UV/visible spectroscopy involves using electromagnetic radiation in the UV and visible light range to analyze samples. Key principles are that different functional groups and molecular structures absorb radiation at characteristic wavelengths. Absorption of light causes electronic transitions between molecular orbitals. The Beer-Lambert law states that absorbance is directly proportional to concentration, with molar absorptivity coefficients describing this relationship. Absorption spectra provide information to identify compounds and determine concentrations.

1. UV / VISIBLE SPECTROSCOPY
Dr. Nilesh Thakare

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.

3. Electromagnetic
Radiation

4. Electromagnetic Radiation
• Electromagnetic radiation consist of discrete
packages of energy which are called as
photons.
• Aphoton consists of an oscillating electric field
(E)& an oscillating magnetic field (M) which
are perpendicular to eachother.

6. Electromagnetic Radiation
• Frequency(ν):
– It is defined as the number of times electrical field
radiation oscillates in onesecond.
– Theunit 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 canbe written as:
c=ν λ
• Asphoton is subjected to energy,so
E=h ν =h c/ λ

8. Electromagnetic Radiation

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

10. Principles of
Spectroscopy

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 acompound.

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, IRSpectroscopy (0.76 - 15μm)

13. Interaction of
EMR with
Matter

14. Interaction of EMRwith matter
1. Electronic EnergyLevels:
• At room temperature the molecules are in the
lowest energy levelsE0.
• 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 isas:
∆E = h ν = En - E0 where (n = 1, 2, 3, … etc)
∆E = 35 to 71 kcal/mole

15. Lambert’
s Law

16. 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 incidentlight.
• Let I be the intensity of incident radiation.
xbe the thickness of thesolution.
Then

17. Lambert’s Law
 dI
I
dx
So,  KI
 dI
dx
Integrate equation betweenlimit
I = Io at x = 0 and
I = I at x=l,
We get,
I
I 0
l n   K l

18. Lambert’s Law
I
  K l
I 0
2 . 3 0 3 lo g
l
KI
2 . 3 0 3
lo g
I 0
 
Absorbance
I
 A
I 0
Where, l o g
K
 E
2.303
A  E .l
Absorption coefficient
Lambert’s Law

19. Beer’
s Law

20. Beer’sLaw
• When a monochromatic radiation is passed
through a solution, the decrease in the
intensity of radiation with thickness ofthe
solution
intensity
is directly proportional
of the incident light as
to the
well as
concentration of thesolution.
• Let I be the intensity of incident radiation.
xbe the thickness of thesolution.
Cbe the concentration of thesolution.
Then

21. Beer’sLaw
 dI
 C .I
dx
So,  K 'C .I
 dI
dx
Integrate equation betweenlimit
I = Io at x = 0 and
I = I at x=l,
We get,
I
I 0
ln   K ' C .l

22. Beer’sLaw
I
2 .303 log
I0
 K .C .l
K
I
C .l
2 .303
log
I 0

Where, Absorbance
I
 A
I 0
lo g
K
 E
2.303
A  E .C .l
Molar extinction
coefficient
Beer’sLaw

23. Beer’sLaw
A  E .C .l
I
T  OR
I
 A log T  log
I 0 I 0
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 ofasolution.

24. PRINCIPLES OF
UV - VISIBLE
SPECTROSCOPY

25. Principle
• The UVradiation region extends from 100 nm
to 400 nm and the visible radiation region
extends from 400 nm to 800nm.
Near UVRegion:200 nm to 400nm
FarUVRegion: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.

26. Electronic
Transitions

27. transitions canThe possible electronic
graphically shown as:

28. • σ → σ* transition
• π → π* transition
• n → σ* transition
• n → π* transition
• σ → π* transition
• π → σ* transition
1
2
3
4
5
6
Thepossible electronic transitions are

29. this
• σ electron from orbital is excited to
corresponding anti-bonding orbital σ*.
• The energy required is large for
transition.
• e.g. Methane (CH4) has C-H bond only and
can undergo σ → σ* transition and shows
absorbance maxima at 125nm.
• σ → σ* transition1

30. • 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 205nm.
• π → π* transition2

31. • 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* peaksin UVregion is small (150
– 250 nm).
• n → σ* transition3

32. • 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 suchtransitions.
• n → π* transitions require minimum energy
and show absorption at longer wavelength
around 300 nm.
• n → π* transition4

33. •These electronic transitions are
transitions & are only theoreticallypossible.
•Thus,n →π* & π→π* electronic transitions
show absorption in region
which is accessible
above 200 nm
to UV-visible
spectrophotometer.
•The UV spectrum is of only a few broad of
absorption.
• σ → π* transition
• π → σ* transition 6
forbidden
5
&

34. Termsused
in
UV/ Visible
Spectroscopy

35. Chromophore
Thepart of amolecule responsible for imparting
color, are called aschromospheres.
OR
Thefunctional 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

36. Chromophore
Tointerpretate UV– visible spectrum following
points should benoted:
1. Non-conjugated alkenes show an intense
absorption below 200 nm & are therefore
inaccessible to UVspectrophotometer.
2. Non-conjugated carbonyl group compound
give a weak absorption band in the 200 - 300
nm region.

37. Chromophore
e.g.
C
H3C CH3
and that cyclohexane hasλmax =291nm.
When double bonds are conjugated in
a compound λmax is shifted to
longer wavelength.
O
e.g. Acetone which has λmax =279 nmO
H2C
1,5 - hexadiene hasλmax =178 nm
2,4 - hexadiene hasλmax =227nm
CH2
H3C
CH3

38. Chromophore
3. Conjugation of C=Cand carbonyl group shifts
the λmax of both groups to longerwavelength.
e.g. Ethylene hasλmax =171 nm
Acetone hasλmax =279nm
C
H3C CH3
O
H 2C CH2
C
CH3
Crotonaldehyde hasλmax =290 nm
O
H2C

39. Auxochrome
The functional groups attached to a
chromophore which modifies the ability of the
chromophore to absorb light , altering the
wavelength or intensity ofabsorption.
OR
Thefunctional group with non-bondingelectrons
that does not absorb radiation in near UVregion
but when attached to achromophore alters the
wavelength & intensity ofabsorption.

40. Auxochrome
e.g. Benzeneλmax =255nm
Phenol λmax =270nm
Aniline λmax =280nm
OH
NH2

41. Absorption
& Intensity
Shifts

43. • When absorption maxima (λmax) of a
compound shifts to longer wavelength, it is
known asbathochromic shift or redshift.
• The effect is due to presence of an auxochrome
or by the changeofsolvent.
• e.g. An auxochrome group like –OH, -OCH3
causes absorption of compound at longer
wavelength.
• Bathochromic Shift (Red Shift)1

44. • In alkaline medium, p-nitrophenol shows red
shift. Because negatively charged oxygen
delocalizes more effectively than the unshared
pair of electron.
p-nitrophenol
λmax =255nm λmax =265nm
• Bathochromic Shift (Red Shift)1
O H
N
+
O O
-
O H
Alk a lin e
m e d iu m -
O
N
+
- -
O O

45. • When absorption maxima (λmax) of a
compound shifts to shorter wavelength, it is
known ashypsochromic shift or blueshift.
• The effect is due to presence of an group
causes removal of conjugation or by the
changeof solvent.
• Hypsochromic Shift (Blue Shift)2

46. • Aniline shows blue shift in acidic medium, it
loses conjugation.
Aniline
λmax =280nm λmax =265nm
• Hypsochromic Shift (Blue Shift)2
N H 2 +
H
A cid ic
m e d iu m
+
N H 3 Cl
-

47. • When absorption intensity (ε) of a compound is
increased, it is known ashyperchromicshift.
• If auxochrome introduces to the compound,
the intensity of absorption increases.
Pyridine
λmax =257nm
2-methyl pyridine
λmax =260nm
• Hyperchromic Effect3
N N CH3

48. • When absorption intensity (ε) of acompoundis
decreased, it is known ashypochromicshift.
Naphthalene
ε =19000
CH3
2-methyl naphthalene
ε =10250
• Hypochromic Effect4

49. Absorbance(A)
Shifts and Effects
Hyperchromic shift
Red
shift
Blu
e
shift
Hypochromic shift
λmax
Wavelength ( λ)

50. APPLICATIONS OF
UV / VISIBLE
SPECTROSCOPY

51. Applications
• Qualitative & QuantitativeAnalysis:
– It is used for characterizing aromatic compounds
and conjugated olefins.
– It can be used to find out molar concentration of the
solute understudy.
• Detection of impurities:
– It is one of the important method to detect
impurities in organicsolvents.
• Detection of isomers arepossible.
• Determination of molecular weight using Beer’s
law.

52. REFERENCES

53. ReferenceBooks
• Introduction to Spectroscopy
– DonaldA. Pavia
• Elementary OrganicSpectroscopy
– Y.R.Sharma
• PhysicalChemistry
– Puri, Sharma& Pathaniya

54. 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