Rotational spectra, vibrational spectra, rotation–vibration spectra of diatomic and linear molecules, Raman spectra - experimental techniques and classical theory of Raman scattering - Question Bank

1. What is the effect of increasing temperature on the intensity of Raman spectra?
A) Stokes line intensity increases, Anti-Stokes line intensity decreases.
B) Stokes line intensity decreases, Anti-Stokes line intensity increases.
C) Both Stokes and Anti-Stokes line intensities decrease.
D) Both Stokes and Anti-Stokes line intensities increase.
2. Which statement best describes the relationship between IR and Raman activity for vibrational modes in linear molecules?
A) Modes that are IR active are also Raman active.
B) Modes that are Raman active are also IR active.
C) IR and Raman activity are often complementary, with some modes active in one but not the other.
D) IR and Raman activity are identical for all vibrational modes.
3. For a linear molecule, a vibrational mode that causes a change in polarizability parallel to the molecular axis will be:
A) IR active and Raman active
B) IR inactive and Raman active
C) IR active and Raman inactive
D) IR inactive and Raman inactive
4. What is the experimental technique used for studying vibrational spectra?
A) Microwave Spectroscopy
B) UV-Vis Spectroscopy
C) Infrared (IR) Spectroscopy
D) X-ray Diffraction
5. The Q branch in rotation–vibration spectra is typically observed for vibrational modes that are:
A) Parallel to the molecular axis in linear molecules
B) Perpendicular to the molecular axis in linear molecules
C) Not involving a change in the dipole moment
D) Involving a change in polarizability only
6. In rotation–vibration spectra of linear molecules, the P and R branches are generally observed when the vibrational mode is:
A) Parallel to the molecular axis
B) Perpendicular to the molecular axis
C) Involved in a bending motion
D) Symmetric
7. Vibrational Raman spectra provide information about:
A) Bond polarizability derivatives
B) Bond lengths
C) Bond strengths
D) Electronic transitions
8. Rayleigh scattering refers to:
A) Inelastic scattering with frequency decrease
B) Inelastic scattering with frequency increase
C) Elastic scattering with no frequency change
D) Scattering due to electronic transitions
9. Which of the following is NOT a type of Raman scattering?
A) Stokes scattering
B) Anti-Stokes scattering
C) Rayleigh scattering
D) Resonance Raman scattering
10. What is the role of the spectrograph in Raman spectroscopy?
A) To generate the excitation light
B) To disperse the scattered light by frequency
C) To amplify the detected signal
D) To cool the sample
11. The frequency of a vibrational transition in a diatomic molecule is approximately given by:
A) ω = sqrt(k/μ)
B) ω = sqrt(μ/k)
C) ω = sqrt(kμ)
D) ω = k/μ
12. How does centrifugal distortion affect the rotational energy levels?
A) It makes the energy levels more closely spaced
B) It makes the energy levels less closely spaced
C) It causes the energy levels to become degenerate
D) It has no effect on energy levels
13. What does the term 'centrifugal distortion' refer to in non-rigid rotor models?
A) The tendency of bonds to lengthen at higher rotational speeds
B) The tendency of bonds to shorten at higher rotational speeds
C) The effect of molecular collisions on rotation
D) The interaction between vibrational and electronic states
14. What is the primary information obtained from rotational spectra?
A) Bond strengths
B) Molecular geometry and bond lengths
C) Electronic configuration
D) Reaction rates
15. A molecule that is IR active is not necessarily Raman active, and vice versa. This is due to differences in:
A) Energy levels
B) Selection rules
C) Required radiation type
D) Molecular symmetry
16. The intensity of Raman scattering is generally:
A) Stronger than IR absorption
B) Weaker than IR absorption
C) Equal to IR absorption
D) Dependent on the electronic transition
17. Which of the following is a common source used in Raman spectroscopy?
A) Globar emitter
B) Deuterium lamp
C) Argon ion laser
D) Tungsten filament lamp
18. What experimental technique is used to measure rotational spectra?
A) Fourier Transform Infrared Spectroscopy (FTIR)
B) UV-Vis Spectrophotometry
C) Microwave Spectroscopy
D) Nuclear Magnetic Resonance (NMR)
19. For linear molecules, the selection rule for vibrational Raman activity is:
A) Change in polarizability
B) Change in dipole moment
C) Change in moment of inertia
D) Change in electronic state
20. The spacing between lines in the P and R branches of a rotation–vibration spectrum is approximately equal to:
A) The rotational constant B
B) Twice the rotational constant B
C) Half the rotational constant B
D) The vibrational frequency
21. What is the Q branch in a rotation–vibration spectrum?
A) The branch where ΔJ = +1
B) The branch where ΔJ = -1
C) The branch where ΔJ = 0
D) The branch where ΔJ = ±2
22. What are the P and R branches in a rotation–vibration spectrum?
A) Branches corresponding to ΔJ = 0 and ΔJ = +1 respectively
B) Branches corresponding to ΔJ = +1 and ΔJ = -1 respectively
C) Branches corresponding to ΔJ = -1 and ΔJ = +1 respectively
D) Branches corresponding to ΔJ = ±2 and ΔJ = 0 respectively
23. In rotation–vibration spectra, what is observed for diatomic molecules?
A) Sharp lines corresponding to pure rotation
B) Sharp lines corresponding to pure vibration
C) A series of lines (a band) with fine structure due to rotation
D) Broad absorption bands due to electronic transitions
24. Rotation–vibration spectra combine information from which two types of molecular motion?
A) Electronic and vibrational
B) Rotational and electronic
C) Vibrational and rotational
D) Translational and vibrational
25. What is the main advantage of Raman spectroscopy over IR spectroscopy for certain applications?
A) It can study homonuclear diatomic molecules
B) It requires less sample
C) It uses visible light which is easier to handle
D) It can be performed in aqueous solutions more easily
26. The selection rules for rotational Raman spectra involve changes in the rotational quantum number J by:
A) ΔJ = 0, ±1
B) ΔJ = ±1
C) ΔJ = ±2
D) ΔJ = ±3
27. Which molecules can exhibit pure rotational Raman spectra?
A) All molecules
B) Only molecules with a permanent dipole moment
C) Only molecules with a changing polarizability during rotation
D) Only linear molecules
28. What is the experimental setup for Raman spectroscopy typically characterized by?
A) A broadband IR source and a detector
B) A laser source and a spectrograph/monochromator
C) A microwave source and a detector
D) A UV lamp and a photomultiplier tube
29. In the context of Raman scattering, what is polarizability?
A) The permanent dipole moment of a molecule
B) The ability of a molecule's electron cloud to be distorted by an electric field
C) The resistance of a molecule to rotation
D) The vibrational frequency of a bond
30. What is the classical theory of Raman scattering based on?
A) Quantum mechanics
B) Wave mechanics
C) Polarizability theory
D) Dipole moment theory
31. The frequency shift in Raman scattering is related to:
A) The frequency of the incident light
B) The vibrational or rotational energy levels of the molecule
C) The electronic energy levels of the molecule
D) The temperature of the sample
32. Anti-Stokes lines in Raman spectra correspond to:
A) Inelastically scattered photons with lower energy than incident photons
B) Inelastically scattered photons with higher energy than incident photons
C) Elastically scattered photons
D) Rayleigh scattered photons
33. Stokes lines in Raman spectra correspond to:
A) Inelastically scattered photons with lower energy than incident photons
B) Inelastically scattered photons with higher energy than incident photons
C) Elastically scattered photons
D) Rayleigh scattered photons
34. What is the phenomenon observed in Raman spectroscopy?
A) Absorption of radiation
B) Emission of radiation
C) Scattering of radiation with a frequency shift
D) Scattering of radiation with no frequency shift
35. Which type of molecules can exhibit vibrational Raman spectra?
A) Only molecules with a changing dipole moment
B) Only molecules with a changing polarizability
C) Molecules with either a changing dipole moment or polarizability
D) Only homonuclear diatomic molecules
36. Overtones in vibrational spectra arise from transitions where Δv is:
A) ±1
B) ±2
C) ±3
D) ±1, ±2, ±3, ...
37. What does the term 'anharmonicity' imply for vibrational spectra?
A) Equal spacing between vibrational levels
B) Decreasing spacing between vibrational levels
C) Increasing spacing between vibrational levels
D) No spacing between vibrational levels
38. The fundamental vibrational transition corresponds to which change in the vibrational quantum number?
A) 0 → 1
B) 1 → 2
C) 0 → 2
D) 1 → 3
39. What is the selection rule for vibrational transitions in the harmonic oscillator approximation?
A) Δv = ±1
B) Δv = ±2
C) Δv = 0, ±1
D) Δv = ±1, ±2
40. In the harmonic oscillator model, what is the energy of a vibrational level given by?
A) E_v = ħω(v + 1/2)
B) E_v = ħωv
C) E_v = ħω(v + 1)
D) E_v = ħω
41. For a vibrational mode to be IR active, what must change during the vibration?
A) Polarizability
B) Dipole moment
C) Moment of inertia
D) Electronic configuration
42. What is the primary region of the electromagnetic spectrum for vibrational spectroscopy?
A) Microwave
B) Far-infrared
C) Near-infrared
D) Visible
43. Which type of molecules can exhibit pure rotational spectra?
A) Homonuclear diatomic molecules
B) Heteronuclear diatomic molecules
C) Linear polyatomic molecules with a dipole moment
D) All of the above
44. What physical property of a molecule is directly related to the rotational constant 'B'?
A) Bond length
B) Molecular mass
C) Moment of inertia
D) Vibrational frequency
45. The spacing between adjacent lines in a pure rotational spectrum of a rigid diatomic molecule is approximately:
A) Constant
B) Increasing with J
C) Decreasing with J
D) Zero
46. What is the selection rule for rotational transitions in a rigid rotor?
A) ΔJ = 0
B) ΔJ = ±1
C) ΔJ = ±2
D) ΔJ = 0, ±1
47. In a rigid rotor model, what is the energy of a rotational level given by?
A) E_J = BJ(J+1)
B) E_J = B(J+1)
C) E_J = J(J+1)
D) E_J = B J
48. For a molecule to exhibit a pure rotational spectrum, what must be true about its dipole moment?
A) It must be zero
B) It must be non-zero
C) It can be zero or non-zero
D) It must be induced
49. What type of electromagnetic radiation is primarily used to study rotational spectra?
A) Visible light
B) Infrared radiation
C) Microwave radiation
D) Ultraviolet radiation
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