🔬 1. Fundamentals of Vibrational Spectroscopy
Q1. Vibrational spectroscopy involves transitions in which spectral range?
a) 100–1000 cm⁻¹
b) 300–3000 cm⁻¹
c) 400–4000 cm⁻¹
d) 500–5000 cm⁻¹
Answer: c) 400–4000 cm⁻¹
Explanation: This range corresponds to the infrared region where most molecular vibrations are observed.Sanfoundry
Q2. Which of the following is not a type of bending molecular vibration?
a) Twisting
b) Stretching
c) Wagging
d) Rocking
Answer: b) Stretching
Explanation: Stretching involves changes in bond lengths, while bending (twisting, wagging, rocking) involves changes in bond angles. Sanfoundry
Q3. The vibrational degrees of freedom for a non-linear molecule are:
a) 3N – 6
b) 3N – 5
c) 2N – 5
d) 2N – 4
Answer: a) 3N – 6
Explanation: Non-linear molecules have 3N total degrees of freedom, subtracting 3 for translations and 3 for rotations, leaving 3N – 6 vibrational modes.San
Q4. In vibrational spectroscopy, the first overtone occurs at:
a) 2ν₁
b) 4ν₁
c) 3ν₁
d) None of the above
Answer: a) 2ν₁
Explanation: Overtones are multiples of the fundamental frequency; the first overtone is twice the fundamental frequency.
Q5. Which of the following cannot show a vibrational absorption spectrum?
a) OCS
b) H₂O
c) CO₂
d) C₂H₄
Answer: d) C₂H₄
Explanation: C₂H₄ (ethylene) lacks a change in dipole moment during certain vibrations, making some modes IR inactive.Sanfoundry
🌈 2. Infrared (IR) Spectroscopy
Q6. Which region of the IR spectrum is known as the fingerprint region?
a) 4000–2500 cm⁻¹
b) 2500–1500 cm⁻¹
c) 1500–600 cm⁻¹
d) 600–400 cm⁻¹
Answer: c) 1500–600 cm⁻¹
Explanation: This region contains complex absorption patterns unique to each molecule, aiding in their identification.
Q7. What is the absorption frequency (in cm⁻¹) of the –C≡N functional group?
a) 3610–3640
b) 1690–1760
c) 1180–1360
d) 2850–2960
Answer: c) 1180–1360
Explanation: The –C≡N triple bond exhibits a strong absorption in this range due to its bond strength and dipole moment.
Q8. Which technique is commonly used to determine the presence of functional groups in a compound?
a) Chromatography
b) IR Spectroscopy
c) Mass Spectrometry
d) X-ray Diffraction
Answer: b) IR Spectroscopy
Explanation: IR spectroscopy identifies functional groups based on characteristic absorption bands.
Q9. In the IR spectrum of benzoyl chloride, a weak band near 1750 cm⁻¹ is formed due to:
a) Inductive effect
b) Fermi resonance between C=O band and first overtone
c) Conjugation effect
d) Hyperconjugation effect
Answer: b) Fermi resonance between C=O band and first overtone
Explanation: The weak band arises from Fermi resonance, an interaction between a fundamental vibration and an overtone or combination band.
Q10. Why do ketenes absorb in IR at a very high frequency (~2150 cm⁻¹)?
a) The inner carbon is sp-hybridized
b) The more s character in a bond, the stronger it is
c) Inner carbon is sp²-hybridized
d) Inner carbon is sp³-hybridized
Answer: a) The inner carbon is sp-hybridized
Explanation: The sp-hybridization leads to stronger bonds and higher stretching frequencies.
💡 3. Raman Spectroscopy
Q11. Raman effect is the scattering of:
a) Atoms
b) Molecules
c) Protons
d) Photons
Answer: d) Photons
Explanation: The Raman effect involves the inelastic scattering of photons by molecules, leading to a change in the energy (and thus frequency) of the scattered photons.
Q12. The selection rule for rotational Raman transitions is:
a) ΔJ = ±1
b) ΔJ = ±2
c) ΔJ = 0
d) ΔJ = ±3
Answer: b) ΔJ = ±2
Explanation: In rotational Raman spectroscopy, the selection rule requires a change in the rotational quantum number of ±2.
Q13. Which of the following molecules is infrared inactive but Raman active?
a) CO₂
b) N₂
c) HCl
d) CH₄
Answer: b) N₂
Explanation: N₂ is a homonuclear diatomic molecule with no permanent dipole moment, making it IR inactive. However, it can be Raman active due to changes in polarizability during vibration.
Q14. The Raman shift depends upon:
a) Incident wavelength
b) Incident intensity
c) Resolving power of spectrograph
d) Molecular energy levels of the scatterer
Answer: d) Molecular energy levels of the scatterer
Explanation: The Raman shift corresponds to the energy difference between the incident and scattered photons, which is determined by the energy levels of the molecule.
Q15. The elastic scattering of photons is called:
a) Atmospheric scattering
b) Rayleigh scattering
c) Conserved scattering
d) Raman scattering
Answer: b) Rayleigh scattering
Explanation: Rayleigh scattering involves the elastic scattering of photons, where there is no change in energy between the incident and scattered photons.