3-Cotton effect, Faraday and Kerr effects.pdf

3-Cotton effect, Faraday and Kerr effects.pdf

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.