opticalrotatorydispersion

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Presented by,

Sujit R. Patel,

Dept. Of Pharmaceutics,

M.M.C.P. ,Belgaum.


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Fundamentals Of ORD.

Optical Rotatory Dispersion.

Cotton Effect.

The ORD Curve.

Circular Dichroism.

Octant Rule.

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ORD refers to the change in optical rotation withthe change in wavelength of light source. i.e. appliedonly in optically active compounds.

Optical rotation caused by compound changed withwavelength of light was first noted by Biotin 1817.

ORD curves in recent years are made use in

structural determination by comparing the curveobtain from compound believed to have relatedstructures particularly applied to carbonylcompounds.

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UNPOLARISEDLIGHT

POLARISED LIGHT

NATURAL LIGHT

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I. Linear or plane polarized light.

II. Circular polarized light.

III. Elliptically polarized light.

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A polarized light vibrating in a single plane

perpendicular to the direction of propagation

is called plane polarised light. A light wave is characterized by an electric

field vector and a magnetic field vector which are perpendicular to each other and in

phase. The two vectors are also perpendicular

to the direction of propogation.

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Linearly polarized light can be regarded as the resultantof two equal and opposite beams of circularly polarizedlight i.e.., as a combination of left and right circularlypolarized light.

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When vibration are along a ellipse lying in a plane

perpendicular to the direction of propagation the light is called

elliptically polarized light

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The difference in indices of refraction for right

circularly polarized light (RCPL) & left circularly

polarized light (LCPL) is know as circular

birefringence.

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The specific rotationof a chemical compound[] is defined as the observed angle of opticalrotation when plane-polarized light is passedthrough a sample with a path length of 1decimeter and a sample concentration of 1 gramper 1 millilitre

where,

Angle of Rotation in degreeslpath length is in decimetersddensity of liquid is in g 100 ml 1TTemperature.

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A negative value means levorotatory rotation and a

positive value means dextrorotatory rotation.

Some examples: Sucrose +66.47 cholesterol 31.5 Camphor +44.26 Penicilin V +223

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FARADAY EFFECT :-

Any liquid or solution, when placed in amagnetic field , rotates the plane of polarized light

because of the effect of magnetic field upon the motion ofelectrons in the molecule.

INDEX OF REFRACTION ():-

The ratio of velocity of ray of light invacuum (c) to its velocity in medium (v) i.e. =C/V

Small differences in the refractive indices for right & left

circularly polarized light cause an appreciable rotation ofthe plane polarized light.

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Optical activity :-

Ability to rotate plane of polarized light .

Optically active compound :-

Certain organic compound when placed in a path of

a plain polarized light, it rotates the plain polarized light

through a certain angle. This property of the compoundto rotate plane polarized light is called optical activity.

For a compound to be optically active, it must be chiral

in nature(Asymmetric character).

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Optical activity is not seen in symmetrical molecules

due to equal rotation in opposite directions getting

cancelled. The isomer that rotates the plane of

polarization to the left is called levo isomer (-) and to the

right is called dextroisomer (+).

To be optically active , a molecule must not possessed

any one of following symmetry elements :

1) Centre of symmetry

2) Plane of symmetry

3) An Proper axis

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The specific rotation [] changes withwavelength is called optical rotatory

dispersion (ORD) .OR

The rate of change of specific rotation with

wavelength is called Optical rotatory

dispersion (ORD) .

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The rate of change of specific rotation withwavelength is known as optical rotary dispersion .

Drude has shown that the specific rotation may be

expressed as a function of wavelength by an equation,[]= k1 + k2 + k3 +-----212 222 223

where - Wavelength of measurement k1,k2 ,k3 constants that can be identified with the

wavelength of maximum absorption of optically activeabsorption bands.

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SpecificRotation

Wavelength

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From graph,

A-Represents the plainpositiveORD curve :

The specific Rotationincreases with decreasing wavelength.

BRepresents the plain negative ORD curve :

Plain implies that there exist no maximum or minimain the curve.

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The combination of circular birefringence and circulardichroism in which the optically active bands are observed ,such phenomenon called as cotton effect.

Optically active bands are absorption bands of thechromophores which are either intrinsically asymmetric orwhich becomes asymmetric because of the interaction withasymmetric environment.

E.g.. 1) Hexahelicene molecule which itself act as

chromophore.

2) Carbonyl group which is symmetric but becomes opticallyactive in an asymmetric environment.

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They are of two types 1) Plain curves

2) Anamolous curves

a) Single cotton effect curves

b) Multiple cotton effect curves

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The curves obtained do not contain any peak and

that curve do not cross the zero rotation line

Such waves are obtained for compounds which

do not have absorption in the wavelength region

where optical activity is being examined

E.g. .compounds exhibiting such plane curves are

alcohols and hydrocarbons

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These curve on the other hand shows a

number of extreme peaks and troughs

depending on the number of absorbing groupsand therefore known as anomolous dispersion

of optical rotation.

This type of curve is obtained for compounds

,which contain an asymmetric carbon atom

and also contains chromophore.

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These are anomolous dispersion curves whichshows maximum and minimum both of themoccurring in the region of maximum absorption.

If an approaching the region of cotton effect fromthe long wavelength ,one passes first throughmaximum (peak) and then a minimum (trough)

,the cotton effect said to be positive . (PositiveCotton effect is where the peak is at a higherwavelength than the trough).

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If the trough is reached first and the peak it is called a

negative cotton effect curves.

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The vertical distance between peak and trough iscalled amplitude a and is convently expressed inhundreds of degrees.

Molecular amplitude, a =2- 1100

Where,

2molar rotation of extreme peak or trough fromlarge wavelength.

1

- molar rotation of extreme peak or trough from

shorter wavelength.

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In this type of curves two or more peaks and

trough are obtained .

E.g. functional group i.e. Ketosteriods ,

Camphor etc exhibits such curves .

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Some materials posses special properties of absorption of the leftcirculary polarised light to different extent than the right circularly

polarised light.This phenomenon is called as circularly dichroism.

The electric field of a light beam causes a linear displacement ofcharge when interacting with a molecule, whereas the magnetic field of it

causes a circulation of charge.

These two motions combined result in a helical displacement when

light impinges on a molecule (both field vectors in the same place are ofthe same direction but at different moments of time).

The magnitude of circular dichroism is expressed by the ellipticiy.

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CIRCULAR DICHROISM

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An ordinary spectrophotometer can be adapted

to measure circular dichroism. It is only

necessary to provide some means of productionr and l circularly polarized radiation.

For this purpose a plane polarized beam can be

passed through a quartz-wave plate. If the plate

is rotated from -45o to +45o , first r and l

circularly polarized light is produced

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APPLICATIONS OF CIRCULAR DICHROISM (CD) SPECTROSCOPY


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( )

PROTEIN CONFORMATION:-

The CD spectrum of a protein can provide a information about the relative amounts ofthe major types of secondary structure within the protein in solution.

CD spectra of the -helix conformation and the random coil of poly-L-amino acidsare distinctly different form each other.

NUCLEIC ACID CONFORMATION:-

The CD spectrum of a single stranded nucleic acid may be calculated fairly accurately

form a knowledge of its nearest neighbor frequency. Thus any differences between the

calculated and measured CD spectrum must be due to variation in structure, such asdouble-strandness.

The CD spectra are frequently used to study changes in the structure of nucleic acids

such as: the los of helicity of single stranded nucleic acids as a function of temperature

or pH structural changes on binding cation and proteins.4 December 2013

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OPTICAL ROTATORY DISPERSION

(ORD)

CIRCUALR DICHROISM

(CD)

ORD is If the refractive indices of thesample for the left and right handed

polarized light are different, when the

components are recombined , the plane

polarized radiation will be rotated through

and angle

Circular dichroism is the differentialabsorption of left and right handed

circularly polarized light

ORD spectra are dispersive CD spectra are absorptive

In ORD the circular polarized light is used

is not converted to elliptical light

In CD the circular polarized lgiht is used

and is converted to elliptical light

ORD graphs are obtained by plotting

specific rotation vs wavelength

CD graphs are obtained by ploting molar

ellipticity vs wavelength.

DIFFERENECES BETWEEN ORD AND CD

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The octant rule was first formulated by Moffittet alfor correlation of the sign of the cottoneffect of chiral cylohexanone derivatives with

their absolute configration.

The space around carbonyl group is dividedinto eight sector about x,y,z axis.

This rule only applies to substitutedcyclohexanone

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It states that atoms lying on the back upper left and backlower right, then octant make positive contribution.

(Positive rotation).

If groups on back upper right and back lower left thenthe octants make negative contribution. (Negative

rotation).

Substituents lying in the co-ordinate planes make no

contribution to the rotatory dispersion.

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Axial and equatorial group

Positivie cotton effect


negativie cotton effect


negative cotton effect

Axial and equatorial group positive

cotton effect

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If an optically active chromophore takes part in a

reaction the extent of the reaction can sometimes be

followed by observing the reduction of the cotton effect.

Eg:- if hydrochloric acid is added to a solution of (+) 3-

methylcyclehexanone in methanol, the cotton effect gets

reduced by 93% because of dimethy ketal formation.In

alcohol only 33% reduction and in isopropyl alcoholthere occurs no reduction.

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Determination of position of keto group and the

confrigration of ring system in a sterodial ketone of

unknown constitution.

Study of conformational changes:

when molecule exist in more than one confirmation insolution, each conformer will have its own ORD or CD

curve and the sign and magnitude of the cotton effect

will change with the change of conformer population,

caused either by a change of solvent polarity or by achange in temperature

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A. Chetwal GR & Anand SK, Instrumental methodof chemical analysis, 5TH edition page no:-2.468-2.481.

B. Kalsi PS, Spectroscopy Of Organic Compounds,New age international limited, sixth edition:611-627.

C. Jag Mohan, Organic Spectroscopy PrinciplesAnd Applications, Narosa publishing hose:482-

496.D. Sharma BK, Instrumental method of chemical analysis,

26THedition:-M-286 to M-307.

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1. What is meant by ORD ? Discuss the principle and

application of ORD .(Nov 98).

2. What is optical rotatory dispersion & how is it useful

in characterization of substances.(Jul 98).3. ORD & Circular Dicroism.(2000).

4. Cotton effect .(2004).

5. Note on the principle & application of ORD &

Circular Dicroism .(2004,Oct 08).

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6. Write an account on fundamental principles of

ORD.(2005).

7. Explain Circular Dicroism.(2006).

8. Describe the instrumentation used for measuring

circular dichroism.(Oct 09).

9. Explain the phenomena of optical rotation & circular

polarization. Giving a schematic diagram, writeabout an ORD instrument. What are the applications

of ORD & CD.(May 10 & 12).

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