ATR-IR and IR interpretation of functional groups MCQs With Answer

Introduction: ATR-IR and IR interpretation of functional groups MCQs With Answer is a focused practice resource for M.Pharm students preparing for advanced spectral analysis. This blog emphasizes Attenuated Total Reflection (ATR) infrared spectroscopy principles, practical sample considerations, crystal choices, penetration depth, and characteristic vibrational frequencies of common functional groups. The MCQs are designed to reinforce spectral interpretation skills, identify diagnostic bands (carbonyls, hydroxyls, nitriles, alkynes, etc.), understand hydrogen-bonding and conjugation effects, and contrast ATR advantages with transmission IR. Detailed, exam-oriented questions with answers help consolidate theoretical understanding and improve problem-solving for both coursework and practical laboratory analysis.

Q1. What does ATR in ATR-IR stand for?

• Attenuated Total Reflection
• Attenuated Transmission Resonance
• Angular Total Refraction
• Absorbance Transfer Response

Correct Answer: Attenuated Total Reflection

Q2. Which of the following best describes the typical penetration depth of the evanescent wave in ATR-IR for mid-IR wavelengths?

• 0.5–5 micrometers
• 50–500 nanometers
• 10–50 micrometers
• 0.01–0.1 millimeters

Correct Answer: 0.5–5 micrometers

Q3. Which ATR crystal is preferred when analyzing very hard, abrasive solids and aggressive chemicals?

• Germanium
• Silicon
• Zinc selenide (ZnSe)
• Diamond

Correct Answer: Diamond

Q4. A broad, strong O–H stretching band due to hydrogen-bonded alcohol appears typically in which wavenumber region?

• 3600–3650 cm⁻¹
• 3200–3500 cm⁻¹
• 2850–2950 cm⁻¹
• 1700–1750 cm⁻¹

Correct Answer: 3200–3500 cm⁻¹

Q5. The C=O stretching frequency for a non-conjugated aliphatic ketone commonly appears near which wavenumber?

• 1715 cm⁻¹
• 1745 cm⁻¹
• 1685 cm⁻¹
• 1600 cm⁻¹

Correct Answer: 1715 cm⁻¹

Q6. Which band is diagnostic for a nitrile (C≡N) stretching vibration?

• 2100 cm⁻¹
• 2250 cm⁻¹
• 3300 cm⁻¹
• 1600 cm⁻¹

Correct Answer: 2250 cm⁻¹

Q7. What is the commonly accepted range for the IR “fingerprint region”?

• 4000–2500 cm⁻¹
• 2500–2000 cm⁻¹
• 2000–1500 cm⁻¹
• 1500–400 cm⁻¹

Correct Answer: 1500–400 cm⁻¹

Q8. Fermi resonance in IR spectroscopy typically causes which observable effect?

• Complete disappearance of the fundamental band
• Splitting and intensity redistribution between interacting bands
• Conversion of an IR band into a Raman band
• No measurable change in the spectrum

Correct Answer: Splitting and intensity redistribution between interacting bands

Q9. Which statement about ATR-IR is FALSE?

• ATR requires the ATR crystal to have a higher refractive index than the sample
• ATR allows direct analysis of solids and pastes with minimal preparation
• ATR spectra require preparation of a thin KBr pellet
• ATR penetration depth depends on wavelength and angle of incidence

Correct Answer: ATR spectra require preparation of a thin KBr pellet

Q10. Which physical property change governs whether a vibrational mode is Raman active?

• Change in dipole moment during vibration
• Change in polarizability during vibration
• Change in mass of atoms during vibration
• Change in bond energy during vibration

Correct Answer: Change in polarizability during vibration

Q11. Which IR feature is especially diagnostic for the presence of an aldehyde group?

• A single strong C–H stretch at 3000 cm⁻¹
• Two weak bands near 2720 and 2820 cm⁻¹ (aldehydic C–H)
• A very broad OH band centered at 3400 cm⁻¹
• A strong sharp band at 2250 cm⁻¹

Correct Answer: Two weak bands near 2720 and 2820 cm⁻¹ (aldehydic C–H)

Q12. The typical C=C stretching vibration for an unconjugated alkene appears around which wavenumber?

• 2100 cm⁻¹
• 1980 cm⁻¹
• 1650 cm⁻¹
• 1450 cm⁻¹

Correct Answer: 1650 cm⁻¹

Q13. A very broad, strong absorption extending roughly from 2500 to 3000 cm⁻¹ together with a C=O band near 1700 cm⁻¹ is characteristic of which functional group?

• Alcohol
• Carboxylic acid
• Aldehyde
• Amide

Correct Answer: Carboxylic acid

Q14. How does conjugation of a carbonyl group with a C=C double bond generally affect the C=O stretching frequency?

• It increases the C=O frequency by ~30–50 cm⁻¹
• It decreases the C=O frequency by ~20–30 cm⁻¹
• It has no effect on C=O frequency
• It causes the C=O band to split into multiple peaks

Correct Answer: It decreases the C=O frequency by ~20–30 cm⁻¹

Q15. Which IR features would indicate a terminal alkyne?

• Sharp ≡C–H stretch near 3300 cm⁻¹ and C≡C stretch ≈2100–2260 cm⁻¹
• Broad O–H band at 3200–3500 cm⁻¹ and strong C=O at 1700 cm⁻¹
• Two weak aldehydic C–H bands at 2720 and 2820 cm⁻¹ only
• Sharp nitrile band at 2250 cm⁻¹ and no ≡C–H stretch

Correct Answer: Sharp ≡C–H stretch near 3300 cm⁻¹ and C≡C stretch ≈2100–2260 cm⁻¹

Q16. Which ATR crystal material is typically chosen to achieve the shallowest penetration depth for samples with high refractive index?

• Zinc selenide (ZnSe)
• Silicon
• Germanium
• Sodium chloride (NaCl)

Correct Answer: Germanium

Q17. Which factor does NOT significantly affect the penetration depth of the evanescent wave in ATR-IR?

• Wavelength of incident IR radiation
• Angle of incidence at the crystal/sample interface
• Refractive indices of crystal and sample
• Sample concentration (for homogeneous bulk material)

Correct Answer: Sample concentration (for homogeneous bulk material)

Q18. Overtones and combination bands in IR spectra are typically observed:

• At lower wavenumbers than fundamentals
• At approximately the same wavenumber as fundamentals
• At higher wavenumbers than fundamentals (multiples or sums)
• Only in the fingerprint region below 1000 cm⁻¹

Correct Answer: At higher wavenumbers than fundamentals (multiples or sums)

Q19. A strong, sharp absorption near 2250 cm⁻¹ is most indicative of which functional group?

• Carbonyl (C=O)
• Nitrile (C≡N)
• Alcohol (O–H)
• Amide (N–H)

Correct Answer: Nitrile (C≡N)

Q20. For improving sensitivity in quantitative ATR-IR measurements which approach is most effective?

• Using a crystal with much lower refractive index than the sample
• Increasing the number of internal reflections (multi-bounce ATR accessory)
• Decreasing sample contact pressure to avoid deformation
• Using a thicker bulk sample to increase penetration depth

Correct Answer: Increasing the number of internal reflections (multi-bounce ATR accessory)

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