED Spectroscopy –Functional group & Finger print region…
Functional group
region
Fingerprint
region
62
Infra RED Spectroscopy – IR Active and IR Inactive
For a molecule to absorb infra red radiation, the molecule has to fulfill the following two
requirements.
1. Correct wavelength of radiation
The molecule absorbs IR radiations only when the natural frequency of vibration of atoms or
groups of atoms or functional groups of molecule is same as the frequency of the incident
radiation. i.e., If infrared radiations are passed through the naturally vibrating molecules then
IR waves matching with the vibrating frequencies of molecule will be absorbed and increases
the amplitude of vibration in molecules.
For example: The natural frequency of HCl is
8.7  1013 sec1
When IR radiation is passed through the HCl and the spectrum is recorded. The spectrum
consists of a absorption band at a frequency 8.7  1013 sec 1
Hence,
8.7  1013 sec1

is the natural or characteristic frequency of the HCl molecule.
  2.886 10 cm
3
1
63
Infra RED Spectroscopy – IR Active and IR Inactive…..
2. Electric dipole 
For absorbing IR radiations a molecule should
possess electric dipole (dipole moment) and the
absorption should causes change in electric dipole.
Molecules with electric dipole will have slight positive
and slight negative charge on its component atoms.
For example:
In HCl, due to the high electro
negativity of Cl, the electron clouds will be more
towards chlorine and less towards hydrogen. So,
hydrogen will get slight positive charge and Cl will get
slight negative charge, i.e.,
But IR radiation absorption is not possible in the
symmetric diatomic molecules like H2, O2, and N2 etc.,
because, symmetric diatomic molecules do not
possess electric dipole. , Therefore, IR radiation
absorption and IR spectrum is not possible in these
molecules.


H  Cl
dipole moment
OO
IR Active
llly H2O, NO
IR Inactive
No dipole moment
llly H2, Cl2
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Infra RED Spectroscopy – Theory - Various types of molecular vibrations
bond length changes
bond angle changes
65
Infra RED Spectroscopy – Theory - Various types of molecular vibrations
Stretching: Rhythmical movement along the bond axis, which leads to the
increase or decrease of interatomic distance.
Symmetrical stretching
Anti-symmetrical stretching
66
Infra RED Spectroscopy – Theory - Various types of molecular vibrations
Bending:
Bending vibrations takes place when a three atom system is a
part of a molecule.
Types of Bending vibrations:
• Scissoring
• Wagging
• Rocking
• Twisting
67
Infra RED Spectroscopy – Theory - Various types of molecular vibrations
Scissoring: Two atoms attached to a central atom move away from and towards each other.
68
Infra RED Spectroscopy – Theory - Various types of molecular vibrations
Wagging: Vibrating group swings back and forth out of plane of molecule.
69
Infra RED Spectroscopy – Theory - Various types of molecular vibrations
Rocking: Vibrating group swings back and forth in the plane of molecule.
70
Infra RED Spectroscopy – Theory - Various types of molecular vibrations
Twisting: Vibrating group rotates about the chemical bond which is attached
to rest of molecule.
71
Infra RED Spectroscopy – Theory - Various types of molecular vibrations
bond length changes
bond angle changes
72
Infra RED Spectroscopy – Theory - Various types of molecular vibrations
bond length changes
bond angle changes
73
Infra RED Spectroscopy – Energy required for molecular vibrations
Energy required
Stretching
bond length changes
>
Bending
bond angle changes
Energy needed to stretch a spring is more than that needed to bend it so the stretching
absorption of a bond will appear at a higher frequency than the bending absorption of the
same bond.
Stretching > Bending > Wagging / Twisting
74
Infra RED Spectroscopy – Number of Fundamental vibrations
• A molecule containing n atoms has 3n degrees of freedom (translational, rotational, vibrational)
• In
Linear molecule with n number of atoms has:
3 degrees called the translational degrees of freedom (specify the centre of mass of
molecule representing the translational motion of molecule
2 degrees called the rotational degrees of freedom (specify the orientation of
molecule about its centre of mass, representing the rotational motion of molecule
3n – 5 degrees called the vibrational degrees of freedom (specify the relative
positions of the ‘n’ nuclei representing vibrational motion
• For Example: In CO2 the number of theoretical absorption bands could be (3x3-5 = 4) four.
Degrees of freedom
Molecule
Total Translational Rotational Vibrational
3n-5
HCl
6
3
2
1
CO2
9
3
2
4
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Infra RED Spectroscopy – Instrumentation
Nernst glower:
mixture of rare
earth
metal
oxides
rod
maintained at
1500C.
Prism and
grating made
of metal
halides like
NaCl, KBr
Thermocouple
or bolometer
gas, liquid or
solution, solid
substance can be
used as samples and
the solvents like
CCl4, CS2 and CHCl3
can be used as
solvents
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Infra RED Spectroscopy - Applications
(i) Structure elucidation of organic compound
We know that, If IR radiations are passed through a molecule the different functional groups in
the molecule absorb IR radiations at their natural frequencies. This phenomenon is helpful in
elucidating the structure of organic compounds.
For example,
• If the IR spectrum of a organic molecule shows absorption band at 5.82 (wave number
range 1800cm-1 and 1650cm-1) then it is possible to identify that a carbonyl group should be
present in it.
• On the other hand if the IR spectrum contains no absorption band in between 5.4 to 6.3,
then it is certain that no carbonyl group is present in the compound.
• Likewise, if the presence of different functional groups in a compound is identified using the
IR spectrum then the structure of unknown compound can be deduced with the help of other
available data.
77
Infra RED Spectroscopy – Applications…
(ii) Structure identification
• The IR spectrum is useful in the identification of a newly synthesized compound.
• The IR spectrum of unknown compound can be compared with the absorption spectrum of
several known compounds in the literature.
• If the IR spectrum of unknown compound matches with a known compound then the
structures of both will also be similar. This method is called finger printing technique.
(iii) Identification of impurities
• If any extra absorption bands are recorded in the IR spectrum of a known compound then it
is certain that the compound is not pure.
• Therefore, the compound should be purified further to remove all the impurities.
• After purification, if the IR spectrum is recorded then there will not be any extra absorption
bands.
• Thus IR spectrum is useful in determining the purity of a compound. .
78
Infra RED Spectroscopy – Applications…
(iv) Study of reaction kinetics
• The rate of a reaction can be studied by recording IR spectrum of the reaction mixture at
definite intervals of time.
• From the IR spectrum disappearance of absorption bands recorded in the beginning (due to
the presence of reactants) and appearance of new absorption bands (due to the formation of
products) can be observed in the course of time.
• Thus the progress of organic reaction can be studied.
(v) To ascertain hydrogen bonding in a molecule
• In a sample with inter and intramolecular hydrogen bonding, if the dilution is increased and
the IR spectrum is recorded, the absorption band, due to intermolecular hydrogen bonding
disappears and the absorption band due to intramolecular hydrogen bonding remains
unchanged.
79
Flame Photometry
Principle
When an atomized metal is passed through a flame, the outer shell electrons
of metals are excited and emit energy of characteristic wavelengths. The
emitted radiations are measured directly.
Theory
When the test solution is introduced into a flame, then solvent evaporates out
and leaves the solute particles as salt. The salt particle dissociates in to
atoms and reaches the vapourised state due to the heat of flame. The atoms
in ground state get excited to the higher energy state by absorbing thermal
energy from the flame. If these excited atoms returns back to the ground
state emits radiation at particular wavelength. On passing the emitted
radiations through optical filter permits the exits of characteristic wavelength
of a metal under study.
80
Flame Photometry - Instrumentation
The test solution containing metal ions is passed in to the atomizer through the nebuliser. Test solution sucked
through the nebuliser, air from compressor and fuel gas from the fuel cylinder mix together in the mixing chamber.
Now, the mixture is introduced into the flame. Here, all the excited atoms emits radiations of different wavelength.
If the emitted radiations are passed through optical filter then, the optical filter permits only the characteristic
wavelength of element under study. The transmitted light falls on the photo cell, amplifier and then on the digital
recorder to get plot.
81
Flame Photometry - Instrumentation
Flame photometry is useful in analyzing the elements which are easily
excited in the flame heat and to emit radiations.
To analyze alkali-metals. Example: Li, Na, K etc.
To analyze alkaline-earth metals. Example: Ca
82
83
Good Luck!
Dr. K. SIVAKUMAR
Department of Chemistry
84
SCSVMV University
[email protected]