The document discusses optical rotatory dispersion and circular dichroism. It defines optical rotatory dispersion as the rate of change of specific rotation with a change in wavelength, which is used for structural determination of carbonyl compounds. Circular dichroism is the phenomenon where left and right circularly polarized light are absorbed to different extents, resulting in elliptically polarized light. Cotton effects arise from the combination of circular birefringence and dichroism near absorption bands, appearing as anomalous dispersion curves with peaks and troughs.

1. OPTICAL ROTATORY DISPERSION AND CIRCULAR
DICHROISM
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2. OPTICAL ROTATORY DISPERSION
 It can be defined as the rate of change of specific rotation with change in
wavelength.
 It is used for the structural determination of carbonyl compounds.
 Measuring optical rotation as a function of wavelength is termed Optical rotatory
dispersion (ORD) spectroscopy.
 It can be measured by polarimeter.
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3. FUNDAMENTALS OF ORD
 Plane polarized light.
 Optical activity.
 Specific rotation.
 Circular Birefringence.
 Optical Rotation.
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4. PLANE POLARISED LIGHT
 Light from ordinary lamp consists of waves vibrating in many different planes.
 When it is passed through polaroid lens, it is found to vibrate in one plane and is said
to be plane polarised light or polarised light.
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5. OPTICAL ACTIVITY
 The compounds which are having the ability to rotate the plane of polarised light
are called optically active compounds.
 This property of compound is called optical activity.
 It is measured by polarimeter.
 Compound which rotates plane of polarised light to right (clock wise) is called
DEXTROROTATORY & is denoted by (+) sign.
 If the compound rotates plane of polarised light towards left side (anti clock wise) is
called LEVOROTATORY & is denoted by (-) sign.
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6.  Enantiomers are optically active compounds.
 Optically active molecules have different refractive indices, and different extinction
coefficients for L and R circularly polarised light.
 For a compound to be optically active it must be devoid of the following properties
1. Plane of symmetry (σ)
2. Center of symmetry (i)
3. Alternating rotation – reflection axis of symmetry or an improper axis (s)
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7. PLANE OF SYMMETRY
 It is the plane, which divides a molecule such that one half forms the mirror images
of other.
 Ex: Meso-tartaric acid.
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8. CENTER OF SYMMETRY
 It is the point in the compound from which a line drawn from one side & extended
equally in the opposite side reaches the same group.
 Ex: Di-keto dimethyl piperazine.
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9. ALTERNATING ROTATION – REFLECTION AXIS OF SYMMETRY OR AN
IMPROPER AXIS (S)
 A molecule possess n-fold-alternating axis of symmetry, if then rotated through an
angle 3600/n about the axis followed by a reflection in a plane perpendicular to the
axis, the molecule is same as original one.
 Eg. 1, 2, 3, 4 – tetramethyl cyclobutane.
 The isomer that rotates the plane polarized light to the left is called levo-isomer (-) &
to the right is called as dextro-isomer (+)
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10. ROTATION OF PLANE POLARISED LIGHT (FRESNEL’S EXPLANATION)
 According to Fresnel, a plane polarized light may be considered as the combination
of two circularly polarized light of which one is right circularly polarized light (RCPL)
& other is left circularly polarized light (LCPL) which are in equal & opposite in
nature.
 RCPL+LCPL= PLANE POLARIZED LIGHT
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11. SPECIFIC ROTATION
 It is defined as the rotation produced by a solution of length 10cm and unit
concentration (1gm/ml) for given wavelength of light at the given temperature.
 It is denoted by α
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12. FACTORS AFFECTING SPECIFIC ROTATION
 Nature of substance
 Length of the column
 Concentration of the solution
 Nature of the solution
 Wavelength of the light used
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13. CIRCULAR BIREFRINGENCE
 If two equal & opposite beams of CPL & PPL passes through an optically active
compound it result in characteristic phenomenon called Circular Birefringence.
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14. OPTICAL ROTATION
 When a plane polarized light (PPL) is passed through optically active compound due
to it’s Circular Birefringence results in unequal rate of propagation of left & right
circularly polarized rays.
 This unequal rate of propagation of both left & right circularly polarized light
deviates the PPL from it’s original direction & it is called as OPTICAL ROTATION.
 It was first noted by Biot in 1817.
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15.  The rotation angle is given by
 Where,
l is the path length traversed by the light
nL and nR are the Refractive Indices for left and right circularly polarized light of
wavelength
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16. CIRCULAR DICHROISM
 Some materials posses special properties of absorption of the left circular polarised
light to different extent than the right circularly polarised light.
 When the component emerges out there is an imbalance in their strength & the
resultant two will not be linearly polarized but elliptically polarized & this
phenomenon is called as CIRCULAR DICHROISM.
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17. COTTON CURVES
 Any medium which is exhibiting circular birefringence may also exhibit circular
dichroism.
 The combination of these two effects in the region in which the optically active
absorption bands are observed gives rise to the phenomenon called cotton effect
and the curves arises.
 There are two types of cotton curves.
1. Plain curve
2. Anomalous curve
 Single cotton effect curve
 Multiple cotton effect curve
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18. PLAIN 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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19. ANOMALOUS CURVE
 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 anomalous
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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20. SINGLE COTTON EFFECT CURVE
 These are anomalous dispersion curves which shows maximum and minimum both
of them occurring in the region of maximum absorption.
 While 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).
 The opposite is called the negative cotton effect.
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22. MULTIPLE COTTON EFFECT CURVE
 In this type of curves two or more peaks and trough are obtained.
 E.g. functional group i.e. Keto-steriods, Camphor, etc., exhibits such curves
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