TRB PG Assistant - Polarimetry, circular dichroism and optical rotatory dispersion principles Question Bank with Options

1. The molar absorptivity difference (Δε) in CD spectroscopy is directly proportional to the differential absorption (ΔA) and inversely proportional to:

A) Path length (l) and concentration (c)

B) Path length (l) only

C) Concentration (c) only

D) The wavelength (λ)

2. Which of the following is a key advantage of CD spectroscopy for studying biomolecules?

A) It can determine the precise 3D coordinates of all atoms.

B) It provides information about the secondary and tertiary structure of proteins.

C) It is effective for all types of organic molecules, regardless of chromophores.

D) It is insensitive to solvent effects.

3. A chiral molecule that has a plane of symmetry and is therefore optically inactive is called a:

A) Racemic mixture

B) Enantiomer

C) Meso compound

D) Diastereomer

4. What is the primary principle behind Optical Rotatory Dispersion (ORD)?

A) The absorption of linearly polarized light

B) The difference in refractive index for left and right circularly polarized light

C) The rotation of plane-polarized light by a chiral substance

D) The scattering of light by chiral molecules

5. Optical Rotatory Dispersion (ORD) is a more detailed analysis than simple polarimetry because it:

A) Measures the intensity of light directly

B) Analyzes the rotation at multiple wavelengths

C) Only works for achiral compounds

D) Provides information about the molecular weight

6. Which of the following statements about Circular Dichroism (CD) is true?

A) It measures the difference in refractive index for left and right circularly polarized light.

B) It is primarily used to determine boiling points of chiral compounds.

C) It requires the presence of a chromophore within the chiral molecule.

D) It is identical to polarimetry but uses circularly polarized light.

7. In the context of polarimetry, the 'dark field' is observed when:

A) The analyzer is set at the same angle as the polariser

B) The analyzer is set at 90 degrees to the polariser

C) The analyzer is set to allow maximum light transmission

D) The light source is turned off

8. The specific rotation of a compound is usually measured under standard conditions. What is a typical standard temperature and wavelength used?

A) 25°C and 589 nm (Sodium D line)

B) 0°C and 400 nm

C) 100°C and 700 nm

D) Room temperature and 365 nm

9. CD spectroscopy can be used to monitor conformational changes in biomolecules. A change in the CD spectrum often signifies:

A) A change in molecular weight

B) A change in the overall charge

C) A change in the three-dimensional structure

D) A change in the elemental composition

10. What is the term for a mixture containing equal amounts of two enantiomers?

A) Diastereomeric mixture

B) Racemic mixture

C) Meso compound

D) Chiral aggregate

11. The observation that (+) lactic acid is dextrorotatory and (-) lactic acid is levorotatory indicates that:

A) They are identical compounds

B) They are enantiomers

C) They are diastereomers

D) One is achiral

12. A sample exhibits strong positive CD signals in the far-UV region (around 190-250 nm). This is often characteristic of:

A) A random coil structure in a protein

B) A beta-sheet structure in a protein

C) An alpha-helix structure in a protein

D) A disordered polypeptide chain

13. Optical Rotatory Dispersion (ORD) is a plot of specific rotation versus:

A) Concentration

B) Temperature

C) Wavelength

D) Path length

14. What is the function of the polariser in a polarimeter?

A) To detect the angle of rotation

B) To convert ordinary light into plane-polarized light

C) To increase the intensity of light

D) To analyze the wavelength of light

15. 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) A carbonyl group in a chiral environment

B) A disulfide bond in a protein

C) An aromatic ring attached to a chiral center

D) A simple alkene without any chiral influence

16. Which of the following statements about polarimetry is INCORRECT?

A) It requires the sample to be optically active.

B) It measures the angle of rotation of plane-polarized light.

C) It can distinguish between enantiomers and diastereomers.

D) It is a quantitative technique.

17. The 'zero crossing' in an ORD spectrum near an absorption band is often indicative of:

A) The presence of an achiral chromophore

B) The absolute configuration of the chiral center

C) The concentration of the sample

D) The solvent used

18. What is the primary advantage of using CD spectroscopy over polarimetry for analyzing complex biological molecules like proteins?

A) CD can analyze samples in the solid state

B) CD provides information about secondary structure (alpha-helix, beta-sheet) which polarimetry does not

C) CD is much simpler to operate

D) CD does not require a chromophore

19. A chiral molecule lacking a chiral center but exhibiting optical activity is known as:

A) A meso compound

B) An allene or atropisomer

C) A racemic compound

D) An achiral molecule

20. ORD and CD are related techniques. What is the relationship described by the Kramers-Kronig relations?

A) The specific rotation is equal to the molar ellipticity

B) The real part of the optical activity (ORD) can be derived from the imaginary part (CD), and vice versa

C) Both techniques measure the same phenomenon

D) They are completely independent spectroscopic methods

21. What does the term 'ellipticity' refer to in CD spectroscopy?

A) The angle of rotation of plane-polarized light

B) The degree to which circularly polarized light is converted into elliptically polarized light

C) The intensity of light absorbed

D) The wavelength at which maximum absorption occurs

22. The Jasco J-715 is a common instrument used for which spectroscopic technique?

A) Nuclear Magnetic Resonance (NMR) Spectroscopy

B) Mass Spectrometry (MS)

C) Circular Dichroism (CD) Spectroscopy

D) Electron Paramagnetic Resonance (EPR) Spectroscopy

23. Which of the following is NOT a factor affecting the observed optical rotation?

A) Wavelength of light

B) Temperature

C) Concentration of the solution

D) Pressure of the surrounding atmosphere

24. What is the term for the phenomenon where a chiral molecule absorbs left and right circularly polarized light to different extents?

A) Optical Rotation

B) Circular Birefringence

C) Circular Dichroism

D) Optical Dispersion

25. If the solution in the previous question were a racemic mixture, what would be the observed rotation?

A) +15 degrees

B) -15 degrees

C) 0 degrees

D) Cannot be determined

26. 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?

A) +15 degrees mL / (g dm)

B) -15 degrees mL / (g dm)

C) 1.5 degrees mL / (g dm)

D) Cannot be determined without solvent information

27. What is the primary application of ORD in organic chemistry?

A) Determining the exact mass of a molecule

B) Determining the absolute configuration of chiral molecules, especially those with carbonyl groups

C) Measuring the rate of a chemical reaction

D) Identifying functional groups based on vibrational frequencies

28. How does the presence of a chiral chromophore affect CD spectra?

A) It leads to absorption of all wavelengths equally

B) It causes differential absorption of left and right circularly polarized light

C) It increases the overall absorbance without any differential effect

D) It causes the sample to become achiral

29. The phenomenon of optical isomerism is a consequence of:

A) The presence of lone pairs of electrons

B) The ability to form hydrogen bonds

C) Molecular chirality and the absence of a plane of symmetry

D) The high electronegativity of certain atoms

30. 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)?

A) θ is directly proportional to ΔA

B) θ is inversely proportional to ΔA

C) θ is independent of ΔA

D) θ is proportional to the square root of ΔA

31. Which of the following is a limitation of using polarimetry for quantitative analysis?

A) It can only be used for liquids

B) It is sensitive to impurities that are also optically active

C) It requires very high concentrations of the analyte

D) It cannot distinguish between enantiomers

32. A polarimeter measures the angle of rotation (α) over a path length (l) and concentration (c). The observed rotation is directly proportional to:

A) l and c

B) l but inversely proportional to c

C) c but inversely proportional to l

D) Neither l nor c

33. What is the 'anomalous dispersion' observed in ORD?

A) The dispersion pattern far from absorption bands

B) The dispersion pattern near absorption bands of chiral chromophores

C) The dispersion of achiral molecules

D) The dispersion of racemic mixtures

34. The 'plain curve' in ORD refers to the dispersion observed:

A) In the absence of any absorption bands

B) Only at very low wavelengths

C) Only at very high wavelengths

D) Near the absorption maximum

35. Which technique is most sensitive for determining the absolute configuration of a chiral molecule with a suitable chromophore?

A) Basic polarimetry

B) UV-Vis Spectroscopy

C) Infrared Spectroscopy

D) Circular Dichroism Spectroscopy

36. How does CD spectroscopy differ fundamentally from polarimetry?

A) Polarimetry measures rotation of plane-polarized light, CD measures differential absorption of circularly polarized light

B) Polarimetry uses monochromatic light, CD uses white light

C) Polarimetry requires chiral samples, CD can analyze achiral samples

D) Polarimetry measures intensity, CD measures wavelength

37. ORD spectra typically show a characteristic pattern near absorption bands of chiral chromophores. This pattern is called a:

A) Gaussian peak

B) Lorentzian peak

C) Cotton effect

D) Hyperchromic shift

38. Optical Rotatory Dispersion (ORD) relates the optical rotation of a chiral compound to:

A) Its molar mass

B) The wavelength of the incident light

C) Its solubility in water

D) Its boiling point

39. What does a positive Cotton effect in CD spectroscopy typically indicate?

A) A specific absolute configuration of a chromophore

B) The absence of chirality

C) A racemic mixture

D) Low concentration of the sample

40. CD spectroscopy is particularly useful for studying the conformation of which type of biomolecules?

A) Simple sugars

B) Fatty acids

C) Proteins and nucleic acids

D) Amino acids

41. In CD spectroscopy, what type of molecular structure is primarily responsible for generating a CD signal?

A) Aromatic rings

B) Carbonyl groups

C) Chiral centers or chiral environments

D) Alkenes

42. 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:

A) Absorbance (A)

B) Transmittance (T)

C) Molar ellipticity (Δε)

D) Specific rotation ([α])

43. What is the role of the analyzer in a polarimeter?

A) To generate polarized light

B) To measure the intensity of light

C) To block light that has been rotated

D) To determine the angle of rotation of polarized light

44. A solution shows no optical activity. What can be concluded about the sample?

A) It is a pure chiral compound

B) It is a racemic mixture or an achiral compound

C) It must be an optically pure enantiomer

D) It has a very high concentration

45. Which type of light source is typically used in a polarimeter for accurate measurements?

A) Incandescent bulb

B) Laser pointer

C) Monochromatic light source (e.g., sodium lamp)

D) UV-Vis lamp

46. The specific rotation ([α]) of a chiral compound is dependent on which of the following factors?

A) Concentration of the solution, wavelength of light, and temperature

B) Pressure of the atmosphere and humidity

C) Color of the solvent used

D) Molecular weight of the solvent only

47. What is the unit of specific rotation in polarimetry?

A) Degrees

B) Radians

C) Degrees mL / (g dm)

D) Moles per liter

48. A compound that rotates the plane of polarized light to the right (clockwise) is designated as:

A) Levorotatory (l- or -)

B) Dextrorotatory (d- or +)

C) Mesomeric (meso)

D) Racemic (dl- or ±)

49. What is the primary phenomenon measured by a polarimeter?

A) The change in refractive index of a solution

B) The rotation of the plane of polarized light by a chiral substance

C) The absorption of light at specific wavelengths

D) The scattering of light by colloidal particles

Polarimetry, circular dichroism and optical rotatory dispersion principles - Question Bank