Theory of IR spectroscopy MCQs With Answer

Theory of IR Spectroscopy MCQs With Answer (M. Pharm)

Infrared (IR) spectroscopy is a cornerstone of Modern Pharmaceutical Analytical Techniques, enabling rapid identification, functional group analysis, and solid-state characterization in drug substances and formulations. This MCQ set focuses on the theoretical foundations M. Pharm students must master—selection rules, Hooke’s law, anharmonicity, Fermi resonance, group frequencies, fingerprint region, and FTIR instrumentation (sources, detectors, interferometers, and ATR). Each question is designed to deepen conceptual understanding and connect principles to pharmaceutical applications such as identity testing, polymorph differentiation, and excipient compatibility. Work through the questions to strengthen your grasp of how molecular vibrations produce diagnostic spectra and how instrument design enhances sensitivity, accuracy, and throughput in routine and advanced analysis.

Q1. What fundamental condition must a molecular vibration satisfy to be IR-active?

• Presence of at least one polar bond in the molecule
• Change in polarizability during the vibration
• Change in dipole moment during the vibration
• Existence of hydrogen bonding in the sample

Correct Answer: Change in dipole moment during the vibration

Q2. For a harmonic oscillator, the primary selection rule for vibrational transitions in IR spectroscopy is:

• Δv = 0 only
• Δv = ±1
• Δv = ±2, ±3 only
• Δv = any integer

Correct Answer: Δv = ±1

Q3. According to the Hooke’s law model, increasing the reduced mass (μ) of a diatomic bond will:

• Increase the vibrational wavenumber
• Decrease the vibrational wavenumber
• Have no effect on vibrational wavenumber
• Broaden the absorption band without shifting it

Correct Answer: Decrease the vibrational wavenumber

Q4. Overtones (Δv ≥ 2) observed in IR spectra primarily arise due to:

• Fermi resonance
• Anharmonicity of molecular vibrations
• Isotopic substitution
• Hydrogen bonding

Correct Answer: Anharmonicity of molecular vibrations

Q5. Fermi resonance in IR spectroscopy refers to:

• Coupling between a fundamental vibration and a rotational transition
• Splitting due to magnetic interactions in paramagnetic molecules
• Interaction between a fundamental and a nearby overtone/combination band of the same symmetry
• Intensity loss from anharmonic damping

Correct Answer: Interaction between a fundamental and a nearby overtone/combination band of the same symmetry

Q6. The “fingerprint region” in a mid-IR spectrum is typically defined as:

• 4000–2500 cm⁻¹
• 2500–1500 cm⁻¹
• 1500–500 cm⁻¹
• 500–50 cm⁻¹

Correct Answer: 1500–500 cm⁻¹

Q7. Conjugation of a carbonyl group (C=O) with a double bond or aromatic ring generally:

• Increases the C=O stretching wavenumber by ~20–30 cm⁻¹
• Decreases the C=O stretching wavenumber by ~20–30 cm⁻¹
• Eliminates IR activity of the C=O stretch
• Converts the C=O stretch into a bending mode

Correct Answer: Decreases the C=O stretching wavenumber by ~20–30 cm⁻¹

Q8. Hydrogen bonding typically causes the O–H stretching band to:

• Shift to higher wavenumber and sharpen
• Shift to lower wavenumber and broaden
• Remain at the same wavenumber with decreased intensity
• Disappear due to selection rule violation

Correct Answer: Shift to lower wavenumber and broaden

Q9. Replacing 12C with 13C in a C=O group will most likely:

• Shift the C=O stretch to higher wavenumber
• Shift the C=O stretch to lower wavenumber
• Leave the C=O stretch unchanged
• Double the intensity of the C=O stretch

Correct Answer: Shift the C=O stretch to lower wavenumber

Q10. In ATR-FTIR, which change increases the effective penetration depth into the sample?

• Using a higher refractive index ATR crystal (e.g., Ge instead of ZnSe)
• Increasing the angle of incidence further above the critical angle
• Using longer wavelength (lower wavenumber) radiation
• Reducing the number of scans collected

Correct Answer: Using longer wavelength (lower wavenumber) radiation

Q11. The Fellgett (multiplex) advantage of FTIR refers to:

• Improved resolution due to longer path length
• Simultaneous measurement of all wavelengths improving SNR for many spectra
• Higher source brightness than dispersive instruments
• Better sample throughput due to larger apertures

Correct Answer: Simultaneous measurement of all wavelengths improving SNR for many spectra

Q12. Accurate wavenumber calibration in an FTIR spectrometer is provided by the:

• Globar source
• DTGS detector
• He–Ne reference laser
• KBr beam splitter

Correct Answer: He–Ne reference laser

Q13. Spectral resolution in FTIR is primarily determined by the instrument’s:

• Slit width of the monochromator
• Detector time constant
• Maximum optical path difference in the interferometer
• Laser power

Correct Answer: Maximum optical path difference in the interferometer

Q14. Which is a common mid-IR radiation source in FTIR instruments?

• Deuterium lamp
• Tungsten–halogen filament
• Globar (silicon carbide) source
• Xenon arc lamp

Correct Answer: Globar (silicon carbide) source

Q15. Which bond typically exhibits the highest stretching frequency in IR spectra?

• C=O
• O–H
• N–H
• C–C (single)

Correct Answer: O–H

Q16. Rotational fine structure is commonly visible in gas-phase IR spectra because of:

• Vibration–rotation coupling
• Electron–phonon coupling
• Hyperfine splitting
• Surface plasmon resonance

Correct Answer: Vibration–rotation coupling

Q17. The fingerprint region is powerful for identity testing in pharmaceuticals primarily because it:

• Contains only overtone and combination bands
• Is free from hydrogen-bonding effects
• Is dominated by solvent absorptions that standardize spectra
• Comprises many coupled skeletal vibrations unique to each molecular structure

Correct Answer: Comprises many coupled skeletal vibrations unique to each molecular structure

Q18. Homonuclear diatomic molecules like N₂ and O₂ are IR-inactive because they:

• Have very low polarizability
• Undergo no change in dipole moment during vibration
• Vibrate only at microwave frequencies
• Are fully transparent to all IR radiation

Correct Answer: Undergo no change in dipole moment during vibration

Q19. In the KBr pellet technique, a critical practical limitation relevant to pharmaceutical samples is that KBr:

• Is opaque in the mid-IR region
• Is strongly fluorescent under IR radiation
• Is hygroscopic and introduces water bands if not dried
• Reactively forms esters with carboxylic acids

Correct Answer: Is hygroscopic and introduces water bands if not dried

Q20. Which detector offers high sensitivity and fast response in the mid-IR but typically requires cooling?

• Photomultiplier tube
• DTGS (deuterated triglycine sulfate)
• MCT (HgCdTe, mercury cadmium telluride)
• Silicon photodiode

Correct Answer: MCT (HgCdTe, mercury cadmium telluride)

Download