The document discusses various topics related to electromagnetic radiation and UV-visible spectroscopy. It defines electromagnetic radiation and its origin from atomic and molecular processes. It describes the electromagnetic spectrum and different regions. UV-visible spectroscopy involves electronic transitions in molecules from ground state to excited state. The types of electronic transitions that can occur are σ-σ*, n-σ*, π-π* and n-π* depending on the orbitals involved. Beer's law states that absorbance is directly proportional to concentration, and deviations from Beer's law can occur due to chemical associations or dissociations, or instrumental factors.

1. 1
BY – VISHAL SINGH SOLANKI
IDEAL INSTITUTE OF PHARMACY, MH

2.  Electromagnetic radiation
Energy of an EMR
Electromagnetic spectrum & it’sregions
 Spectroscopy & it’stypes
 Colorimetry
UV-spectroscopy
 Electronic transitions
Terms used in UV-Visiblespectroscopy
Absorption band & it’stypes 2

3.  Electromagneticradiation is a typeof energy that is
transmitted through spaceatenormousvelocities.
 Radiant energy has wave nature and being associated
with electric as well as magnetic field,these radiations
are called electromagneticradiation.
 Electromagnetic radiation has it’s origin in atomic and
molecularprocesses.
3

4.  The field may be represented as electricand magnetic
vectors oscillating in mutually perpendicular planes.
4

5.  The energy of an EMR can be given by the following
equation: E=hµ
Where E= Energy of radiation
h=Planck’sconstant
µ=Frequency of radiation
5

6. Frequency(µ)= c/λ
6
:-Hence, E=hµ
E=hc/λ
Therefore,energy of a radiation depends uponfrequency
and wavelength of radiation.

7. The arrangement obtained by arranging various types of
electromagneticwavesorradiations in orderof their
increasing wavelegth or decreasing frequencies is called
electromagnetic spectrum.
The electromagnetic spectrum is divided into a number
of regions; these are artificial divisions in the sense that
they have been defined solely as a result of differences in
the instrumentation required for producing and detecting
radiation of agiven frequency range.
7

8. 8

9.  Ultraviolet: 190~400nm
 Violet:
 Indigo:
400 - 420 nm
420 - 440 nm
 Blue:
 Green:
440 - 490 nm
490 - 570 nm
 Yellow:
 Orange:
570 - 585 nm
585 - 620 nm
 Red: 620 - 780 nm 9

10. 10

11. Spectroscopy is the measurement and
interpretation of electromagnetic radiation
absorbed oremitted when the moleculesor
atoms or ions of a sample moves from one
energy state toanotherenergy state
11

12. 1) Atomic spectroscopy:Here, the changes in energy takes
place at atomic level.
Eg: Atomic absorption spectroscopy, Flame
photometry
2)Molecular spectroscopy:Here,the changes in energy takes
place at molecular level.
Eg: UV spectroscopy, colorimetry, infrared
spectroscopy
12

13. Absorption spectrophotometry can be
defined as the measurement of absorption of radiant
energy by various substances.It involves the
measurementof absorptivecapacity forradiant energy
in thevisible,UVand IR regions of thespectrum.
13

14. o λ- 400-800nm
o Coloured substance absorbs light of different λin
different manner and hence get an absorption curve
o Theλatwhich maximum absorption takes place is called as
λmax
o λmax is characteristic for every colouredsubstance
o On plotting a graph of concentration v/s absorbance, we
getacalibration curve that is useful in determining the
concentration or amount of a drug substance in the given
samplesolution. 14

15. 15

16.  UV spectroscopy is concerned with thestudyof absorption
of uv radiation which ranges from200-400nm.
 Valenceelectronsabsorb theenergy thereby molecules
undergoes transition from ground state to excited
state.
 This absorption ischaracteristic and dependson the
nature of electronspresent.
 Types of e- σ electrons:
π electrons:
n electrons:
in saturated compounds
in unsaturatedcompounds
in non bondedelectrons 16

17. 17

18.  σ electron from orbital is excited tocorresponding
anti-bonding orbitalσ*.
 Theenergy required is large for this transition.
 The organic compounds in which all the valence shell
electrons are involved in the formation of σ bond do
notshowabsorption in normal uvregion (200-400nm)
 This transition is observed with saturated compounds. 18

19.  Eg: Methane(CH₄) has C-H bond only and can undergoσ-
σ* transition and shows absorption maximaat 122nm.
 The usual spectroscopic technique cannot beused below
200 nm.
 Tostudy this high energy transition,theentire region
should be evacuated (Vacuum uvregion)
 Here,theexcitation ocuurs with net retention of electronic
spin
 This region is lessinformative
19
1) σ-σ*

20.  π electron in a bonding orbital is excited to
corresponding anti-bonding orbitalπ*.
 Energy required is less whencompared to n-σ*
 Compounds containing multiple bondslike
alkenes,alkynes,carbonyls,nitriles,aromatic
compounds etc undergo π-π*transition.
Eg:Alkenesgenerallyabsorb in the region 170-205nm.
20

21.  Absorption usuallyoccurs in theordinary uv
spectrophotometer
 Absorption bands in unconjugated alkenes(170-
190nm)
 Absorption bands in carbonyls (180nm)
 Introductionof alkyl group in olefinic linkage
produces bathochromicshift
21
2) π-π*

22.  Saturated compoundscontaining one heteroatom
with unshared pair of electrons(n) like O,N,S and
halogens arecapableof n-σ* transition.
 These transition require lessenergy than σ-σ*
transition.
 In saturated alkyl halides, the energy required for
transition decreasewith increase in the size of halogen
atom (or decrease inelectronegativity)
22

23.  Eg:Methyl chloride has aλmax of 173nm.
Methyl iodide has aλmax of 258nm.
 This typeof transition isvery sensitive to hydrogen
bonding
Eg: Alcohol & amines
 Hydrogen bonding shift the uvabsorptions to
shorterwavelength.
23
3) n-σ*

24.  An electron from non-bonding orbital is promoted to
anti-bonding π* orbital.
 Compounds containing double bondsinvolving
hetero atoms(C=O,N=O) undergo such type of
transitions.
 This transition require minimumenergy outof all
transitions and shows absorption band at longer
wavelength around 300nm.
24

25.  Eg:Saturated aldehydes shows both type oftransitions
(n-π*, π-π*) at {lowenergy and high energy}
around 290 and 180 nm.
25
4) n-π*

26. 26

27.  Chromophore is defined as the nucleus or any
isolated covalently bonded group responsible for the
absorption of lightradiation.
 Any group which exhibits absorption of
electromagneticradiations in thevisibleorultraviolet
region.
C=C , C=O ,NO2 etc
 Some of the important chromophores are
carbonyls,acids,esters,nitrile,ethylenicgroups.
27

28. 28

29.  These are co-ordinatively saturated or un-saturated
groupswhich themselvesdo notabsorb radiations,but
when presentalongwith achromophoreenhances the
absorbing properties of chromophore.
 Also knownas colourenhancing group.
 All auxochromes haveoneor more non-bonding pair
of electrons.
-NH2 ,-OH ,-OR,-COOH etc
 Itextend theconjugationof a chromophore bysharing
the non-bonding electrons. 29

30. 30

31. 31

32.  When the absorption maxima(λmax)of acompound
shifts to longer wavelength,it is known as
bathochromic shift orred shift.
 The effect is due to the presenceof auxochromeor by
change of solvent.
 Eg: The n-π* transition for carbonyl compounds
experiences bathochromic shift when the polarityof
solvent isdecreased.
32

33.  When theabsorption maxima (λmax) of a compound
shifts to a shorter wavelength,it is known as
hypsochromic shift or blueshift.
 Theeffect is due to the presenceof a groupcauses
removal of conjugationor bychangeof solvent.
33

34. Eg:
Aniline shows blueshift in acidic medium since it loses
conjugation. Aniline(280nm) & Anilinium ion (-
203nm).
34

35. 35

36. 36

37. 37

38. 38

39.  The spectrum consist of sharp peaks and each peak will
correspond to the promotion of electron from one
electronic level toanother.
 During promotion,the electron moves from a given
vibrational and rotational level within one electronicmode
to theotherwithin the nextelectronic mode.
 Thus,therewill bea large no of possible transitions
 Hence,not justone but a large no. of wavelengthswhich
are close enough will be absorbed resulting in the
formation of bands
39

40. 40
K-Bandsoriginatedue toπ-π* transition from a
compound containing a conjugatedsystem
Such typeof bandsarise in compounds like
dienes,polyenes and enonesetc.
Compound Transition λmax(nm) εmax
Acetophenone π-π* 240 13,000
1,3-butadiene π-π* 217 21,000

41. R-Band transition originatedue to n-π* transition of a
single chromophoric group and having atleast one
lone pairof electronson the heteroatom
Theseare less intensewith εmax value below 100
Compound Transition λmax(nm) εmax
Acetone n-π* 270 15
Acetaldehyde n-π* 293 12
41

42. Such typeof bandsarisedue toπ-π* transition in
aromatic or hetero-aromaticmolecules.
Benzene shows absorption peaks between 230-
270nm.when achromophoricgroup is attached to the
benzene ring ,the B-Bands are observed at longer
wavelengths than the more intenseK-Bands.
Compound Transition λmax(nm) εmax
Benzene π-π* 255 215
Phenol π- π* 270 1450
42

43. E-Band originate due to the electronic transitions in
the benzenoid systemsof threeethylenic bondswhich
are in closed cyclicconjugation.
Theseare furthercharacterized as E1 and E2 bands
E1 band which appearat shorterwavelength is usually
more intense than the E2 band for the same
compound which appearsat longerwavelength.
Compound E1 Band E1 Band E2 Band E2 Band
λmax(nm) εmax λmax(nm) εmax
Benzene 184 50,000 204 79,000
Napthalene 221 133,000 286 9,300
43

44. 44

45. BEER’S LAW
According to this law,when a beam of monochromatic
radiation is passed through a solution of absorbing species,the
intensity of beam of monochromatic light decreases exponentially
with increase in concentration of absorbing species
LAMBERT’S LAW -dI/dc α I
Lambert’s law states that the rate of decrease of
intensity of monochromatic light with the thickness of the
medium is directly proportional to the intensity of incident
light.
-dI/dt α I
45

46. According to beer’s law,
-dI α I
dc
The decrease in the intensity of light (I) with
concentration(c) is proportional to intensityof
incident light(I)
-dI = K.I
dc
{ removing & introducing the
constant of proportionality
“K”}
-dI = K.dc { rearranging terms}
I 46

47.  When concentration =0, there is no
absorbance,Hence I=I₀ Substituting in
equation 
 -ln I₀ = K*0+b
 -ln I₀ = b
 Substituting the value of b in
equation
I
-ln I = K.c + b  { b constantof integration}
47

48. I₀ = ekc { removing natural log }
I
I
I₀
= e-kc {making inverse on bothsides}
 [equation for beer’slaw]
According to lambert’s law,
-dI α I
dt
This eqn can be simplified by replacing ‘c’ with ‘t’ in
I = I₀e-kt 
Eqn  &  can be combined toget I = I₀e-kct 48

49. I = ₁₀-kct { rearranging terms }
I₀
{ inverse on bothsides}
I₀ = ₁₀kct
I
Taking log on both sides,
log I₀ = Kct 
I
Here, TransmittanceT = I₀ , Absorbance,A = log 1
I T 49

50. A = log 1
T
A = log 1 { Since T = I₀ }
I/I₀ I
A =log I₀ 
I
Using eqn  & , since A =log I₀ and
I
log I₀ = Kct ,
I
A= Kct
Instead of K, we can useε
A = εct {Mathematical eqn for beerlambert’s
law} 50

51. 51

52. 52

53. There are 3 typesof deviations usuallyobserved
A)The real limitationof the law is that the beer’s law is
successful in describing the absorption behaviour of
dilute solutionsonly.
B)CHEMICAL DEVIATIONS:
Association of molecules
This can be explained by taking the examples of
methylene blueat small concentration(10‾⁵ molar) and 53

54.  Dissociation of molecules
This can beexplained by the fact thatdichromate
ions posses their maximum absorbance at 450nm
which is orange in colour .Butupondilution,itwill be
dissociated to chromate ions having maximum
absorbanceat 410nm which is yellow in colour.
This law is notvalid in case if theabsorbing material
is coagulated intoa small numberof large units.
This law showsdeviation if theabsorbing material at 54

55.  This law is notapplicable in caseof suspension.
C)INSTRUMENTAL DEVIATIONS:
Strict adherence of an absorbing system to this law is
observed only when theradiation used is monochromatic.
Strayradiation,slitwidth also causes deviation.
Hence,the reasons for the deviation depends on
environment such astemperature,pressure,solvent, 55

56.  Absorbance
0.800
λ=430nm
0.600 λ=570nm
0.400
0.200
0.00
0.00 4.00 8.00 12.00 16.00 concentration
56

57.  Absorbance
0.800 ε=1000 ε=1500
0.600
ε=1750
0.400
0.200
0.00 2.0 4.0 6.0 8.0 10.0 concentration
57

58.  2.0
0.0%
0.2%
 1%
 1.0 5%
 0

2.5 5.0 7.5 10 concentration
58

59. 1)Elementary organic spectroscopy,principles &chemical
applications,Y.R Sharma,Revised edition,pg n.o
18,26,27
2) Pharmaceutical chemistry,Instrumental
techniques,vol 2,Leslie.G.chatten,pg n.o 21-24
3) Principles and practice of analyticalchemistry,F.W
Fifield & D.kealey, 5th edition ,pg n.o270-274
4) Pharmaceutical analysis,P.Parimoo, pg n.o
147,151,152,165 59

60.  6)Instrumental analysis,Skoog,Holler,Crouch, pg
no.383,386
 7)Practical pharmaceutical chemistry,4th edition,partv
2, Beckett ,stenlake, pg n.o275-277
60

61. 
61
