This means that the molar extinction coefficients of the two enantiomers

This means that the molar extinction coefficients of the two enantiomers (ϵl𝜖𝑙 and ϵr𝜖𝑟) are unequal in circularly polarized light. These differences in absorption (ϵl𝜖𝑙 and ϵr𝜖𝑟) can be measured as a function of wavelength, and the curves obtained are called circular dichroism curves. They have positive or negative signs (Cotton effect) just as for optical rotatory dispersion curves

1. UNIT 4: ORD and CD
M.Sc. Semester II
Dr Anil Kumar Singh
Department of Chemistry
Mahatma Gandhi Central University
(Part III: Cotton effect, Faraday and Kerr effects)

2. COTTON EFFECT
▪Inthelatenineteenthcentury,Cottonstudiedopticalrotatorydispersioncurvesof
compoundsintheregionoftheirabsorptions,wherebothopticalrotationandcircular
dichroismoccursimultaneously.
▪Henoticedthatnotonlyisitpossibletoobservethecirculardichroismandellipticity
inanabsorptionregion,butalsothattheshapeofopticalrotatorydispersion(ORD)
curvesinsidetheabsorptionregiondiffersmarkedlyfromtheirshapeoutsidethese
regions.
▪FormanyyearsORDcurvesinsideabsorptionregionswerereferredtoas
“anomalous”ORDcurves,butthisiserroneous,sincethisshapeistheusualonefor
ORDcurvesinsidetheseregions.
▪Thecombinationoftheappearanceofcirculardichroism(andellipticity)andanS-
shapedORDcurveforanopticallyactivecompoundinsideitsabsorptionregionis
knownastheCottonEffect.

3. Cotton Effect Curves
•Inawavelengthregionwherethelightisabsorbed,absolutemagnitudeoftheoptical
rotationatfirstvariesrapidlywithwavelength,crosseszeroatabsorptionmaximaand
thenagainvariesrapidlywithwavelengthbutinoppositedirection.
•TheeachCottoneffectconsistsoftwoextremes,ageometricmaximumcalleda"peak"
andageometricminimumcalleda"trough".
•TheCottoneffectiscalledpositiveiftheopticalrotationfirstincreasesasthe
wavelengthdecreasesthatmeanspositiveCottoneffectcurvehasitspeakinthelonger
wavelengthregion,whileitiscallednegativeifopticalrotationfirstdecreasesasthe
wavelengthdecreases.
•NegativeCottoneffectcurveisdefinedashavingitstroughappearingatthelonger
wavelength.
•OpticallypureenantiomersalwaysdisplayoppositeCottoneffectORDcurvesof
identicalmagnitude.

4. 0
-1
-2
-3
1
2
3
Wavelength [λ]
[]
----------
----------
Peak
Trough
Positive cotton Effect Curve
0
-1
-2
-3
1
2
3
Wavelength [λ]
[]
----------
----------
Peak
Trough
Negative cotton Effect Curve
Cotton Effect Curves

5. •Cottoneffectcurvesareobtainedforopticallyactivecompoundshaving
chromophoresabsorbinginthenearUVregion.
•Theyshowpeaksandtroughsdependingontheabsorbinggroupspresentinthe
systemandthusthesearecalledananomalousdispersionofopticalrotation.These
areagaindividedintotwocategories;
•(i)SingleCottoneffectcurves
•(ii)MultipleCottoneffectcurves
Cotton Effect Curves

6. Theseareanomalousdispersioncurveswhichshow
maximumandminimumbothoccurringintheregion
ofmaximumabsorption.Sothereappearsonlyone
peakandonetroughintheSingleCottoneffect
curves.Theverticaldistancebetweenpeakand
troughiscalledamplitude“a”.Amplitudeis
measuredindegrees.
Molecularamplitude,??????=
??????1−??????2
100
Where??????
1ismolarrotationoftroughorpeakfrom
shorterwavelengthand??????
2isthemolarrotationof
extremepeakortroughfromhigherwavelength.
Single Cotton effect curves
0
-2
-3
3
Wavelength [λ]
[]
Peak
Trough
Single cotton Effect Curve
-1
1
2
………….
………….
Amplitude, a

7. These are a little different from the single Cotton effect curves. They contain more than
one peak and one trough.
Multiple Cotton effect curves
0
-2
-3
3
Wavelength [λ]
[]
-1
1
2
0
-2
-3
3
Wavelength [λ]
[]
-1
1
2
Positive Multiple CE Curve Negative Multiple CE Curve

8. Faraday Effect
•An interaction between lightand amagneticfield in a medium. The Faraday effect
causes a rotation of the plane ofpolarizationwhich is linearly proportional to the
component of the magnetic field in the direction of propagation.
•Incircularlypolarizedlightthedirectionoftheelectricfieldrotatesatthefrequency
ofthelight,eitherclockwiseorcounter-clockwise.Inamaterial,thiselectricfield
causesaforceonthechargedparticlescomprisingthematerial(becauseoftheirlow
mass,theelectronsaremostheavilyaffected).
•Themotionthuseffectedwillbecircular,andcircularlymovingchargeswillcreate
theirown(magnetic)fieldinadditiontotheexternalmagneticfield.
•Therewillthusbetwodifferentcases:thecreatedfieldwillbeparalleltothe
externalfieldforone(circular)polarization,andintheopposingdirectionforthe
otherpolarizationdirection–thusthenetBfieldisenhancedinonedirectionand
diminishedintheoppositedirection.

9. Faraday Effect
Thechangeinthedynamicsofthe
interactionforeachbeamandone
ofthebeamswillbesloweddown
morethantheother,causinga
phasedifferencebetweentheleft-
andright-polarizedbeam.When
thetwobeamsareaddedafterthis
phaseshift,theresultisagaina
linearlypolarizedbeam,butwitha
rotationinthepolarization
direction.
Picture credit: https://commons.wikimedia.org/w/index.php?curid=1945979

10. where,
βistheangleofrotation(inradians)
Bisthemagneticfluxdensityinthedirectionof
propagation(inteslas)
disthelengthofthepath(inmeters)wherethe
lightandmagneticfieldinteract
νistheVerdetconstantforthematerial.This
empiricalproportionalityconstant(inunitsof
radiansperteslapermeter)varieswith
wavelengthandtemperature.
The relation between theangle of rotationof the polarization and the magnetic field in
a transparent material is:
β=νBd
Faraday Effect

11. Kerr Effect
TheKerreffectisachangeintherefractiveindexofamaterialinresponsetoan
appliedelectricfield.
Itisthespecialcaseinwhichaslowlyvaryingexternalelectricfieldisappliedby,for
instance,avoltageonelectrodesacrossthesamplematerial.Underthisinfluence,the
samplebecomesbirefringent,withdifferentindicesofrefractionforlightpolarized
paralleltoorperpendiculartotheappliedfield.Thedifferenceinindexof
refraction,Δn,isgivenby
Δn= λΚΕ
2
whereλisthewavelengthofthelight,KistheKerrconstant,andEisthestrengthof
theelectricfield.Thisdifferenceinindexofrefractioncausesthematerialtoactlike
awaveplatewhenlightisincidentonitinadirectionperpendiculartotheelectric
field.

12. References
•Module25:SpectroscopicmethodsfordeterminationofAbsoluteConfiguration
ofCoordinationComplexes,e-pathshala.
•Wikipedia

13. THANK YOU
Next…..
Part IV: Application of ORD & CD in determining absolute
configuration of metal complexes.