UV/Visible spectroscopy involves the interaction of electromagnetic radiation in the ultraviolet-visible spectral region with matter. Key points: 1. Electromagnetic radiation consists of photons that interact with molecules through electronic, vibrational, and rotational energy transitions. 2. UV/Vis spectroscopy follows Beer's law - absorbance is directly proportional to concentration and path length. It can be used to determine concentrations. 3. Chromophores are functional groups that absorb UV-Vis radiation through n→π* and π→π* transitions. Common chromophores include C=O, C=C, C≡N. 4. Auxochromes are functional groups that modify the absorption properties of chromoph

1. 3. UV / Visible Spectroscopy
Kanhaya lal kumawat
M.tech.
Ict (UDCT), Mumbai
Advance Characterization Techniques

2. Spectroscopy
It is the branch of science that deals with the study of
interaction of electromagnetic radiation with matter.

3. Spectroscopy
Absorption
spectroscopy
UV
spectroscopy
IR
spectroscopy
NMR
spectroscopy
Emission
spectroscopy
Fluorimetry
Flame
photometery

4. UV / VISIBLE SPECTROSCOPY

5. 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.

7. 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.

8. 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

10. 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. Interaction of EMR
with Matter

12. 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

13. 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.

14. 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

15. Principles of
Spectroscopy

16. Lambert’s Law

17. Schematic diagram of modern UV spectrometers
sample
reference
detector
I0
I0 I0
I
log(I0/I) = A
200 700
l, nm
monochromator/
beam splitter optics
UV-VIS sources

18. Lambert’s Law
Statement-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 and x be the thickness of the
solution.
Then I
dx
dI
D
KI
dx
dI
(Integrate equation between limit I = Io at x = 0 and I = I at x=l)
Kl
I
I
0
ln
We get,
Kl
I
I
0
log
303
.
2
l
K
I
I
303
.
2
log
0
Lambert’s Law
l
E
A .
Where, is Absorption coefficient
E
K
303
.
2

19. Beer Lambert’s Law

21. Beer Lambert’s Law
• Statement-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, Io be the intensity of incident radiation, x be the
thickness of the solution,C be the concentration of the solution
Then
I
C
dx
dI
.
D
I
C
K
dx
dI
.
'
(Integrate equation between limit I = Io at x = 0 and I = I at x=l)
l
C
K
I
I
.
'
ln
0
l
C
E
A .
.
l
C
K
I
I
.
.
log
303
.
2 0
l
C
K
I
I
.
303
.
2
log 0
A
I
I0
log
Where is Molar extinction coefficient
E
K
303
.
2

22. Continue
0
I
I
T
A
I
I
T
0
log
log
l
C
E
A .
.
‰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 its concentration .
optical density (OD)

23. ‰Chemical deviations
9 High concentrations (affect the charge distribution extent of absorption)
9 Chemical interactions (dissociation,association or solvent interaction)
‰Instrument deviations
9 non monochromatic light, Stray lights
‰Experimental Deviation
9 Reflections, scattering etc
Limitation of Beer Lambert’s Law

24. PRINCIPLES OF
UV - VISIBLE
SPECTROSCOPY

25. 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.

26. Instrumentation
1. The construction of a traditional UV-VIS spectrometer is very similar to
an IR, as similar functions – sample handling, irradiation, detection and
output are required
2. Here is a simple schematic that covers most modern UV spectrometers:
sample
reference
detector
I0
I0 I0
I
log(I0/I) = A
200 700
l, nm
monochromator/
beam splitter optics
UV-VIS sources

27. Light Source:
‰Deuterium lamp for UV range
‰Tungsten/halogen for visible range
Combined sources used to cover a range (200-800 nm)

28. Cuvettes:
‰ Glass or plastic – visible
‰ Quartz – visible, UV
‰ Acrylic – visible, UV

29. Solvents Solutions
• Solvents should not absorb UV-radiation within same range as the substance.
• (A strong absorbing solvent allows very little amount of light to pass through the sample)
• Non-polar solvents-
• no solute-solvent interactions so “fine structure”
• Polar solvents-
• solute-solvent interactions through H-bonding “no fine structure”

30. Solvatochromic effect
(Solvatochromism)
change in absorption spectra (λmax, intensity, and shape)
‰ Positive solvatochromism
9 corresponds to a bathochromic shift (or red shift) with increasing solvent polarity.
‰ Negative solvatochromism
9 corresponds to a Hypsochromic Shift (Blue Shift) with increasing solvent polarity.

31. Example

32. •
The normalized UV-vis spectra of N1 photographs of N1 in different solvents

33. The normalized emission spectra and of N1 in different solvents under UV irradiation

34. The possible electronic transitions can
graphically shown as:

35. ‰The higher is the energy gap, the lower is the wavelength of
the light absorbed.
‰Bigger jumps requires more energy, so absorb light with a
shorter wavelength

36. Effect of conjugation

37. Assignment
what is reason behind the variation of λmax?

38. Assignment
Arrange the following set of compounds in increasing order
conjugation, λmax, energy?

39. Electronic
Transitions

40. • 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

41. • 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

42. • 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

43. • 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

44. • 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

45. •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

46. Example
Electronic transition in Formaldehyde

48. Possible transitions in different functionalities

49. Typical transitions in different functionalities

50. Other possible transitions
‰charge transfer
‰d-d transition

51. Terms used
in
UV / Visible
Spectroscopy

52. 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

53. 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.

54. Chromophore
e.g. Acetone which has λmax = 279 nm
and that cyclohexanone 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

55. 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

56. 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.

57. Auxochrome
e.g. Benzene λmax = 255 nm
Phenol λmax = 270 nm
Aniline λmax = 280 nm
OH
NH2

58. Absorption
Intensity
Shifts

60. Terminologies

61. • 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

62. • 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

63. • 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

64. • Aniline shows blue shift in acidic medium, it
looses conjugation.
Aniline
λmax = 280 nm λmax = 265 nm
• Hypsochromic Shift (Blue Shift)
2
NH2
H
+
Acidic
medium
NH3
+
Cl
-

65. • 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

66. • 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

67. Wavelength ( λ )
Absorbance
(
A
)
Shifts and Effects
Hyperchromic shift
Hypochromic shift
Red
shift
Blue
shift
λmax

68. Woodward–Fieser Rules
‰ In 1941, Woodward proposed a set of rules to predict λmax values for the lowest
energy transition called Woodward–Fieser Rules
‰Applications
™ To predict λmax values of generally Dienes, Enones, Aromatics

69. λmax of dienes
‰ Concept of homoannular and heteroannular diene system
a-homoannular
b-heteroannular

70. Woodward–Fieser Rules for
1.Dienes
‰ Base value for heteroannular diene (unsubstituted, conjugated) - 214 nm
‰ Base value for homoannular diene (unsubstituted, conjugated) - 253nm
‰ Increments for
9 Each extra double bonds in conjugation + 30 nm
9 Exocyclic double bond (effect is two fold if the bond is exocyclic to two
rings) + 5 nm
‰ Substituent effect:
9 Simple alkyl substituents(ring residue) + 5 nm
9 Halogen (-Cl, -Br) + 5 nm
9 OR (R=Alkyl) + 6 nm
9 SR (R=Alkyl) + 30 nm
9 NR2 (R=Alkyl) + 60 nm

71. Find Woodward–Fieser Rules
1- 2-
λmax = 229 nm λmax = 229 nm
3- 4-
λmax = 234 nm λmax = 229 nm
Hint-Apply concept of base value, alkylchain/ring residue, exocylcic double
bond,double bond extending conjugation(DEC)

72. Assignment
Find λmax of following dienes using Woodward–Fieser Rules
Hint-Apply concept of base value, alkylchain/ring residue, exocylcic double
bond,double bond extending conjugation(DEC)
Answer a-239 nm, b-273 nm, c-234 nm, d-240 nm, e-303 nm, f-284 nm, g-278 nm

73. Woodward–Fieser Rules for
2. Enone Dienone
‰ Acyclic α,β-unsaturated ketones-215 nm
‰ 6-membered cyclic α,β-unsaturated ketones-215 nm
‰ 5-membered cyclic α,β-unsaturated ketones-202 nm
‰ α,β-unsaturated aldehydes-210 nm
‰ α,β-unsaturated carboxylic acid esters-195 nm
‰ Increments for:
9 Alkyl group/ring residue: α +10, β +12
9 Polar groups: -OH: α +35 β +30 δ +50

74. Find λmax of following dienes using Woodward–Fieser Rules
a-Base value: 215 nm α substituent: +10 β substituent: +12 =237 nm
b-Base value: 215 nm2 β substituents:+24 1 exocyclic C=C: + 5 =244 nm
c-Base value: 202 nm β substituent: +12 α-OH: +35 =249 nm
d-Base value: 215 nm 2 β substituents: +24 α-OH: +35 =274 nm
e-Base value: 215 nm α alkyl substituent: +10 β alkyl substi: +12 =237 nm
f-Base value: 215 nm α alkyl: +10 β alkyl: +12 .= 237 nm

75. Limitations
9 In case of Biphenyls, Substitutions cause loss of co-planarity of orbitals loss of
overlap, blue shift with reduced intensity.
9 applicable only if there is no strain around chromophore.
9 (calculated and experimental values of λmax do not match means molecule
must have some strain)
9 trans isomers at longer wavelength and high intensity
‰Fieser-Kuhn rules for Conjugated Polyenes can be
alternate

76. Thanku for your kind patience