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Fundamental principles of ORD cotton effect curves, their characteristics and interpretation HARSHA AC DEPT. OF PHARMACEUTICES KRUPANADHI COLLEGE OF PHARMACY

Principles of ORD

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Page 1: Principles of ORD

Fundamental principles of ORDcotton effect curves, their

characteristics and interpretation

HARSHA AC

DEPT. OF PHARMACEUTICES

KRUPANADHI COLLEGE OF PHARMACY

Page 2: Principles of ORD

Polarimeter

• An optical device used to measure the rotation of the plane of vibration of polarized light

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Page 4: Principles of ORD

Optical Rotatory Dispersion (ORD)

• Defined as "the rate of change of specific rotation or Rotatory power with the change in wavelength"

Or

• "The variation of the molecular rotation of optically active substance with the variation in the wavelength of plane polarized light used is called as ORD"

Or

• "It is the measurement of angle of rotation as a function of wavelength"

Page 5: Principles of ORD

Principle Of ORD• According to Fresenel a PPL may be considered as the

combination or two CPL (RCPL &LCPL). Which are equal and opposite in nature, 'When a PPL is passed through an optically active compound, due to its circular birefringence (unequal refractive indices for RCPL and LCPL) results an unequal rate of propagation of (RCPL &LCPL)

• This unequal propagation of both RCPL and LCPL deviates the PPL from its original direction and called to be" optical rotation". In the same way unequal absorption coefficient of the substance for the RCPL and LCPL is also observed (circular dichroism) which changes CPL to optically polarized light

Page 6: Principles of ORD

Instrumentation of ORD

• Polarimeter

• Spectropolarimeter (combination of spectrophotometer and

polarimeter)

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Instrumentation of ORD (Cont.)

• Components of polarimeter: Light source Polarizer· Sample tube Analyzer Detector

Page 8: Principles of ORD

Instrumentation of ORD (Cont.) Light source

• common light source for polarimeter

• sodium vapor lamp and mercury vapor lamps Other light sources are used

SOURCE WAVELENGTH

Xenon arc lamp 260-340nm.

Zirconium lamp 290-300nm.

Sodium Lamp 589-589 .6nm

Mercury lamp 435.8nm, 491.6nm, 546.lnm, 577.0 nm and 579.1 nm.

Page 9: Principles of ORD

Monochromators• used in case of spectropolarimeter. Prisms and gratings are

mostly used as monochromator

• Litrrow monochromators are used for manually operated spectropolarimeter

Polariser• production of plane-polarized radiation

Page 10: Principles of ORD

Types of polariser

Depending on the type of output

• Linear polariser

• Circular polariser

• Elliptical polarizerLinear polariser• In linear polarizer EMR filtered so that its electric

field vectors oscillate in one plane, in which the radiation is propagated. A linearly polarized light may be represented mathematically and graphically as a combination of right and left beams of CPL

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Circular polariser• In circular polarizer EMR filtered so that the tip of the electric

field vector describes a helix, along which axis the radiation is propagated: It can be described by examining the movement of electric field vectors only. The tip of the electric vectors E follows a helical path along surface of cylinder and helix as being pushed out of the light source in the direction of propagation but not rotated.

Elliptical polarizer• It is the most general form and circular polarizer is special

cases of elliptical polarizer. During elliptical polarized light passes through ·a sample in a region where absorption takes place, the incident unpolarised light is converted into elliptically polarized light that is the resultant electric field vector takes an elliptical path.

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Mechanism of action:• Polarizer are based on four fundamental mechanisms

• Selective absorption (Dichroism)

• Reflection

• Scattering

• Birefringence (double refraction)

Page 13: Principles of ORD

Selective absorption (Dichroism)

• Here the polarized light is produced by the used of Polaroid filters which are composed of strongly dichroitic crystal oriented in a plastic material. These components strongly absorb light vibrating in one direction and weakly absorb light vibrating in the perpendicular direction

Page 14: Principles of ORD

Reflection:• Mirrors are used for production of PPL. When the light strikes

the mirror it gets polarized by reflection from a mirror it gets polarized by reflection from a mirror at Brewster's angle i.e.

Tan i= ή

Where

i= angle of incidence

ή = refractive index of mirror material

The only component vibrating perpendicular to the plane of incidence (parallel to mirror surface) will be reflected

Page 15: Principles of ORD

• Birefringence (double refraction):

• It is an optical phenomenon in which a crystal shows different refractive index for the light with the plane of polarization in two perpendicular orientations

• Uniaxial crystals have only one optical axis and two different principle indices of refractions about which atoms are arranged symmetrical

Page 16: Principles of ORD

• A birefrigent crystal divides an entering ray of monochromatic radiation into two rays, which are perpendicular to each other in two different directions. Two types of wavelength expand through a uniaxial crystal.

• Spherical-o- wavelet (Ordinary ray or O-ray)

• Ellipsoidal-E- wavelet (extraordinary ray or E-ray)

Page 17: Principles of ORD

• The O-ray has its vibrations perpendicular to the principle section of the crystal (opposite faces of the crystal). E-ray has its vibration parallel to the principle section. Precisely, the ray traveling along the optical axis is O-ray and that along the principle plane is E-ray.

• Several different varieties of polarizing and analyzing prism are known. They vary in the angels of faces of the prism and of the cut diagonally through the prism

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Different Types of Prisms:

• Nicol Prism.

• Wollaston prism.

• Glan Thomson Prism.

• Nomarski Prism

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• Nicol Prism• It is a type of polarizer, an optical device used to generate a beam of

polarized light. It was the first type of polarizing prism to be invented, in 1828 by William Nicol (1770-1851) It consists of a rhombohedral crystal of calcite that has been cut at a 68° angle, split diagonally, and then joined again using Canada balsam

• Nicol prisms were once widely used in microscopy and polarimetry, and the term "crossed Nicols" (abbreviated as XN) is still used to refer to observation of a sample between orthogonally orientated polarizer

Page 20: Principles of ORD

• Wollaston prism:

• It is an optical device, invented by William Hyde Wollaston, that manipulates polarized light. It separates randomly polarized or Unpolarized light into two orthogonal, linearly polarized outgoing beams

• It consists of two orthogonal calcite prisms, cemented together on their base (typically with Canada balsam) to form two right triangle prisms with perpendicular optic axes. Outgoing light beams diverge from the prism, giving two polarized rays, with the angle of divergence determined by the prisms' wedge angle and the wavelength of the light

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Sample tubes:

• These are cylindrical tubes having plan parallel glass discs at tile ends. The glass 'must be free from stains. The length' of the tube may be determined by measuring the rotation and known strongly rotating liquid usually 5-25 cms in length.

Eg: Nicotine in ethanol

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Analyzer:

• They are used to determine the direction of rotation of plane polarized light. They have same construction as that of polarizer but are rotatable and have vernier scale fixed to it to measure the angle of rotation. A polarizer is fixed to the instrument

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Detectors:

• Eye serves as a detector for most of the polarimeter

• For UV and Visible -Photo multiplier is usually chosen

• For IR thermocouple, bolometer is used

Page 24: Principles of ORD

• Operating procedure:

• Initially no sample is placed in the sample tube. The polarizer produces a PPL when the analyzer is in the same position as that of the polarizer, the light waves from the polarizer passes through the analyzer is rotated at right angles, then no waves passes through it and the field view will be dark.

• The same phenomenon occurs, when an optically active compound in the sample tube rotates the PPL. Hence the analyzer has to be rotated to that particular angle (a) to view for the PPL. This angle of rotation can be measured by vernier scale fixed to the analyzer, which gives the measure of the optical rotation.

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spectropolarimeter is based upon null balance instrument in which null balance reading is achieved in one of the two ways

Some instruments used mechanical null balance device in which the analyzer and polarizer is turned by servomotor

Other instrument uses an electrical Faraday effect in which polarizer and analyzer are at fixed position while the rotation of sample is carried out electrically using Faraday effect

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• Rudolf photoelectric spectropolarimeter:

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• Cary's Spectropolarimeter:

• This is similar to Rudolf instrument, except that in this instrument the mechanical oscillation of the analyzer is replaced by magnetic optical effect. To achieve this, a Faraday cell is placed a head of the analyzer. A motor energized by the amplified current form the photo multiplier moves the polarizer by means of mechanical linkage.

• The polarizer and analyzer of this instrument is double image prism. The wavelength range is 185 - 600 nm, rotation changes 0'.02 - 2, full scale sensitivity is 0.5 mille degree with favorable conditions and sample densities of 2 to 18nm can, be used

Page 28: Principles of ORD

COTTON EFFECT A french physicist A.cotton discovered this effect ,the

combination of circular dichroism and cicrular birefringence known as cotton effect .which may be studied by observing the change of optical with the wavelength so called ORD

The curve obtained by plotting optical rotation v/s wavelength down to above 220 nm using photoelectric spectropolrimeter is known as ORD-curves or cotton curves

Cotton discovered a relation between RP and light absorption in optically active compound. As one approches certain optical activity absorption band in a compound from long wavelength side the rotatory at first increases strongly ,then fall off and changes sign. This is known as cotton effect and the curves describing such effect is called cotton curves

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COTTON EFFECT They are of two types: 1.plain curves 2.anmalous curves

a)Single cotton effect curve

b)Multiple cotton effect curve Plane curves Cotton effects is not seen for compounds ,which is far UV well bellow

220nm because it occurs only near absorption maximum The curve obtained do not contain any peak or inflections and the curve

do not have absorption in the wavelength region where optical activity is being examined. The increase optical activity is directly proportional to the decrease in the wavelength ,so the plot of ф against λ is a plane curve this curve show no minimum i.e. they are smooth

e.g. of the compound s exhibiting such plane curve are alcohol and hydrocarbons.

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Page 31: Principles of ORD

2. Anamolous curves

• These curves shows a number of extreme peaks and troughs depending on the number of absorbing groups and there fore known as anamolous dispersion of optical rotation

• This type of curves is obtained for the compounds which contain an asymmetric carbon atom and also contains chromophore which absorbs near UV region.

Page 32: Principles of ORD

Single cotton effect curves These are anomalous dispersion curves which shows

minimum and maximum both of them occurring in the region of maximum absorption .if in approaching the region of cotton effect from the longer wavelength, one passes first through maximum (peak)and then a minimum (trough),the cotton effect is said to be positive. If the trough is reached first and then the peak it is called a negative cotton effect curve. the vertical distance between the peak and trough is called the amplitude “a” and it is conventionally expressed in hundreds of degrees

Molecular amplitude, a=φ2-φ1/100

Page 33: Principles of ORD

Where,

• φ2=molar rotation of extreme peak or trough from larger wavelength & φ1= molar rotation of peak or trough from shorter wavelength

• b)multiple cotton effect curves:

• in this type of ORD curve two or more peak or trough are obtained .

• e.g. functional groups i.e. ketosteroides ,champhor etc. exhibit such curves.

Page 34: Principles of ORD

Applications of ORD• In the recognition of optical activity• Measurement of the optical rotation of a pure compound may

be used as one of the physical constants & which will also distinguish (+) or (-) isomer

• In qualatative analysis • In ORD techniques can be applied to any optically compound

giving rise to a measurable CE. This includes naturally occuring as well systhetic substance belonging to terpens, steorides, alkaloids, amino acids, protines, carbohydrates, nucleosides and plant hormones

Page 35: Principles of ORD

• In quantitative analysis• Optical rotation may be used in quantative analysis of

optically active compounds

• It is possible to quantify any compound among the mixture or 2 substance at single wavelength where both substance are optically active and when a concentration of one substance is known

• Quantification of an optically active compound from an optically inactive compound can be carried out

Page 36: Principles of ORD

• In structural elucidation:

• Recognition of functional groups• The functional groups which absorb light in the UV/Visible

region give characteristic ORD curves, when attached to an asymmetric molecule even though they arc symmetric by themselves

• Eg: Aryl, xanthate, nitrate, sulfoxides, conjugated double bonds and hetero aromatic rings

• Presence of carbonyl groups and acetonyl groups in a compound can be easily established by ORD curves. When it is found to be very difficult to establish by any other methods.

• Eg: Spectrophotometry, chemical analysis

Page 37: Principles of ORD

• Determination of location and Position of Functional groups in a compound

• It is possible to locate the position of functional groups in a compound by using ORD curves

• Eg: Cholestane-l-.one, Cholestane-2-one. And Cholestane-3-one

Page 38: Principles of ORD

• By taking the ORD curves for above compounds we can easily distinguish cholestane-lone from cholestane-2-one, but it is not possible to distinguish cholestane-2- one and cholestane-3-one, because the curves arc more closely placed and moreover the 'shapes of the curves are similar (but, differences in amplitude).

Page 39: Principles of ORD

• Applications of optical rotatary dispersion -With respect to structure:

• One of the applications of the ORD is a tool in structural elucidation, especially in the location of functional grous

• The structure of a molecule affects the shape of the ORD curve

• As we are able to predict the location of functional groups by ORD curves based on the shape of dispersion curves. One such example is rubijervine; the location of the hydroxyl groups in rubijervine was not fully confirmed. Partial oxidation of the alkaloid, keeping the c-3 hydroxyl group intact, yielded a ketone, the ORD curve of this ketone proved to be typical of the 12-keto steroid type. By this we can conclude that the second hydroxyl group in rubijervine is situated at C-l2 position

Page 40: Principles of ORD

• Addition or deletion of functional groups also affects the shape of the curve and cotton effect

• Ex: introduction of a single methyl group at <2-3or C4 in cholestanone or the introduction of 2 methyl groups at C2 does not change the sign of the cotton effect curve in 3-cholestanone. But introduction of a gem dimethyl group at C4 leads to formation of 4, 4-dimethyl cholestanone and inversion of the CE

Page 41: Principles of ORD

• Applications of ORD with respect to configuration:

• It is the arrangement of atoms or groups in space around an asymmetric carbon", When the standard ORD curves or dispersion curves of known compounds are with us we can assign the exact configuration of unknown compound

• Eg: Degradation product of penta cyclic diterpene Cofestol gave an opposite (inverted) ORD curves to that of 4a-ethyl. 5a-cholestan-3-one, whose configuration is well established. So the written configuration is ascertained for the degradation product (ketone B) in the vicinity of the chromophore

Page 42: Principles of ORD

• A change in the configuration in the vicinity of the chromophore brings the change in the shapes of the ORD curves, Therefore configurational changes in the vicinity of an active chromophore can be identified by ORD curves

• As the structure affects the shape of the dispersion curve, so does the configuration in the vicinity of the chromophore

• The cis and trans androstan-17β-ol-3-ones. The curves for both cis and trans isomers are shown in the figure and clearly indicates that the position of ketone chromophore and its configuration in the vicinity of ketone group

Page 43: Principles of ORD

• Applications of ORD with respect to conformation:

• Conformation of (+)-cis-l O-methyl-2-decalone. This may exist in either of the two conformations (I and II) that exhibit different contributions (positive or negative) to the cotton effect and hence can be identified

• The application of the CE reveals that the conformation 1 should give the negative CE (ring B being in the negative sector), while the structure II displays a positive cotton effect (ring B being in the positive sector). Actually, it gives a negative CE and so has the conformation l. This is illustrated in fig.

Page 44: Principles of ORD

References • Stereochemistry of carbon compounds-Ernest L.Eliel

• ORD and CD in chemistry and biochemistry – Pierre Crabbe

• Instrumental method of analysis – Galen W.Ewing

• Application of optical activity to stereochemistry determination by M.Legrand and M J Rougler

• Spectroscopy by John C. Lindon

• Organic chemistry – vol. 2 I. L. Finar

• Spectroscopy of organic compounds-P.S.Kalsi