NMR signals and interpretation MCQs With Answer

NMR signals and interpretation MCQs With Answer

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NMR signals and interpretation MCQs With Answer

Nuclear Magnetic Resonance (NMR) spectroscopy is a cornerstone of Modern Pharmaceutical Analytical Techniques, enabling precise structural elucidation, stereochemical analysis, and purity assessment of drug substances. This quiz targets M. Pharm students who need to confidently interpret 1H and 13C NMR spectra, understand chemical shift trends, splitting patterns, coupling constants, integration, anisotropy effects, solvent and temperature influences, and essential 2D techniques (COSY, HSQC, HMBC, NOE). Each question emphasizes practical interpretation skills common in pharmaceutical problem-solving—from identifying functional groups to decoding complex spin systems and leveraging decoupling or DEPT for clarity. Work through the MCQs to strengthen your signal assignment logic and reinforce mechanistic understanding behind shifts, multiplicities, and correlations.

Q1. In 1H NMR, which internal standard is most commonly used and why?

  • Tetramethylsilane (TMS), δ = 0.00 ppm, inert and highly shielded
  • Sodium 3-(trimethylsilyl)propionate (TSP), δ = 0.00 ppm, hydrophilic
  • Acetone-d6, δ = 2.05 ppm, miscible with most solvents
  • Residual CHCl3, δ = 7.26 ppm, easily visible

Correct Answer: Tetramethylsilane (TMS), δ = 0.00 ppm, inert and highly shielded

Q2. Which proton type typically resonates furthest downfield in 1H NMR?

  • Aromatic proton (δ 6.5–8.5 ppm)
  • Aldehydic proton (δ 9–10 ppm)
  • Carboxylic acid OH (δ 10–13 ppm)
  • Benzylic methylene (δ 2.2–2.8 ppm)

Correct Answer: Carboxylic acid OH (δ 10–13 ppm)

Q3. The unusual upfield shift of a terminal acetylenic proton (δ ~2–3 ppm) is primarily due to which effect?

  • Magnetic anisotropy of the C≡C bond producing a shielding cone along the axis
  • Strong hydrogen bonding with solvent
  • Enhanced electron withdrawal by sp-hybridized carbon
  • Scalar coupling with 13C nuclei

Correct Answer: Magnetic anisotropy of the C≡C bond producing a shielding cone along the axis

Q4. A vicinal trans-alkenyl coupling in 1H NMR typically has what coupling constant (J)?

  • 0–2 Hz
  • 6–8 Hz
  • 12–18 Hz
  • 20–25 Hz

Correct Answer: 12–18 Hz

Q5. The ethyl fragment –O–CH2–CH3 in ethyl acetate is recognized in 1H NMR by which pattern and integration?

  • Triplet (2H) near δ 4.1 ppm and quartet (3H) near δ 1.3 ppm
  • Quartet (2H) near δ 4.1 ppm and triplet (3H) near δ 1.3 ppm
  • Singlet (3H) near δ 2.05 ppm and triplet (2H) near δ 1.3 ppm
  • Doublet (6H) near δ 1.0 ppm and septet (1H) near δ 2.7 ppm

Correct Answer: Quartet (2H) near δ 4.1 ppm and triplet (3H) near δ 1.3 ppm

Q6. After a D2O shake, which signal is most likely to disappear from a 1H NMR spectrum?

  • Phenolic OH
  • Benzylic CH2
  • Aromatic CH
  • Vinylic CH

Correct Answer: Phenolic OH

Q7. First-order (n+1) splitting approximation holds best under which condition?

  • Δν/J ≥ 10 between coupled nuclei
  • Δν/J ≈ 1 between coupled nuclei
  • J increases with magnetic field strength
  • At very high sample concentration only

Correct Answer: Δν/J ≥ 10 between coupled nuclei

Q8. Which system typically exhibits an AA′BB′ (A2B2) pattern rather than simple doublets in 1H NMR?

  • p-Disubstituted benzene with two identical substituents
  • o-Disubstituted benzene with unequal substituents
  • Isolated ethyl group
  • Aliphatic –CH2–CH3 under rapid exchange

Correct Answer: p-Disubstituted benzene with two identical substituents

Q9. In 13C DEPT-135, how do different carbon types appear?

  • CH and CH3 positive; CH2 negative; quaternary absent
  • Only CH appears; CH2 and CH3 negative
  • All carbons appear as positive peaks
  • Quaternary carbons are strongly positive

Correct Answer: CH and CH3 positive; CH2 negative; quaternary absent

Q10. Which 13C chemical shift range best corresponds to a ketone carbonyl carbon?

  • 0–60 ppm
  • 70–90 ppm
  • 100–150 ppm
  • 190–220 ppm

Correct Answer: 190–220 ppm

Q11. When moving from a 300 MHz to a 600 MHz spectrometer, which statement is correct?

  • J values (Hz) stay the same; chemical shift in Hz doubles; ppm values remain constant
  • J values (Hz) double; chemical shift in Hz halves; ppm values change
  • J values (Hz) and ppm both double
  • J values (Hz) and ppm both remain unchanged

Correct Answer: J values (Hz) stay the same; chemical shift in Hz doubles; ppm values remain constant

Q12. Which 2D experiment correlates each proton with its directly bonded carbon?

  • COSY
  • HSQC
  • HMBC
  • NOESY

Correct Answer: HSQC

Q13. HMBC primarily shows which type of correlations?

  • 1JCH one-bond H–C correlations only
  • Long-range 2–3 bond H–C correlations, often to quaternary carbons
  • H–H through-space correlations within 5 Å
  • H–H scalar couplings across two bonds only

Correct Answer: Long-range 2–3 bond H–C correlations, often to quaternary carbons

Q14. The Nuclear Overhauser Effect (NOE) in 1H NMR provides what kind of information?

  • Through-bond scalar coupling relationships
  • Through-space proximity enhancements between protons within about 5 Å
  • Direct proton–carbon connectivity
  • Absolute configuration without a chiral reference

Correct Answer: Through-space proximity enhancements between protons within about 5 Å

Q15. Which 1H NMR pattern is most diagnostic of an isopropyl group, –CH(CH3)2?

  • Triplet integrating to 3H and quartet integrating to 2H
  • Doublet integrating to 6H and septet integrating to 1H
  • Singlet integrating to 9H
  • Doublet of doublets integrating to 2H

Correct Answer: Doublet integrating to 6H and septet integrating to 1H

Q16. In CDCl3, which residual solvent signal is typically observed in 1H NMR?

  • δ 7.26 ppm for CHCl3
  • δ 2.50 ppm for DMSO-d6
  • δ 3.31 ppm for CD3OD
  • δ 4.79 ppm for H2O in D2O

Correct Answer: δ 7.26 ppm for CHCl3

Q17. Which 13C acquisition mode collapses proton couplings to simplify spectra and enhance sensitivity?

  • Proton-coupled 13C
  • Off-resonance decoupled 13C
  • Broadband proton-decoupled 13C
  • Inverse-gated decoupled 13C with NOE suppression

Correct Answer: Broadband proton-decoupled 13C

Q18. A broad amide NH signal sharpens and shifts upon lowering temperature. What is the primary reason?

  • Slowing of exchange processes reduces line broadening
  • Increase of J coupling at low temperature
  • Formation of TMS complexes at low temperature
  • Enhanced solvent viscosity eliminates all couplings

Correct Answer: Slowing of exchange processes reduces line broadening

Q19. Two aromatic protons appear as doublets with J = 8.5 Hz. What is the most likely relationship?

  • Meta coupling
  • Para coupling
  • Ortho coupling
  • Long-range benzylic coupling

Correct Answer: Ortho coupling

Q20. Which nucleus commonly causes fast relaxation and line broadening that diminishes observable 1H–X couplings in amines?

  • 31P (I = 1/2)
  • 19F (I = 1/2)
  • 14N (I = 1)
  • 13C (I = 1/2)

Correct Answer: 14N (I = 1)

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