Introduction: Vibrational frequency and absorption are central concepts in infrared (IR) spectroscopy, crucial for B.Pharm students analyzing drug molecules and functional groups. Key factors affecting vibrational frequency include bond strength, reduced mass, bond order, hybridization (sp, sp2, sp3), and electronegativity. Spectral shifts arise from conjugation, hydrogen bonding, solvent effects, isotopic substitution, anharmonicity, and vibrational coupling, which also influence absorption intensity and selection rules. Understanding characteristic frequency ranges, fingerprint region interpretation, and how molecular environment alters peaks helps in structural identification, impurity profiling, and formulation analysis. This focused set of questions reinforces conceptual depth and practical interpretation. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. Which two parameters primarily determine the fundamental vibrational frequency of a diatomic bond according to the harmonic oscillator model?
- Bond length and electronegativity
- Force constant (bond strength) and reduced mass
- Solvent polarity and temperature
- Hybridization and conjugation
Correct Answer: Force constant (bond strength) and reduced mass
Q2. How does increasing bond order (single → double → triple) generally affect vibrational frequency?
- Decreases frequency
- First decreases then increases
- Increases frequency
- No change in frequency
Correct Answer: Increases frequency
Q3. Which substitution causes a lower C=O stretching frequency in an ester compared to a ketone?
- Conjugation with a C=C bond
- Attachment to an electron-withdrawing group
- Isotopic substitution with 13C
- Hydrogen bonding to the carbonyl oxygen
Correct Answer: Conjugation with a C=C bond
Q4. What is the effect of replacing hydrogen by deuterium on an X–H stretching frequency?
- No effect
- Frequency increases
- Frequency decreases due to increased reduced mass
- Intensity increases but frequency unchanged
Correct Answer: Frequency decreases due to increased reduced mass
Q5. Why does an sp-hybridized C–H bond vibrate at a higher frequency than an sp3 C–H bond?
- Because sp hybridization reduces bond polarity
- Because sp C–H has a stronger bond (higher force constant)
- Because sp3 carbons have lower mass
- Because of solvent interactions only
Correct Answer: Because sp C–H has a stronger bond (higher force constant)
Q6. Which factor most directly increases IR absorption intensity for a molecular vibration?
- Lack of dipole moment change during vibration
- Greater change in dipole moment during the vibration
- Higher molecular weight
- Lower force constant
Correct Answer: Greater change in dipole moment during the vibration
Q7. Hydrogen bonding typically causes which effect on X–H stretching frequencies?
- Blue shift (higher frequency) and stronger absorption
- Red shift (lower frequency) and broader absorption
- No shift but sharper peaks
- Complete disappearance of the band
Correct Answer: Red shift (lower frequency) and broader absorption
Q8. Conjugation of a carbonyl with a double bond typically results in what change to the C=O stretch?
- Increase in frequency due to resonance stabilization
- Decrease in frequency due to delocalization of electron density
- No change in frequency
- Only intensity changes, frequency remains constant
Correct Answer: Decrease in frequency due to delocalization of electron density
Q9. Which statement about solvent effects on vibrational frequencies is correct?
- Polar solvents never affect frequencies
- Solvent can shift frequencies via specific interactions like hydrogen bonding
- Solvent only affects intensity, not frequency
- Spectrum in solution is identical to gas phase spectrum
Correct Answer: Solvent can shift frequencies via specific interactions like hydrogen bonding
Q10. What is Fermi resonance?
- Coupling between an overtone or combination band and a fundamental of similar energy causing shifts and intensity changes
- Complete cancellation of IR bands due to symmetry
- Resonance in NMR rather than IR
- Absorption due to electronic transitions
Correct Answer: Coupling between an overtone or combination band and a fundamental of similar energy causing shifts and intensity changes
Q11. Which vibrational mode is IR inactive for a perfectly symmetric linear molecule with no change in dipole moment?
- Asymmetric stretch
- Symmetric stretch with no dipole change
- Bending mode that changes dipole
- Any mode that changes dipole moment
Correct Answer: Symmetric stretch with no dipole change
Q12. How does increasing temperature generally affect IR band shapes?
- Sharpens bands
- Broadens bands due to increased vibrational population and collisions
- Eliminates peaks entirely
- Only shifts the baseline
Correct Answer: Broadens bands due to increased vibrational population and collisions
Q13. Which functional group exhibits a strong absorption near 1700 cm−1?
- Alcohol O–H stretch
- Carbonyl C=O stretch
- C–H bending
- N–H bend
Correct Answer: Carbonyl C=O stretch
Q14. What effect does electron-withdrawing substitution adjacent to a C=O group typically have on its stretching frequency?
- Decreases frequency by weakening the bond
- Increases frequency by withdrawing electron density and strengthening the C=O bond
- No effect
- Only affects IR intensity
Correct Answer: Increases frequency by withdrawing electron density and strengthening the C=O bond
Q15. Which phenomenon explains why aromatic C–H stretches are found at slightly higher frequencies than aliphatic C–H stretches?
- Lower reduced mass in aromatic C–H
- Increased s-character and bond strength in aromatic C–H
- Effect of solvent only
- Isotopic substitution in aromatic rings
Correct Answer: Increased s-character and bond strength in aromatic C–H
Q16. Why do nitrile (C≡N) stretches appear in a distinct region around 2200–2300 cm−1?
- Because of weak bond strength and low force constant
- Because triple bonds have high force constants and distinct reduced mass
- Because nitriles are not IR active
- Because of hydrogen bonding only
Correct Answer: Because triple bonds have high force constants and distinct reduced mass
Q17. What is the primary reason coupling between vibrational modes alters observed frequencies?
- Covalent bond breakage
- Mixing of normal modes leads to energy redistribution and shifted frequencies
- Change in molecular weight
- Instrumental artifact only
Correct Answer: Mixing of normal modes leads to energy redistribution and shifted frequencies
Q18. Which effect reduces the C–H stretching frequency in alcohols when compared to isolated C–H bonds?
- Hydrogen bonding to O–H indirectly affecting C–H
- Increased bond order of C–H
- Presence of aromatic ring only
- Isotopic enrichment with 13C
Correct Answer: Hydrogen bonding to O–H indirectly affecting C–H
Q19. In the fingerprint region (below ~1500 cm−1), why is interpretation important for B.Pharm students?
- This region contains characteristic absorptions unique to specific functional groups allowing compound identification
- Fingerprint region has no useful information
- Only overtone bands appear there
- It only shows solvent peaks
Correct Answer: This region contains characteristic absorptions unique to specific functional groups allowing compound identification
Q20. What is anharmonicity in vibrational spectroscopy?
- Deviation from harmonic oscillator behavior causing overtones and non-equally spaced energy levels
- Perfect harmonic motion of bonds
- Only observed in NMR spectroscopy
- Instrumental noise leading to baseline distortion
Correct Answer: Deviation from harmonic oscillator behavior causing overtones and non-equally spaced energy levels
Q21. How does resonance (mesomeric effect) adjacent to a bond generally affect its vibrational frequency?
- Resonance always increases frequency
- Resonance delocalization can lower force constant and decrease frequency
- Resonance changes only intensity, not frequency
- Resonance causes isotopic shifts
Correct Answer: Resonance delocalization can lower force constant and decrease frequency
Q22. What is the effect of heavy atom substitution (e.g., Cl replacing H) on vibrational frequency of a neighboring bond?
- It reduces reduced mass and increases frequency
- It increases reduced mass and generally lowers frequency of related modes
- It has no effect since mass change is negligible
- It always sharpens the peak
Correct Answer: It increases reduced mass and generally lowers frequency of related modes
Q23. Which statement correctly describes selection rules for IR activity?
- A vibrational mode is IR active if it involves a change in molecular polarizability only
- A vibrational mode is IR active if it involves a change in dipole moment
- All vibrational modes are IR active regardless of dipole change
- A vibrational mode is IR active only in Raman spectroscopy
Correct Answer: A vibrational mode is IR active if it involves a change in dipole moment
Q24. In a carbonyl-containing drug molecule, which interaction with the environment most strongly broadens and shifts the C=O peak?
- Van der Waals forces only
- Hydrogen bonding with protic solvents or functional groups
- Nonpolar solvent interactions
- Presence of aromatic rings at distant positions
Correct Answer: Hydrogen bonding with protic solvents or functional groups
Q25. Overtones in IR spectroscopy are due to what characteristic of molecular vibrations?
- Perfect harmonic potentials producing equally spaced levels
- Anharmonicity allowing transitions to higher vibrational levels (2ν, 3ν)
- Electronic transitions incorrectly recorded in IR
- Instrumental harmonics only
Correct Answer: Anharmonicity allowing transitions to higher vibrational levels (2ν, 3ν)
Q26. Which hybridization leads to the highest C–C stretching frequency in general?
- sp3–sp3
- sp2–sp2 (as in double bond)
- sp–sp (triple bond)
- All have identical frequencies
Correct Answer: sp–sp (triple bond)
Q27. How does steric strain in a ring influence vibrational frequencies of bonds within it?
- Steric strain can alter bond angles and force constants, shifting frequencies
- Steric strain has no observable effect on IR spectra
- Steric strain only affects NMR chemical shifts
- Steric strain always eliminates IR activity
Correct Answer: Steric strain can alter bond angles and force constants, shifting frequencies
Q28. Why are symmetric stretches of homonuclear diatomic molecules (e.g., O2) IR inactive?
- They involve a change in dipole moment
- They do not change the dipole moment and thus are IR inactive
- Because homonuclear diatomics have too high frequency
- Because they only show Raman activity due to mass differences
Correct Answer: They do not change the dipole moment and thus are IR inactive
Q29. What causes a blue shift (increase in frequency) of an X–H stretch in some cases upon hydrogen bonding?
- Stronger hydrogen bonding always lowers frequency, blue shift never occurs
- When hydrogen bonding shortens the X–H bond, increasing force constant
- Only isotopic substitution causes blue shifts
- Instrument calibration errors cause apparent blue shifts
Correct Answer: When hydrogen bonding shortens the X–H bond, increasing force constant
Q30. For analyzing drug stability, which IR observation would most likely indicate hydrolysis of an ester to a carboxylic acid?
- Disappearance of C=O band and appearance of a new sharp nitrile band
- Shift of ester C=O frequency and appearance of broad O–H stretch plus a carboxylic acid C=O band
- No change in IR spectrum
- Only changes in fingerprint region unrelated to functional groups
Correct Answer: Shift of ester C=O frequency and appearance of broad O–H stretch plus a carboxylic acid C=O band
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