optical rotatory dispersion

Presented by, Sujit R. Patel, Dept. Of Pharmaceutics,

M.M.C.P. ,Belgaum.

Fundamentals Of ORD. Optical Rotatory Dispersion.

Cotton Effect.

The ORD Curve. Circular Dichroism.

Octant Rule.

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 with the change in wavelength of light source. i.e. applied only in optically active compounds.

• Optical rotation caused by compound changed with

wavelength of light was first noted by Biot in 1817.

• ORD curves in recent years are made use in structural determination by comparing the curve obtain from compound believed to have related structures particularly applied to carbonyl compounds.

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Natural light is having two components electric component and magnetic component, and both are perpendicular to each other in different planes.

Both these electric component and magnetic components are also perpendicular to the direction of propagation of light.

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UNPOLARISED LIGHT

POLARISED LIGHT

NATURAL LIGHT

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I. Linear or plane polarized light.

II. Circular polarized light.

III. Elliptically polarized light.

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 Ē and a magnetic field vector Ĥ which are perpendicular to each other and in phase. The two vectors are also perpendicular to the direction of propogation.

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 Ē and a magnetic field vector Ĥ which are perpendicular to each other and in phase. The two vectors are also perpendicular to the direction of propogation.

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When vibration of light are along a circle lying in a plane perpendicular to the direction of propagation the light is called circular polarized light.

Represent a wave in which the electrical component & the magnetic component spirals around the direction of propagation (vibration) of the ray, either clockwise (or right handed or dextrorotatory.) OR counterclockwise (or left handed or levorotary).

When vibration of light are along a circle lying in a plane perpendicular to the direction of propagation the light is called circular polarized light.

Represent a wave in which the electrical component & the magnetic component spirals around the direction of propagation (vibration) of the ray, either clockwise (or right handed or dextrorotatory.) OR counterclockwise (or left handed or levorotary).

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

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

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 rotation of a chemical compound [α] is defined as the observed angle of optical rotation α when plane-polarized light is passed through a sample with a path length of 1 decimeter and a sample concentration of 1 gram per 1 millilitre

where, α – Angle of Rotation in degrees l –path length is in decimeters d – density of liquid is in g 100 ⁄ ml 1

T–Temperature.

The specific rotation of a chemical compound [α] is defined as the observed angle of optical rotation α when plane-polarized light is passed through a sample with a path length of 1 decimeter and a sample concentration of 1 gram per 1 millilitre

where, α – Angle of Rotation in degrees l –path length is in decimeters d – density of liquid is in g 100 ⁄ ml 1

T–Temperature.

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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

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

a magnetic field , rotates the plane of polarized light because of the effect of magnetic field upon the motion of electrons in the molecule.

INDEX OF REFRACTION (η) :- The ratio of velocity of ray of light in

vacuum (c) to its velocity in medium (v) i.e. η= C/V

Small differences in the refractive indices for right & left circularly polarized light cause an appreciable rotation of the plane polarized light.

FARADAY EFFECT :-Any liquid or solution, when placed in

a magnetic field , rotates the plane of polarized light because of the effect of magnetic field upon the motion of electrons in the molecule.

INDEX OF REFRACTION (η) :- The ratio of velocity of ray of light in

vacuum (c) to its velocity in medium (v) i.e. η= C/V

Small differences in the refractive indices for right & left circularly polarized light cause an appreciable rotation of the 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 compound to rotate plane polarized light is called optical activity. For a compound to be optically active, it must be chiral in nature(Asymmetric character).

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 compound to 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 dextro isomer (+).

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

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 dextro isomer (+).

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 [α] changes with wavelength is called optical rotatory dispersion (ORD) .

OR

The rate of change of specific rotation with wavelength is called Optical rotatory dispersion (ORD) .

The specific rotation [α] changes with wavelength 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 with wavelength is known as optical rotary dispersion .

Drude has shown that the specific rotation may be expressed as a function of wavelength by an equation,[α] = k1 + k2 + k3 +-----

λ2 – λ12 λ2 – λ2

2 λ2 – λ23

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

wavelength of maximum absorption of optically active absorption bands.

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

Drude has shown that the specific rotation may be expressed as a function of wavelength by an equation,[α] = k1 + k2 + k3 +-----

λ2 – λ12 λ2 – λ2

2 λ2 – λ23

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

wavelength of maximum absorption of optically active absorption bands.

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SpecificRotatio

n α

Wavelength λ

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From graph,A-Represents the plain positive ORD curve : The specific Rotation

increases with decreasing wavelength. B – Represents the plain negative ORD curve :

Plain – implies that there exist no maximum or minima

in the curve.

From graph,A-Represents the plain positive ORD curve : The specific Rotation

increases with decreasing wavelength. B – Represents the plain negative ORD curve :

Plain – implies that there exist no maximum or minima

in the curve.

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

Optically active bands are absorption bands of the chromophores which are either intrinsically asymmetric or which becomes asymmetric because of the interaction with asymmetric environment.

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

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

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

Optically active bands are absorption bands of the chromophores which are either intrinsically asymmetric or which becomes asymmetric because of the interaction with asymmetric environment.

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

2) Carbonyl group which is symmetric but becomes optically active 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

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

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 groups and 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 which shows maximum and minimum both of them occurring in the region of maximum absorption.

If an approaching the region of cotton effect from the long wavelength ,one passes first through maximum (peak) and then a minimum (trough) ,the cotton effect said to be positive . (Positive Cotton effect is where the peak is at a higher wavelength 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’ is called amplitude ‘a’ and is convently expressed in hundreds of degrees.

Molecular amplitude, a =ǿ2 - ǿ1

100Where, ǿ2 – molar rotation of extreme peak or trough from

large wavelength.

ǿ1 - molar rotation of extreme peak or trough from shorter wavelength.

The vertical distance between ‘peak’ and ‘trough’ is called amplitude ‘a’ and is convently expressed in hundreds of degrees.

Molecular amplitude, a =ǿ2 - ǿ1

100Where, ǿ2 – molar rotation of extreme peak or trough from

large wavelength.

ǿ1 - 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 left circulary 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 of charge 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 of the same direction but at different moments of time).

The magnitude of circular dichroism is expressed by the ellipticiy.

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CIRCULAR DICHROISMCIRCULAR DICHROISMː–ː–

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An ordinary spectrophotometer can be adapted to measure circular dichroism. It is only necessary to provide some means of production r 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

An ordinary spectrophotometer can be adapted to measure circular dichroism. It is only necessary to provide some means of production r 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

PROTEIN CONFORMATION:-

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

CD spectra of the α-helix β conformation and the random coil of poly-L-amino acids are 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 as double-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.

APPLICATIONS OF CIRCULAR DICHROISM (CD) SPECTROSCOPY

PROTEIN CONFORMATION:-

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

CD spectra of the α-helix β conformation and the random coil of poly-L-amino acids are 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 as double-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.

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OPTICAL ROTATORY DISPERSION (ORD)

CIRCUALR DICHROISM (CD)

ORD is If the refractive indices of the sample 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 α

Circular dichroism is the differential absorption 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 DIFFERENECES BETWEEN ORD AND CD

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The octant rule was first formulated by Moffitt et al for correlation of the sign of the cotton effect of chiral cylohexanone derivatives with their absolute configration.

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

This rule only applies to substituted cyclohexanone

The octant rule was first formulated by Moffitt et al for correlation of the sign of the cotton effect of chiral cylohexanone derivatives with their absolute configration.

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

This rule only applies to substituted cyclohexanone

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It states that atoms lying on the back upper left and back lower right, then octant make positive contribution. (Positive rotation).

If groups on back upper right and back lower left then the octants make negative contribution. (Negative rotation).

Substituents lying in the co-ordinate planes make no contribution to the rotatory dispersion.

It states that atoms lying on the back upper left and back lower right, then octant make positive contribution. (Positive rotation).

If groups on back upper right and back lower left then the 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 groupPositivie cotton effect

Axial and equatorial group negativie cotton effect

Axial and equatorial group 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 alcohol there 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 in

solution, 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 a change in temperature

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 in

solution, 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 a change in temperature

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A. Chetwal GR & Anand SK, Instrumental method of 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 Principles And Applications, Narosa publishing hose:482-496.

D. Sharma BK, Instrumental method of chemical analysis,

26TH edition:-M-286 to M-307.

A. Chetwal GR & Anand SK, Instrumental method of 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 Principles And Applications, Narosa publishing hose:482-496.

D. Sharma BK, Instrumental method of chemical analysis,

26TH edition:-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).

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, write about an ORD instrument. What are the applications of ORD & CD.(May 10 & 12).

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, write about an ORD instrument. What are the applications of ORD & CD.(May 10 & 12).

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