IR spectroscopy – principles and vibrational modes in molecules MCQs With Answer

Infrared (IR) spectroscopy is a fundamental analytical technique for B.Pharm students that probes molecular vibrations to identify functional groups and confirm drug structures. Based on absorption of IR radiation at characteristic wavenumbers (cm⁻¹), IR reveals stretching and bending vibrational modes, fingerprint regions, and effects of conjugation, hydrogen bonding, and isotopic substitution. Key concepts include selection rules (change in dipole moment), group frequencies (C=O, O–H, N–H, C≡N), instrument types (FTIR vs dispersive), and sampling methods (ATR, KBr pellet, Nujol mull). Mastery of IR interpretation aids purity assessment, formulation analysis, and structural elucidation in pharmaceutical research. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the primary principle behind IR spectroscopy?

• Absorption of ultraviolet light to excite electrons
• Emission of photons due to nuclear transitions
• Absorption of infrared radiation by molecular vibrations that change dipole moment
• Scattering of X-rays by electron clouds

Correct Answer: Absorption of infrared radiation by molecular vibrations that change dipole moment

Q2. Which unit is conventionally used to report IR frequencies?

• Hertz (Hz)
• Nanometers (nm)
• Wavenumbers (cm⁻¹)
• Electronvolts (eV)

Correct Answer: Wavenumbers (cm⁻¹)

Q3. What is the IR selection rule for a vibrational mode to be IR active?

• The vibration must conserve molecular symmetry
• The vibration must change the molecular dipole moment
• The vibration must involve only hydrogen atoms
• The vibration must be degenerate

Correct Answer: The vibration must change the molecular dipole moment

Q4. Which description best differentiates stretching from bending vibrations?

• Stretching changes bond angles; bending changes bond lengths
• Stretching changes bond lengths; bending changes bond angles
• Stretching involves only heavy atoms; bending involves hydrogens
• Stretching is always IR inactive; bending is always IR active

Correct Answer: Stretching changes bond lengths; bending changes bond angles

Q5. How many normal vibrational modes does a nonlinear molecule with N atoms have?

• 3N
• 3N − 5
• 3N − 6
• N − 1

Correct Answer: 3N − 6

Q6. How many normal modes does a linear molecule with N atoms possess?

• 3N − 3
• 3N − 6
• 3N − 5
• N

Correct Answer: 3N − 5

Q7. Which statement explains why O2 is IR inactive?

• O2 has too many vibrational modes
• O2 is homonuclear and its vibrations do not change dipole moment
• O2 has vibrations only in the far-IR region outside instrument range
• O2 undergoes only rotational transitions, not vibrational

Correct Answer: O2 is homonuclear and its vibrations do not change dipole moment

Q8. A typical strong C=O stretching absorption for a saturated ketone appears near which wavenumber?

• 3300 cm⁻¹
• 2250 cm⁻¹
• 1700 cm⁻¹
• 1100 cm⁻¹

Correct Answer: 1700 cm⁻¹

Q9. How does hydrogen bonding affect the O–H stretching band in IR spectra?

• It sharpens and shifts the band to higher frequency
• It broadens and shifts the band to lower frequency
• It eliminates the O–H band completely
• It converts stretching into bending modes only

Correct Answer: It broadens and shifts the band to lower frequency

Q10. Where does a typical C≡N (nitrile) stretch appear in the IR spectrum?

• ~1700 cm⁻¹
• ~2250 cm⁻¹
• ~3000 cm⁻¹
• ~600 cm⁻¹

Correct Answer: ~2250 cm⁻¹

Q11. The fingerprint region of an IR spectrum is usually defined as:

• 4000–3000 cm⁻¹
• 3000–2000 cm⁻¹
• 2000–1500 cm⁻¹
• 1500–400 cm⁻¹

Correct Answer: 1500–400 cm⁻¹

Q12. One main advantage of FTIR over dispersive IR spectrometers is:

• FTIR has no need for detectors
• FTIR provides simultaneous measurement of all frequencies (better signal-to-noise)
• FTIR cannot measure solids
• FTIR removes the need for sample preparation entirely

Correct Answer: FTIR provides simultaneous measurement of all frequencies (better signal-to-noise)

Q13. What is the primary benefit of ATR (attenuated total reflectance) sampling?

• Requires derivatization of samples
• Allows analysis with minimal sample preparation and little destruction
• Only works for gaseous samples
• Produces higher resolution than FTIR interferometers

Correct Answer: Allows analysis with minimal sample preparation and little destruction

Q14. Which sampling technique is commonly used to analyze solid pharmaceutical powders in IR?

• Nujol mull between IR plates only
• KBr pellet method
• Direct gas cell measurement
• UV-visible cuvette

Correct Answer: KBr pellet method

Q15. For quantitative IR analysis, Beer–Lambert law implies:

• Absorbance is independent of concentration
• Absorbance is proportional to concentration (A = ε b c)
• Transmittance increases linearly with concentration
• Only peak position matters, not intensity

Correct Answer: Absorbance is proportional to concentration (A = ε b c)

Q16. Conjugation of a carbonyl group with a C=C double bond typically causes the C=O stretching frequency to:

• Increase significantly (>200 cm⁻¹)
• Decrease slightly (shift to lower wavenumber)
• Remain completely unchanged
• Disappear from the spectrum

Correct Answer: Decrease slightly (shift to lower wavenumber)

Q17. Replacing hydrogen with deuterium in a molecule generally causes vibrational frequencies to:

• Increase due to heavier mass
• Decrease due to heavier mass
• Remain identical
• Become IR inactive

Correct Answer: Decrease due to heavier mass

Q18. The term “scissoring” describes which type of vibrational motion?

• Out-of-plane wagging of hydrogens
• In-plane bending where two bonded atoms move toward and away like scissors
• Symmetric stretching of a triple bond
• Rotation around a single bond

Correct Answer: In-plane bending where two bonded atoms move toward and away like scissors

Q19. In CO2, which vibrational mode is IR active?

• Symmetric stretch
• Antisymmetric stretch
• All modes are IR inactive
• Only rotational modes are IR active

Correct Answer: Antisymmetric stretch

Q20. The intensity of an IR absorption band is primarily related to:

• The mass of the entire molecule only
• The magnitude of dipole change during vibration
• The color of the sample
• The number of isotopes present

Correct Answer: The magnitude of dipole change during vibration

Q21. A sharp strong peak near 2250 cm⁻¹ in a drug sample typically indicates:

• A carbonyl group
• An aromatic C–H stretch
• A nitrile (C≡N) group
• An O–H hydrogen-bonded stretch

Correct Answer: A nitrile (C≡N) group

Q22. Aromatic C–H out-of-plane bending vibrations are diagnostic in which approximate region?

• 700–900 cm⁻¹
• 1500–1600 cm⁻¹
• 3000–3100 cm⁻¹
• 2000–2200 cm⁻¹

Correct Answer: 700–900 cm⁻¹

Q23. Which region of an IR spectrum is most useful for identifying functional groups?

• 4000–1500 cm⁻¹ (group region)
• 1500–400 cm⁻¹ (fingerprint region)
• Below 400 cm⁻¹ only
• Only the visible region

Correct Answer: 4000–1500 cm⁻¹ (group region)

Q24. Overtone and combination bands in IR spectra are generally:

• Stronger than fundamental bands
• Absent in organic molecules
• Weaker and often found above fundamental frequencies
• Only observed in liquids

Correct Answer: Weaker and often found above fundamental frequencies

Q25. Broadening of an IR peak is commonly caused by:

• Instrument resolution improvements
• Hydrogen bonding and sample heterogeneity
• Decrease in temperature only
• Use of deuterated solvents exclusively

Correct Answer: Hydrogen bonding and sample heterogeneity

Q26. Which component is central to an FTIR spectrometer for producing an interferogram?

• Diffraction grating
• Prism
• Michelson interferometer
• Monochromator slit

Correct Answer: Michelson interferometer

Q27. Which of the following molecules is IR inactive due to symmetry?

• HCl
• CO
• N2
• H2O

Correct Answer: N2

Q28. Bending vibrations in a molecule typically involve changes in:

• Bond lengths only
• Bond angles
• Molecular electronic spin states
• Nuclear decay pathways

Correct Answer: Bond angles

Q29. In pharmaceutical analysis, IR spectroscopy is particularly useful for:

• Measuring radioactive contamination
• Identifying functional groups and checking compound identity/purity
• Replacing mass spectrometry for exact molecular mass determination
• Counting polymer chain lengths precisely

Correct Answer: Identifying functional groups and checking compound identity/purity

Q30. The fingerprint region is most valuable because it is:

• Highly similar for all organic molecules
• Unique for each compound, enabling exact identification by comparison
• Located above 4000 cm⁻¹
• Only containing O–H stretches

Correct Answer: Unique for each compound, enabling exact identification by comparison

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com