Interpretation of NMR signals MCQs With Answer

Interpretation of NMR signals MCQs With Answer
This question set is designed for M.Pharm students taking Advanced Instrumental Analysis (MPA 201T) who want to strengthen practical interpretation skills in proton and carbon NMR. The questions emphasize real exam-style problems: chemical shifts, coupling constants, multiplicities, spin systems, DEPT interpretation, 2D correlations (HSQC/HMBC/COSY), exchangeable protons and anisotropic effects. Each MCQ focuses on how spectral features relate to molecular structure and common pitfalls encountered during spectral assignment. Use these items to test conceptual understanding and to practice translating NMR data into structural conclusions—essential for formulation, impurity profiling and structural elucidation tasks in pharmaceutical research.

Q1. What chemical shift is assigned to the standard reference tetramethylsilane (TMS) in 1H NMR?

• 0 ppm
• 1.5 ppm
• 7.26 ppm
• 77.0 ppm

Correct Answer: 0 ppm

Q2. According to the simple n+1 rule, the multiplicity of an NMR signal is determined by:

• The number of equivalent neighboring protons plus one
• The total number of protons in the molecule
• The number of carbon atoms attached to the proton
• The magnetic field strength of the spectrometer

Correct Answer: The number of equivalent neighboring protons plus one

Q3. In 1H NMR, the integrated area under a signal is directly proportional to:

• The number of chemically equivalent protons giving that signal
• The coupling constant between the protons
• The molecular weight of the compound
• The solvent polarity

Correct Answer: The number of chemically equivalent protons giving that signal

Q4. What does the coupling constant (J) measured in Hz principally represent?

• Scalar (through-bond) coupling between nuclei, measured in Hz
• Dipolar through-space interactions only
• The absolute concentration of the sample
• The chemical shift difference in ppm

Correct Answer: Scalar (through-bond) coupling between nuclei, measured in Hz

Q5. Which effect generally causes a proton signal to appear downfield (higher ppm) in 1H NMR?

• Presence of electronegative substituents or electron-withdrawing groups
• Increased electron density (electron-donating groups)
• Shielding by nearby alkyl groups
• Lower spectrometer magnetic field

Correct Answer: Presence of electronegative substituents or electron-withdrawing groups

Q6. In which approximate region do aromatic protons typically resonate in 1H NMR?

• 6.0–8.5 ppm
• 0.5–2.0 ppm
• 3.0–4.5 ppm
• 9.5–11.0 ppm

Correct Answer: 6.0–8.5 ppm

Q7. In a DEPT-135 13C NMR experiment, which statement is true?

• CH and CH3 carbons give positive signals; CH2 carbons give negative signals; quaternary carbons do not appear
• All carbons give positive signals regardless of multiplicity
• Quaternary carbons appear as double intensity peaks
• Only quaternary and CH2 carbons are detected

Correct Answer: CH and CH3 carbons give positive signals; CH2 carbons give negative signals; quaternary carbons do not appear

Q8. A quartet observed in a 1H NMR spectrum most commonly arises from coupling to:

• Three equivalent neighboring protons
• One neighboring proton with J = 4 Hz
• Two non-equivalent protons with identical J values
• A carbon atom (13C) at natural abundance

Correct Answer: Three equivalent neighboring protons

Q9. A large vicinal coupling constant (J ~7–10 Hz) between two protons in a sugar ring most likely indicates:

• Axial–axial (trans) coupling in a pyranose ring
• Equatorial–equatorial coupling with a gauche orientation
• Long-range W-coupling across four bonds
• Through-space NOE interaction

Correct Answer: Axial–axial (trans) coupling in a pyranose ring

Q10. How do exchangeable protons (OH, NH) typically appear in 1H NMR and how can they be confirmed?

• As broad signals that often disappear after D2O exchange
• As sharp multiplets unaffected by D2O exchange
• As intense singlets with fixed coupling to all neighboring protons
• They only appear in 13C NMR spectra

Correct Answer: As broad signals that often disappear after D2O exchange

Q11. Which statement best describes the HMBC experiment in heteronuclear 2D NMR?

• HMBC correlates protons with carbons two or three bonds away (mainly 2J and 3J)
• HMBC shows only one-bond 1H–13C correlations
• HMBC is used to detect through-space NOE interactions between protons
• HMBC directly measures molecular diffusion coefficients

Correct Answer: HMBC correlates protons with carbons two or three bonds away (mainly 2J and 3J)

Q12. Two protons that give identical signals under achiral conditions because they are chemically indistinguishable are called:

• Homotopic
• Enantiotopic
• Diastereotopic
• Conformers

Correct Answer: Homotopic

Q13. If a proton is coupled to two non-equivalent neighboring protons with different J values, the observed splitting is typically:

• A doublet of doublets
• A triplet
• A singlet
• A multiplet of seven lines

Correct Answer: A doublet of doublets

Q14. What is the typical chemical shift range for an aldehydic proton in 1H NMR?

• 9–10 ppm
• 0–1 ppm
• 3–4 ppm
• 6–7 ppm

Correct Answer: 9–10 ppm

Q15. In 13C NMR, where do ketone carbonyl carbons typically resonate?

• About 200–220 ppm
• 30–50 ppm
• 100–120 ppm
• 0–30 ppm

Correct Answer: About 200–220 ppm

Q16. The residual proton signal of CDCl3 appears at what chemical shift in 1H NMR?

• 7.26 ppm
• 0.00 ppm
• 2.50 ppm
• 3.35 ppm

Correct Answer: 7.26 ppm

Q17. Which 2D NMR experiment is commonly used to observe one-bond 1H–13C correlations (direct heteronuclear correlation)?

• HSQC (or HMQC)
• HMBC
• COSY
• NOESY

Correct Answer: HSQC (or HMQC)

Q18. The Nuclear Overhauser Effect (NOE) provides information about:

• Through-space interactions indicating spatial proximity (typically <5 Å)
• Through-bond scalar coupling constants only
• The number of chemically equivalent protons
• Absolute chemical shifts of carbons

Correct Answer: Through-space interactions indicating spatial proximity (typically <5 Å)

Q19. What is the typical magnitude of an ortho (vicinal aromatic) 3JHH coupling constant?

• 6–9 Hz
• 0–2 Hz
• 12–15 Hz
• 20–25 Hz

Correct Answer: 6–9 Hz

Q20. Small satellite peaks flanking a main proton signal in a 1H NMR spectrum are usually due to:

• Coupling between 1H and 13C at natural abundance (~1.1%), producing 13C satellites
• Impurities only and never due to isotopic coupling
• Spin–spin relaxation artifacts that indicate no coupling
• Quadrupolar broadening from deuterium in the solvent

Correct Answer: Coupling between 1H and 13C at natural abundance (~1.1%), producing 13C satellites

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