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UV / VISIBLE SPECTROSCOPY
Spectroscopy
• It is the branch of science that deals with the
study of interaction of matter with light.
OR
• It is the branch of science that deals with the
study of interaction of electromagnetic
radiation with matter.
Electromagnetic
Radiation
Electromagnetic Radiation
• Electromagnetic radiation consist of discrete
packages of energy which are called as
photons.
• A photon consists of an oscillating electric field
(E) & an oscillating magnetic field (M) which
are perpendicular to each other.
Electromagnetic Radiation
• Frequency (ν):
– It is defined as the number of times electrical field
radiation oscillates in one second.
– The unit for frequency is Hertz (Hz).
1 Hz = 1 cycle per second
• Wavelength (λ):
– It is the distance between two nearest parts of the
wave in the same phase i.e. distance between two
nearest crest or troughs.
Electromagnetic Radiation
• The relationship between wavelength &
frequency can be written as:
c = ν λ
• As photon is subjected to energy, so
E = h ν = h c / λ
Electromagnetic Radiation
Electromagnetic Radiation
Violet 400 - 420 nm Yellow 570 - 585 nm
Indigo 420 - 440 nm Orange 585 - 620 nm
Blue 440 - 490 nm Red 620 - 780 nm
Green 490 - 570 nm
Principles of
Spectroscopy
Principles of Spectroscopy
• The principle is based on the measurement of
spectrum of a sample containing atoms /
molecules.
• Spectrum is a graph of intensity of absorbed or
emitted radiation by sample verses frequency
(ν) or wavelength (λ).
• Spectrometer is an instrument design to
measure the spectrum of a compound.
Principles of Spectroscopy
1. Absorption Spectroscopy:
• An analytical technique which concerns with
the measurement of absorption of
electromagnetic radiation.
• e.g. UV (185 - 400 nm) / Visible (400 - 800 nm)
Spectroscopy, IR Spectroscopy (0.76 - 15 μm)
Principles of Spectroscopy
2. Emission Spectroscopy:
• An analytical technique in which emission
(of a particle or radiation) is dispersed
according to some property of the emission
& the amount of dispersion is measured.
• e.g. Mass Spectroscopy
Interaction of
EMR with
Matter
Interaction of EMR with matter
1. Electronic Energy Levels:
• At room temperature the molecules are in the
lowest energy levels E0.
• When the molecules absorb UV-visible light
from EMR, one of the outermost bond / lone
pair electron is promoted to higher energy
state such as E1, E2, …En, etc is called as
electronic transition and the difference is as:
∆E = h ν = En - E0 where (n = 1, 2, 3, … etc)
∆E = 35 to 71 kcal/mole
Interaction of EMR with matter
2. Vibrational Energy Levels:
• These are less energy level than electronic
energy levels.
• The spacing between energy levels are
relatively small i.e. 0.01 to 10 kcal/mole.
• e.g. when IR radiation is absorbed, molecules
are excited from one vibrational level to
another or it vibrates with higher amplitude.
Interaction of EMR with matter
3. Rotational Energy Levels:
• These energy levels are quantized & discrete.
• The spacing between energy levels are even
smaller than vibrational energy levels.
∆Erotational < ∆Evibrational < ∆Eelectronic
Lambert’s
Law
Lambert’s Law
• When a monochromatic radiation is passed
through a solution, the decrease in the
intensity of radiation with thickness of the
solution is directly proportional to the
intensity of the incident light.
• Let I be the intensity of incident radiation.
x be the thickness of the solution.
Then
Lambert’s Law
I
dx
dI


So, KI
dx
dI


Integrate equation between limit
I = Io at x = 0 and
I = I at x=l,
We get,
Kl
I
I


0
ln
Lambert’s Law
Kl
I
I


0
log
303
.
2
l
K
I
I
303
.
2
log
0


Where, Absorbance
A
I
I

0
log
E
K

303
.
2
l
E
A .

Absorption coefficient
Lambert’s Law
Beer’s
Law
Beer’s Law
• When a monochromatic radiation is passed
through a solution, the decrease in the
intensity of radiation with thickness of the
solution is directly proportional to the
intensity of the incident light as well as
concentration of the solution.
• Let I be the intensity of incident radiation.
x be the thickness of the solution.
C be the concentration of the solution.
Then
Beer’s Law
I
C
dx
dI
.


So, I
C
K
dx
dI
.
'


Integrate equation between limit
I = Io at x = 0 and
I = I at x=l,
We get,
l
C
K
I
I
.
'
ln
0


Beer’s Law
l
C
K
I
I
.
.
log
303
.
2 0

l
C
K
I
I
.
303
.
2
log 0

Where, Absorbance
A
I
I

0
log
E
K

303
.
2
l
C
E
A .
.

Molar extinction
coefficient
Beer’s Law
Beer’s Law
l
C
E
A .
.

0
I
I
T  OR A
I
I
T 


0
log
log
From the equation it is seen that the absorbance
which is also called as optical density (OD) of a solution
in a container of fixed path length is directly
proportional to the concentration of a solution.
PRINCIPLES OF
UV - VISIBLE
SPECTROSCOPY
Principle
• The UV radiation region extends from 10 nm
to 400 nm and the visible radiation region
extends from 400 nm to 800 nm.
Near UV Region: 200 nm to 400 nm
Far UV Region: below 200 nm
• Far UV spectroscopy is studied under vacuum
condition.
• The common solvent used for preparing
sample to be analyzed is either ethyl alcohol
or hexane.
Electronic
Transitions
The possible electronic transitions can
graphically shown as:
• σ → σ* transition
1
• π → π* transition
2
• n → σ* transition
3
• n → π* transition
4
• σ → π* transition
5
• π → σ* transition
6
The possible electronic transitions are
• σ electron from orbital is excited to
corresponding anti-bonding orbital σ*.
• The energy required is large for this
transition.
• e.g. Methane (CH4) has C-H bond only and
can undergo σ → σ* transition and shows
absorbance maxima at 125 nm.
• σ → σ* transition
1
• π electron in a bonding orbital is excited to
corresponding anti-bonding orbital π*.
• Compounds containing multiple bonds like
alkenes, alkynes, carbonyl, nitriles, aromatic
compounds, etc undergo π → π* transitions.
• e.g. Alkenes generally absorb in the region
170 to 205 nm.
• π → π* transition
2
• Saturated compounds containing atoms with
lone pair of electrons like O, N, S and
halogens are capable of n → σ* transition.
• These transitions usually requires less energy
than σ → σ* transitions.
• The number of organic functional groups
with n → σ* peaks in UV region is small (150
– 250 nm).
• n → σ* transition
3
• An electron from non-bonding orbital is
promoted to anti-bonding π* orbital.
• Compounds containing double bond
involving hetero atoms (C=O, C≡N, N=O)
undergo such transitions.
• n → π* transitions require minimum energy
and show absorption at longer wavelength
around 300 nm.
• n → π* transition
4
•These electronic transitions are forbidden
transitions & are only theoretically possible.
•Thus, n → π* & π → π* electronic transitions
show absorption in region above 200 nm
which is accessible to UV-visible
spectrophotometer.
•The UV spectrum is of only a few broad of
absorption.
• σ → π* transition
5
• π → σ* transition 6
&
Terms used
in
UV / Visible
Spectroscopy
Chromophore
The part of a molecule responsible for imparting
color, are called as chromospheres.
OR
The functional groups containing multiple bonds
capable of absorbing radiations above 200 nm
due to n → π* & π → π* transitions.
e.g. NO2, N=O, C=O, C=N, C≡N, C=C, C=S, etc
Chromophore
To interpretate UV – visible spectrum following
points should be noted:
1. Non-conjugated alkenes show an intense
absorption below 200 nm & are therefore
inaccessible to UV spectrophotometer.
2. Non-conjugated carbonyl group compound
give a weak absorption band in the 200 - 300
nm region.
Chromophore
e.g. Acetone which has λmax = 279 nm
and that cyclohexane has λmax = 291 nm.
When double bonds are conjugated in a
compound λmax is shifted to longer wavelength.
e.g. 1,5 - hexadiene has λmax = 178 nm
2,4 - hexadiene has λmax = 227 nm
C
H3
C
CH3
O
O
C
H2
CH2
C
H3
CH3
Chromophore
3. Conjugation of C=C and carbonyl group shifts
the λmax of both groups to longer wavelength.
e.g. Ethylene has λmax = 171 nm
Acetone has λmax = 279 nm
Crotonaldehyde has λmax = 290 nm
C
H3
C
CH3
O
C
H2 CH2
C
CH3
O
C
H2
Auxochrome
The functional groups attached to a
chromophore which modifies the ability of the
chromophore to absorb light , altering the
wavelength or intensity of absorption.
OR
The functional group with non-bonding electrons
that does not absorb radiation in near UV region
but when attached to a chromophore alters the
wavelength & intensity of absorption.
Auxochrome
e.g. Benzene λmax = 255 nm
Phenol λmax = 270 nm
Aniline λmax = 280 nm
OH
NH2
Absorption
& Intensity
Shifts
• When absorption maxima (λmax) of a
compound shifts to longer wavelength, it is
known as bathochromic shift or red shift.
• The effect is due to presence of an auxochrome
or by the change of solvent.
• e.g. An auxochrome group like –OH, -OCH3
causes absorption of compound at longer
wavelength.
• Bathochromic Shift (Red Shift)
1
• In alkaline medium, p-nitrophenol shows red
shift. Because negatively charged oxygen
delocalizes more effectively than the unshared
pair of electron.
p-nitrophenol
λmax = 255 nm λmax = 265 nm
• Bathochromic Shift (Red Shift)
1
OH
N
+ O
-
O
OH
-
Alkaline
medium
O
-
N
+ O
-
O
• When absorption maxima (λmax) of a
compound shifts to shorter wavelength, it is
known as hypsochromic shift or blue shift.
• The effect is due to presence of an group
causes removal of conjugation or by the
change of solvent.
• Hypsochromic Shift (Blue Shift)
2
• Aniline shows blue shift in acidic medium, it
loses conjugation.
Aniline
λmax = 280 nm λmax = 265 nm
• Hypsochromic Shift (Blue Shift)
2
NH2
H
+
Acidic
medium
NH3
+
Cl
-
• When absorption intensity (ε) of a compound is
increased, it is known as hyperchromic shift.
• If auxochrome introduces to the compound,
the intensity of absorption increases.
Pyridine 2-methyl pyridine
λmax = 257 nm λmax = 260 nm
ε = 2750 ε = 3560
• Hyperchromic Effect
3
N N CH3
• When absorption intensity (ε) of a compound is
decreased, it is known as hypochromic shift.
Naphthalene 2-methyl naphthalene
ε = 19000 ε = 10250
CH3
• Hypochromic Effect
4
Wavelength ( λ )
Absorbance
(
A
)
Shifts and Effects
Hyperchromic shift
Hypochromic shift
Red
shift
Blue
shift
λmax
APPLICATIONS OF
UV / VISIBLE
SPECTROSCOPY
Applications
• Qualitative & Quantitative Analysis:
– It is used for characterizing aromatic compounds
and conjugated olefins.
– It can be used to find out molar concentration of the
solute under study.
• Detection of impurities:
– It is one of the important method to detect
impurities in organic solvents.
• Detection of isomers are possible.
• Determination of molecular weight using Beer’s
law.
REFERENCES
Reference Books
• Introduction to Spectroscopy
– Donald A. Pavia
• Elementary Organic Spectroscopy
– Y. R. Sharma
• Physical Chemistry
– Puri, Sharma & Pathaniya
Resources
• http://www2.chemistry.msu.edu/faculty/reu
sch/VirtTxtJml/Spectrpy/UV-
Vis/spectrum.htm
• http://en.wikipedia.org/wiki/Ultraviolet%E2
%80%93visible_spectroscopy
• http://teaching.shu.ac.uk/hwb/chemistry/tut
orials/molspec/uvvisab1.htm

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uvvisiblespectroscopy-130121115849-phpapp02.pptx

  • 1. UV / VISIBLE SPECTROSCOPY
  • 2. Spectroscopy • It is the branch of science that deals with the study of interaction of matter with light. OR • It is the branch of science that deals with the study of interaction of electromagnetic radiation with matter.
  • 4. Electromagnetic Radiation • Electromagnetic radiation consist of discrete packages of energy which are called as photons. • A photon consists of an oscillating electric field (E) & an oscillating magnetic field (M) which are perpendicular to each other.
  • 5.
  • 6. Electromagnetic Radiation • Frequency (ν): – It is defined as the number of times electrical field radiation oscillates in one second. – The unit for frequency is Hertz (Hz). 1 Hz = 1 cycle per second • Wavelength (λ): – It is the distance between two nearest parts of the wave in the same phase i.e. distance between two nearest crest or troughs.
  • 7. Electromagnetic Radiation • The relationship between wavelength & frequency can be written as: c = ν λ • As photon is subjected to energy, so E = h ν = h c / λ
  • 9. Electromagnetic Radiation Violet 400 - 420 nm Yellow 570 - 585 nm Indigo 420 - 440 nm Orange 585 - 620 nm Blue 440 - 490 nm Red 620 - 780 nm Green 490 - 570 nm
  • 11. Principles of Spectroscopy • The principle is based on the measurement of spectrum of a sample containing atoms / molecules. • Spectrum is a graph of intensity of absorbed or emitted radiation by sample verses frequency (ν) or wavelength (λ). • Spectrometer is an instrument design to measure the spectrum of a compound.
  • 12. Principles of Spectroscopy 1. Absorption Spectroscopy: • An analytical technique which concerns with the measurement of absorption of electromagnetic radiation. • e.g. UV (185 - 400 nm) / Visible (400 - 800 nm) Spectroscopy, IR Spectroscopy (0.76 - 15 μm)
  • 13. Principles of Spectroscopy 2. Emission Spectroscopy: • An analytical technique in which emission (of a particle or radiation) is dispersed according to some property of the emission & the amount of dispersion is measured. • e.g. Mass Spectroscopy
  • 15. Interaction of EMR with matter 1. Electronic Energy Levels: • At room temperature the molecules are in the lowest energy levels E0. • When the molecules absorb UV-visible light from EMR, one of the outermost bond / lone pair electron is promoted to higher energy state such as E1, E2, …En, etc is called as electronic transition and the difference is as: ∆E = h ν = En - E0 where (n = 1, 2, 3, … etc) ∆E = 35 to 71 kcal/mole
  • 16. Interaction of EMR with matter 2. Vibrational Energy Levels: • These are less energy level than electronic energy levels. • The spacing between energy levels are relatively small i.e. 0.01 to 10 kcal/mole. • e.g. when IR radiation is absorbed, molecules are excited from one vibrational level to another or it vibrates with higher amplitude.
  • 17. Interaction of EMR with matter 3. Rotational Energy Levels: • These energy levels are quantized & discrete. • The spacing between energy levels are even smaller than vibrational energy levels. ∆Erotational < ∆Evibrational < ∆Eelectronic
  • 19. Lambert’s Law • When a monochromatic radiation is passed through a solution, the decrease in the intensity of radiation with thickness of the solution is directly proportional to the intensity of the incident light. • Let I be the intensity of incident radiation. x be the thickness of the solution. Then
  • 20. Lambert’s Law I dx dI   So, KI dx dI   Integrate equation between limit I = Io at x = 0 and I = I at x=l, We get, Kl I I   0 ln
  • 23. Beer’s Law • When a monochromatic radiation is passed through a solution, the decrease in the intensity of radiation with thickness of the solution is directly proportional to the intensity of the incident light as well as concentration of the solution. • Let I be the intensity of incident radiation. x be the thickness of the solution. C be the concentration of the solution. Then
  • 24. Beer’s Law I C dx dI .   So, I C K dx dI . '   Integrate equation between limit I = Io at x = 0 and I = I at x=l, We get, l C K I I . ' ln 0  
  • 25. Beer’s Law l C K I I . . log 303 . 2 0  l C K I I . 303 . 2 log 0  Where, Absorbance A I I  0 log E K  303 . 2 l C E A . .  Molar extinction coefficient Beer’s Law
  • 26. Beer’s Law l C E A . .  0 I I T  OR A I I T    0 log log From the equation it is seen that the absorbance which is also called as optical density (OD) of a solution in a container of fixed path length is directly proportional to the concentration of a solution.
  • 27. PRINCIPLES OF UV - VISIBLE SPECTROSCOPY
  • 28. Principle • The UV radiation region extends from 10 nm to 400 nm and the visible radiation region extends from 400 nm to 800 nm. Near UV Region: 200 nm to 400 nm Far UV Region: below 200 nm • Far UV spectroscopy is studied under vacuum condition. • The common solvent used for preparing sample to be analyzed is either ethyl alcohol or hexane.
  • 30. The possible electronic transitions can graphically shown as:
  • 31. • σ → σ* transition 1 • π → π* transition 2 • n → σ* transition 3 • n → π* transition 4 • σ → π* transition 5 • π → σ* transition 6 The possible electronic transitions are
  • 32. • σ electron from orbital is excited to corresponding anti-bonding orbital σ*. • The energy required is large for this transition. • e.g. Methane (CH4) has C-H bond only and can undergo σ → σ* transition and shows absorbance maxima at 125 nm. • σ → σ* transition 1
  • 33. • π electron in a bonding orbital is excited to corresponding anti-bonding orbital π*. • Compounds containing multiple bonds like alkenes, alkynes, carbonyl, nitriles, aromatic compounds, etc undergo π → π* transitions. • e.g. Alkenes generally absorb in the region 170 to 205 nm. • π → π* transition 2
  • 34. • Saturated compounds containing atoms with lone pair of electrons like O, N, S and halogens are capable of n → σ* transition. • These transitions usually requires less energy than σ → σ* transitions. • The number of organic functional groups with n → σ* peaks in UV region is small (150 – 250 nm). • n → σ* transition 3
  • 35. • An electron from non-bonding orbital is promoted to anti-bonding π* orbital. • Compounds containing double bond involving hetero atoms (C=O, C≡N, N=O) undergo such transitions. • n → π* transitions require minimum energy and show absorption at longer wavelength around 300 nm. • n → π* transition 4
  • 36. •These electronic transitions are forbidden transitions & are only theoretically possible. •Thus, n → π* & π → π* electronic transitions show absorption in region above 200 nm which is accessible to UV-visible spectrophotometer. •The UV spectrum is of only a few broad of absorption. • σ → π* transition 5 • π → σ* transition 6 &
  • 37. Terms used in UV / Visible Spectroscopy
  • 38. Chromophore The part of a molecule responsible for imparting color, are called as chromospheres. OR The functional groups containing multiple bonds capable of absorbing radiations above 200 nm due to n → π* & π → π* transitions. e.g. NO2, N=O, C=O, C=N, C≡N, C=C, C=S, etc
  • 39. Chromophore To interpretate UV – visible spectrum following points should be noted: 1. Non-conjugated alkenes show an intense absorption below 200 nm & are therefore inaccessible to UV spectrophotometer. 2. Non-conjugated carbonyl group compound give a weak absorption band in the 200 - 300 nm region.
  • 40. Chromophore e.g. Acetone which has λmax = 279 nm and that cyclohexane has λmax = 291 nm. When double bonds are conjugated in a compound λmax is shifted to longer wavelength. e.g. 1,5 - hexadiene has λmax = 178 nm 2,4 - hexadiene has λmax = 227 nm C H3 C CH3 O O C H2 CH2 C H3 CH3
  • 41. Chromophore 3. Conjugation of C=C and carbonyl group shifts the λmax of both groups to longer wavelength. e.g. Ethylene has λmax = 171 nm Acetone has λmax = 279 nm Crotonaldehyde has λmax = 290 nm C H3 C CH3 O C H2 CH2 C CH3 O C H2
  • 42. Auxochrome The functional groups attached to a chromophore which modifies the ability of the chromophore to absorb light , altering the wavelength or intensity of absorption. OR The functional group with non-bonding electrons that does not absorb radiation in near UV region but when attached to a chromophore alters the wavelength & intensity of absorption.
  • 43. Auxochrome e.g. Benzene λmax = 255 nm Phenol λmax = 270 nm Aniline λmax = 280 nm OH NH2
  • 45.
  • 46. • When absorption maxima (λmax) of a compound shifts to longer wavelength, it is known as bathochromic shift or red shift. • The effect is due to presence of an auxochrome or by the change of solvent. • e.g. An auxochrome group like –OH, -OCH3 causes absorption of compound at longer wavelength. • Bathochromic Shift (Red Shift) 1
  • 47. • In alkaline medium, p-nitrophenol shows red shift. Because negatively charged oxygen delocalizes more effectively than the unshared pair of electron. p-nitrophenol λmax = 255 nm λmax = 265 nm • Bathochromic Shift (Red Shift) 1 OH N + O - O OH - Alkaline medium O - N + O - O
  • 48. • When absorption maxima (λmax) of a compound shifts to shorter wavelength, it is known as hypsochromic shift or blue shift. • The effect is due to presence of an group causes removal of conjugation or by the change of solvent. • Hypsochromic Shift (Blue Shift) 2
  • 49. • Aniline shows blue shift in acidic medium, it loses conjugation. Aniline λmax = 280 nm λmax = 265 nm • Hypsochromic Shift (Blue Shift) 2 NH2 H + Acidic medium NH3 + Cl -
  • 50. • When absorption intensity (ε) of a compound is increased, it is known as hyperchromic shift. • If auxochrome introduces to the compound, the intensity of absorption increases. Pyridine 2-methyl pyridine λmax = 257 nm λmax = 260 nm ε = 2750 ε = 3560 • Hyperchromic Effect 3 N N CH3
  • 51. • When absorption intensity (ε) of a compound is decreased, it is known as hypochromic shift. Naphthalene 2-methyl naphthalene ε = 19000 ε = 10250 CH3 • Hypochromic Effect 4
  • 52. Wavelength ( λ ) Absorbance ( A ) Shifts and Effects Hyperchromic shift Hypochromic shift Red shift Blue shift λmax
  • 53. APPLICATIONS OF UV / VISIBLE SPECTROSCOPY
  • 54. Applications • Qualitative & Quantitative Analysis: – It is used for characterizing aromatic compounds and conjugated olefins. – It can be used to find out molar concentration of the solute under study. • Detection of impurities: – It is one of the important method to detect impurities in organic solvents. • Detection of isomers are possible. • Determination of molecular weight using Beer’s law.
  • 56. Reference Books • Introduction to Spectroscopy – Donald A. Pavia • Elementary Organic Spectroscopy – Y. R. Sharma • Physical Chemistry – Puri, Sharma & Pathaniya