ord

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Optical Rotatory Dispersion

• 
  
• 
  Presented by,
• 
  Sujit R. Patel,
• 
  Dept. Of Pharmaceutics,
• 
  M.M.C.P. ,Belgaum.

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Content

• Fundamentals Of ORD.
• 
  
• Optical Rotatory Dispersion.
• Cotton Effect.
• The ORD Curve.
• Circular Dichroism.
• Octant Rule.

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INTRODUCTION-

• 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?

• 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
NATURAL LIGHT
POLARISED LIGHT
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Types of polarized light?

1. Linear or plane polarized light.
2. Circular polarized light.
3. Elliptically polarized light.

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Plane polarized light?

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

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Circularly polarized light-

• 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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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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Circularly polarized light?
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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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Eliptical Polarised Light-
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Circular Birefringence?

• 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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In practice we measure the specific rotation
a

• The specific rotation of a chemical compound a
  is defined as the observed angle of optical
  rotation a 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,
• a Angle of Rotation in degrees
• l path length is in decimeters
• d density of liquid is in g 100 / ml 1
• TTemperature.

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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 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 (?) -
• The ratio of
  velocity of ray of light in vacuum (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 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).

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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

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OPTICAL ROTATRY DISPERSION?

• The specific rotation a 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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OPTICAL ROTATRY DISPERSION?

• 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,
• a k1 k2
  k3 -----
• ?2 ?12 ?2 ?22
  ?2 ?23
• where
• ?- 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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INSTRUMENTATION FOR ORD-
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The ORD curve?
Specific Rotation a
Wavelength ?
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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.

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COTTON EFFECT?

• 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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Types Of ORD 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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Plain curves (Normal smooth curves or single
curves )

• 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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Anamolous curves?

• 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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Single cotton effect curves?

• 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 ?2 - ?1
• 100
• Where,
• ?2 molar rotation of extreme peak or trough
  from large wavelength.
• ?1 - molar rotation of extreme peak or trough
  from shorter wavelength.

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Multiple cotton effect curves?

• 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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Circular Dichroism?

• 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 DICHROISM?
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Instrument used in CD?

• 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 a-helix ß
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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DIFFERENECES BETWEEN ORD
AND CD
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 a 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.
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Octant Rule

• 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.

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Axial and equatorial group Positivie 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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EQUILIBRIUM STUDIES?

• 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

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References

• Chetwal GR Anand SK, Instrumental method of
  chemical analysis, 5TH edition page no-
  2.468-2.481.
• Kalsi PS, Spectroscopy Of Organic Compounds, New
  age international limited, sixth edition611-627.
• Jag Mohan, Organic Spectroscopy Principles And
  Applications, Narosa publishing hose482-496.
  
  
• D. Sharma BK, Instrumental method of chemical
  analysis,
• 26TH edition-M-286 to M-307.

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IMPORTANT QUESTIONS?

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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Cont.

• 6. Write an account on fundamental principles of
  ORD.(2005).
• Explain Circular Dicroism.(2006).
• Describe the instrumentation used for measuring
  circular dichroism.(Oct 09).
• 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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THANK YOU