Polarimetry, circular dichroism and optical rotatory dispersion principles - One Line Questions

1. If the solution in the previous question were a racemic mixture, what would be the observed rotation? — 0 degrees
2. A solution of a pure enantiomer is found to have an observed rotation of +15 degrees in a polarimeter tube of 1 dm length, using a sodium lamp at 25°C. If the concentration is 1 g/mL, what is its specific rotation? — +15 degrees mL / (g dm)
3. The specific rotation of a compound is usually measured under standard conditions. What is a typical standard temperature and wavelength used? — 25°C and 589 nm (Sodium D line)
4. In CD spectroscopy, the signal is generated by transitions within chiral chromophores. Which of these is NOT typically considered a chiral chromophore in this context? — A simple alkene without any chiral influence
5. CD spectroscopy can be used to monitor conformational changes in biomolecules. A change in the CD spectrum often signifies: — A change in the three-dimensional structure
6. A chiral molecule lacking a chiral center but exhibiting optical activity is known as: — An allene or atropisomer
7. A sample exhibits strong positive CD signals in the far-UV region (around 190-250 nm). This is often characteristic of: — An alpha-helix structure in a protein
8. What does a positive Cotton effect in CD spectroscopy typically indicate? — A specific absolute configuration of a chromophore
9. Circular Dichroism (CD) spectroscopy measures the difference in absorption of left-handed circularly polarized light and right-handed circularly polarized light. This difference is represented as: — Molar ellipticity (Δε)
10. In CD spectroscopy, what type of molecular structure is primarily responsible for generating a CD signal? — Chiral centers or chiral environments
11. Which technique is most sensitive for determining the absolute configuration of a chiral molecule with a suitable chromophore? — Circular Dichroism Spectroscopy
12. What is the primary advantage of using CD spectroscopy over polarimetry for analyzing complex biological molecules like proteins? — CD provides information about secondary structure (alpha-helix, beta-sheet) which polarimetry does not
13. Optical Rotatory Dispersion (ORD) is a plot of specific rotation versus: — Wavelength
14. The specific rotation ([α]) of a chiral compound is dependent on which of the following factors? — Concentration of the solution, wavelength of light, and temperature
15. What is the unit of specific rotation in polarimetry? — Degrees mL / (g dm)
16. What is the primary application of ORD in organic chemistry? — Determining the absolute configuration of chiral molecules, especially those with carbonyl groups
17. What is the term for a mixture containing equal amounts of two enantiomers? — Racemic mixture
18. ORD spectra typically show a characteristic pattern near absorption bands of chiral chromophores. This pattern is called a: — Cotton effect
19. The 'plain curve' in ORD refers to the dispersion observed: — In the absence of any absorption bands
20. Which type of light source is typically used in a polarimeter for accurate measurements? — Monochromatic light source (e.g., sodium lamp)
21. Which of the following is a key advantage of CD spectroscopy for studying biomolecules? — It provides information about the secondary and tertiary structure of proteins.
22. Which of the following is a limitation of using polarimetry for quantitative analysis? — It is sensitive to impurities that are also optically active
23. A solution shows no optical activity. What can be concluded about the sample? — It is a racemic mixture or an achiral compound
24. How does the presence of a chiral chromophore affect CD spectra? — It causes differential absorption of left and right circularly polarized light
25. Which of the following statements about Circular Dichroism (CD) is true? — It requires the presence of a chromophore within the chiral molecule.
26. Which of the following statements about polarimetry is INCORRECT? — It can distinguish between enantiomers and diastereomers.
27. Optical Rotatory Dispersion (ORD) relates the optical rotation of a chiral compound to: — The wavelength of the incident light
28. A polarimeter measures the angle of rotation (α) over a path length (l) and concentration (c). The observed rotation is directly proportional to: — l and c
29. A compound that rotates the plane of polarized light to the right (clockwise) is designated as: — Dextrorotatory (d- or +)
30. Optical Rotatory Dispersion (ORD) is a more detailed analysis than simple polarimetry because it: — Analyzes the rotation at multiple wavelengths
31. The Jasco J-715 is a common instrument used for which spectroscopic technique? — Circular Dichroism (CD) Spectroscopy
32. What is the term for the phenomenon where a chiral molecule absorbs left and right circularly polarized light to different extents? — Circular Dichroism
33. The molar absorptivity difference (Δε) in CD spectroscopy is directly proportional to the differential absorption (ΔA) and inversely proportional to: — Path length (l) and concentration (c)
34. How does CD spectroscopy differ fundamentally from polarimetry? — Polarimetry measures rotation of plane-polarized light, CD measures differential absorption of circularly polarized light
35. A chiral molecule that has a plane of symmetry and is therefore optically inactive is called a: — Meso compound
36. CD spectroscopy is particularly useful for studying the conformation of which type of biomolecules? — Proteins and nucleic acids
37. What is the primary principle behind Optical Rotatory Dispersion (ORD)? — The rotation of plane-polarized light by a chiral substance
38. In the context of polarimetry, the 'dark field' is observed when: — The analyzer is set at 90 degrees to the polariser
39. What does the term 'ellipticity' refer to in CD spectroscopy? — The degree to which circularly polarized light is converted into elliptically polarized light
40. What is the primary phenomenon measured by a polarimeter? — The rotation of the plane of polarized light by a chiral substance
41. What is the 'anomalous dispersion' observed in ORD? — The dispersion pattern near absorption bands of chiral chromophores
42. The 'zero crossing' in an ORD spectrum near an absorption band is often indicative of: — The absolute configuration of the chiral center
43. The phenomenon of optical isomerism is a consequence of: — Molecular chirality and the absence of a plane of symmetry
44. ORD and CD are related techniques. What is the relationship described by the Kramers-Kronig relations? — The real part of the optical activity (ORD) can be derived from the imaginary part (CD), and vice versa
45. The observation that (+) lactic acid is dextrorotatory and (-) lactic acid is levorotatory indicates that: — They are enantiomers
46. What is the function of the polariser in a polarimeter? — To convert ordinary light into plane-polarized light
47. What is the role of the analyzer in a polarimeter? — To determine the angle of rotation of polarized light
48. Which of the following is NOT a factor affecting the observed optical rotation? — Pressure of the surrounding atmosphere
49. In CD spectroscopy, the molar ellipticity (θ) is calculated using the Beer-Lambert law principle, but it accounts for the differential absorption. What is its relationship with the differential absorbance (ΔA)? — θ is directly proportional to ΔA
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