circular dichroism (CD) and Optical Rotatry Dispersion (ORD).pptx

CD and ORD is a topic of spectroscopy for BSc and MSc Chemistry and Physics students

1. CIRCULAR DICHROISM (CD) AND OPTICAL ROTATORY DISPERSION (ORD) Dr. Kiran MGC FGS

2. " Dichroism " is used to denote direction -dependent light absorption. or Property of exhibiting different colors by reflected or transmitted light. Linear dichroism refers to the differential absorption of light polarized parallel or perpendicular to the some reference axis. "Birefringence" refers to the direction - dependent index of refraction (two different refractive index).

3. Plane polarized light The E vector of linearly polarized light also called plane-polarized light has a constant direction and a modulated amplitude . By contrast, the E vector of circularly polarized light has a constant amplitude but a modulated direction.

4. Types of polarized light • Plane polarized light consists two circularly polarized components of equal intensity . • Two circularly polarized components are like left and right -handed springs/helices. • As observed by looking at the source, right-handed circularly polarized light rotates clockwise. • Frequency of rotation is related to the frequency of the light. • Can be resolved into its two circularly polarized components. • When added together after passing through an optically isotropic medium, plane polarized light results.

5. . Linearly polarised light is light whose oscillations are confined to a single plane. To really understand circular dichroism , one must first understand the basics of polarisation Vertically Polarised Light All polarised light states can be described as a sum of two linearly polarised states at right angles to each other, usually referenced to the viewer as vertically and horizontally polarised light. Horizontally Polarised Light

6. 45 Degree Polarised Light If for instance we take horizontally and vertically polarised light waves of equal amplitude that are in phase with each other, the resultant light wave ( blue ) is linearly polarised at 45 degrees .

7. The optical element that converts between linearly polarised light and circularly polarised light is termed a quarter‐wave plate. A quarter‐wave plate is birefringent , i.e. the refractive indices seen by horizontally and vertically polarised light are different . A suitably oriented plate will convert linearly polarised light into circularly polarised light by slowing one of the linear components of the beam with respect to the other so that they are one quarter‐wave out of phase . This will produce a beam of either left‐ or right‐CPL (circularly polarised light).

8. Right Circularly Polarised (RCP) Light Left Circularly Polarised (LCP) Light The difference in absorbance of left‐hand and right‐hand circularly polarised light is the basis of circular dichroism . A molecule that absorbs LCP and RCP differently is optically active , or chiral .

9. W hat are chiral molecules? Chiral molecules exist as pairs of mirror‐image isomers. These mirror image isomers are not super‐imposable and are known as enantiomers . The physical and chemical properties of a pair of enantiomers are identical with two exceptions: the way that they interact with polarised light and the way that they interact with other chiral molecules .

10. Circular birefringence and optical rotation Chiral molecules exhibit circular birefringence, which means that a solution of a chiral substance presents an anisotropic medium through which left circularly polarised (L‐CPL) and right circularly polarised (R‐CPL) propagate at different speeds.

11. A linearly polarised wave can be thought of as the resultant of the superposition of two circularly polarised waves , one left circularly polarised , the other right circularly polarised .

12. On traversing the circularly birefringent medium, the phase relationship between the circularly polarised waves changes and the resultant linearly polarised wave rotates.

13. This is the origin of the phenomenon known as optical rotation, which is measured using a polarimeter . Measuring optical rotation as a function of wavelength is termed optical rotatory dispersion (ORD) spectroscopy.

14. Circular birefringence - the orange cuboid represents the sample Circular Birefringence " two different refractive index

15. Circular dichroism - the orange cuboid represents the sample Circular dichroism : Measurement of unequal absorption of light i.e R or L

16. Although ORD spectra and CD spectra can theoretically provide equivalent information, each technique has been used for very distinct applications . Optical rotation at a single wavelength is used as a general measurement tool for chiral molecules, to determine concentration and as a determinant of chiral purity compared to a known standard. The simplicity and low‐cost of the experiment and instrumentation makes it ideal for this application .

17. Circular dichroism spectra on the other hand are better spectrally resolved than ORD spectra, and consequently more suitable for advanced spectral analysis.

18. Circular dichroism (CD) is the difference in the absorption of left‐handed circularly polarised light (LCPL) and right‐handed circularly polarised light (RCPL) and occurs when a molecule contains one or more chiral chromophores (light‐absorbing groups). Circular dichroism ΔA(λ) = A(λ)LCPL ‐ A(λ)RCPL where λ is the wavelength .

19. Circular dichroism (CD) spectroscopy is a spectroscopic technique where the CD of molecules is measured over a range of wavelengths. CD spectroscopy is used extensively to study chiral molecules of all types and sizes, but it is in the study of large biological molecules where it finds its most important applications.

20. A primary use is in analyzing the secondary structure or conformation of macromolecules, particularly proteins as secondary structure is sensitive to its environment, temperature or pH, circular dichroism can be used to observe how secondary structure changes with environmental conditions or on interaction with other molecules. Structural, kinetic and thermodynamic information about macromolecules can be derived from circular dichroism spectroscopy.

21. Measurements carried out in the visible and ultra‐violet region of the electro‐magnetic spectrum monitor electronic transitions, and, if the molecule under study contains chiral chromophores then one CPL state will be absorbed to a greater extent than the other and the CD signal over the corresponding wavelengths will be non‐zero.

22. A circular dichroism signal can be positive or negative , depending on whether LCPL is absorbed to a greater extent (90%) than RCPL (CD signal positive ) or to a lesser extent (88%) (CD signal negative ). CD varies as a function of wavelength , and that a CD spectrum may exhibit both positive and negative peaks.

23. Circular dichroism spectra are measured using a circular dichroism spectrometer , such as the Chirascan , which is a highly specialised derivative of an ordinary absorption spectrometer. CD spectrometers measure alternately the absorption of L and R CPL , usually at a frequency of 50 kHz , and then calculate the circular dichroism signal.

24. C hemistry of CD and ORD Linear polarized light can be viewed as a superposition of opposite circular polarized light of equal amplitude and phase. A projection of the combined amplitudes perpendicular to the propagation direction thus yields a line . When this light passes through an optically active sample with a different  absorbance  A  for the two components, the amplitude of the stronger absorbed component will smaller than that of the less absorbed component . The consequence is that a projection of the resulting amplitude now yields an ellipse instead of the usual line. Note that the polarization direction has not changed. The occurrence of  ellipticity  is called  Circular Dichroism  - it is not the same as optical rotation.

25. Rotation of the polarization plane (or the axes of the dichroic ellipse) by a small angle ’ a  ’occurs when the phases for the two circular components become different, which requires a difference in the refractive index ’ n’ . This effect is called circular birefringence . It can be shown that when CD exists,  optical rotation  must exist as well,. The change of optical rotation with wavelength is called optical rotary dispersion,  ORD .

26. (a) Linear polarized light can be viewed as a superposition of opposite circular polarized light of equal amplitude and phase. (b) different absorption of the left- and right hand polarized component leads to ellipticity (CD) and optical rotation (OR). The actual effect is minute and using actual numbers the ellipse would still resemble a line. (a) (b)

27. Optical Rotatory Dispersion (ORD) Spectroscopy • The refractive indices for R and L varies as a function of wavelength , as does the difference between them. •One can measure Δn by measuring the rotation of linearly polarized light.

28. 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 the cotton effect . And obtain curve known as cotton effect curve. It gives two types of absorption curve that is positive or negative cotton effect curve.

29. Cotton effect • Positive Cotton effect is where the crest peak is at a higher wavelength than the trough • Negative Cotton effect is the opposite. • Optically pure enantiomerism always display opposite Cotton effect ORD curves of identical magnitude. • Zero crossover point between the peak and the trough closely corresponds to the normal UV λ max.

30. Octant Rule : One of the important uses of CD and ORD is to determine the absolute configuration of ketones and aldehydes . This is accomplished by application of a semi-empirical generalization known as the octant rule. The rule predicts the sign of the n → π * Cotton effect of a ketone due to the contributions of substituents in each of eight octant sectors surrounding the carbonyl chromophore . The rule allows one to determine the absolute configuration and also the likely conformation of a molecule from the sign of the Cotton effect.

31. Octant rule is the one which is used to correlate the sign of the cotton effect. One eighth of a circle . i.e. Quadrant. Robert Hooke invented the octant in 1684 There are 45 degrees in an octant because an octant is 1 eighth of a circle , and a circle is 360 degrees, and 45 degrees is one eighth of 360. Since a circle has 360 degrees you would divide this number by 8 (eight). That may be a big part of the reason why they're called "Octants"

32. This is very useful empirical rule in predicting the sign and magnitude of the cotton effect . This rule applies only to the substituted cyclohexanones . The cyclohexanone molecule is divided into eight octants by three planes A, B and C shown in figure. Plane A passes carbon atoms 1 and 4 ; the substituents attached to carbon atoms 4 thus lie in this plane. Plane B encompasses carbon atoms 1, L2 and R2 where L and R denotes left and right from the observer’s point of view. The substituents in the equatorial positions at L2 and R2 are practically in this plane.

33. Plane C bisects the carbonyl group and is perpendicular to plane A and B . thus plane A and B produce four octants and plane C produce four more . The midpoint of the C = O bond is chosen to be the origin of the coordinate system. The octant rule may be stated that substituents in the lower left and far upper right octants make a negative contribution to the cotton effect , substituents in the far lower right and far upper left make a positive contribution; substituents in any of the three planes do not make contribution.

34. As rarely do substituents bend over the carbonyl group towards the oxygen atom beyond, the four octants in the front of plane C are usually vacant and will not concern us. Thus we shall consider only the four octants defined by planes A and B . as a simple illustration of this rule let us consider 3-methylcyclohexanone molecule.

35. THANKS

36. Physics of CD and ORD Linear polarized light can be viewed as a superposition of opposite circular polarized light of equal amplitude and phase. A projection of the combined amplitudes perpendicular to the propagation direction thus yields a line. When this light passes through an optically active sample with an absorbance A , the amplitude of the stronger absorbed component will be smaller than that of the less absorbed component. The consequence is that a projection of the resulting amplitude now yields an ellipse instead of the usual line. The occurrence of ellipticity is called Circular Dichroism . Rotation of the polarization plane (or the axes of the dichroic ellipse) by a small angle a occurs when the phases for the 2 circular components become different, which requires a difference in the refractive index n . The change of optical rotation with wavelength is called optical rotary dispersion, ORD .

37. Octant Rule One of the important uses of CD and ORD is to determine the absolute configuration of ketones and aldehydes . This is accomplished by application of a semi-empirical generalization known as the octant rule . The rule predicts the sign of the n → π * Cotton effect of a ketone due to the contributions of substituents in each of eight octant sectors surrounding the carbonyl chromophore . The rule allows one to determine the absolute configuration and also the likely conformation of a molecule from the sign of the Cotton effect