NMR interpretation for structure elucidation MCQs With Answer

NMR interpretation for structure elucidation MCQs With Answer

Introduction: This quiz compilation focuses on advanced NMR interpretation skills crucial for M.Pharm students studying MPC 201T Advanced Spectral Analysis. The questions emphasize practical aspects of 1D and 2D NMR—chemical shift assignment, integration, multiplicity, coupling constants, exchangeable protons, and the use of DEPT, HSQC, HMBC and NOE data for structure elucidation. Designed to deepen conceptual understanding and application, these MCQs challenge you to analyze spectral patterns, predict spin systems, and interpret long-range correlations that reveal connectivity and stereochemistry. Use this set to test, revise, and strengthen your ability to deduce organic structures confidently from NMR data.

Q1. Which chemical shift range is most typical for an aldehydic proton (–CHO) in 1H NMR?

• 0.5–2.0 ppm
• 2.0–4.5 ppm
• 6.0–8.5 ppm
• 9.0–10.5 ppm

Correct Answer: 9.0–10.5 ppm

Q2. In a proton NMR spectrum of ethyl acetate (CH3CH2O–), the CH3 group adjacent to CH2 appears as which multiplicity?

• Singlet
• Doublet
• Triplet
• Quartet

Correct Answer: Triplet

Q3. DEPT-135 spectrum shows positive peaks for which types of carbon atoms?

• Quaternary carbons only
• CH and CH3 carbons
• CH2 carbons only
• All carbons appear negative

Correct Answer: CH and CH3 carbons

Q4. Which 2D NMR experiment directly correlates proton and carbon through one-bond (1JCH) coupling?

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

Correct Answer: HSQC (or HMQC)

Q5. HMBC is primarily used to detect correlations between protons and carbons separated by how many bonds?

• Only one bond (1J)
• Two and three bonds (2J and 3J)
• Four or more bonds only
• Only through-space contacts

Correct Answer: Two and three bonds (2J and 3J)

Q6. Which observation most strongly indicates the presence of an exchangeable proton (OH or NH) in 1H NMR?

• A sharp multiplet unaffected by D2O
• A broad singlet that disappears on D2O exchange
• A triplet with constant coupling to adjacent protons
• Integration equal to two protons always

Correct Answer: A broad singlet that disappears on D2O exchange

Q7. The Karplus relationship connects a vicinal 3JHH coupling constant to which structural parameter?

• The electronegativity of substituents
• The dihedral angle between the coupled protons
• The chemical shift of the protons
• The number of neighbouring equivalent protons

Correct Answer: The dihedral angle between the coupled protons

Q8. In a 1H NMR spectrum run in CDCl3, the residual solvent peak appears approximately at which chemical shift?

• 1.25 ppm
• 3.50 ppm
• 7.26 ppm
• 9.50 ppm

Correct Answer: 7.26 ppm

Q9. A proton that couples to two non-equivalent neighbouring protons with different J values will most likely appear as:

• A singlet
• Doublet of doublets
• Triplet of triplets
• A broad unresolved multiplet only

Correct Answer: Doublet of doublets

Q10. In 13C NMR, quaternary carbons typically give weaker signals because:

• They have shorter relaxation times and higher intensity
• They lack attached protons so have lower NOE enhancement and longer relaxation
• They are always absent in 13C spectra
• They overlap with proton peaks in 13C NMR

Correct Answer: They lack attached protons so have lower NOE enhancement and longer relaxation

Q11. Which NMR technique is most useful to determine through-space proximity (typically <5 Å) between two protons?

• COSY
• HSQC
• NOESY (or ROESY)
• DEPT-135

Correct Answer: NOESY (or ROESY)

Q12. A monosubstituted benzene ring normally gives what pattern in the 1H NMR aromatic region?

• One sharp singlet
• Three distinct peaks corresponding to pairs of equivalent protons (complex multiplet)
• Seven separate doublets
• No aromatic signals

Correct Answer: Three distinct peaks corresponding to pairs of equivalent protons (complex multiplet)

Q13. In a proton NMR, second-order (non-first-order) splitting appears when:

• The difference in chemical shift (Δν) between coupled nuclei is much larger than coupling constant J
• The Δν is comparable to or smaller than the coupling constant J, causing distortions
• All protons are exchangeable
• The sample is run at very high temperature only

Correct Answer: The Δν is comparable to or smaller than the coupling constant J, causing distortions

Q14. A DEPT-90 experiment will show signals for which carbon types?

• Only CH carbons
• CH and CH3 carbons
• Only quaternary carbons
• Only CH2 carbons

Correct Answer: Only CH carbons

Q15. Which feature of a 1H NMR spectrum directly gives the relative number of protons contributing to a signal?

• Chemical shift
• Multiplicity
• Integration (area under the peak)
• Line width only

Correct Answer: Integration (area under the peak)

Q16. In COSY spectra, a cross-peak indicates:

• Scalar (through-bond) coupling between the two correlated protons
• A through-space NOE interaction
• A direct 1JCH correlation
• Only long-range 4J couplings

Correct Answer: Scalar (through-bond) coupling between the two correlated protons

Q17. Which observation suggests the presence of an aromatic para-disubstituted benzene?

• A single aromatic proton signal
• Two doublets of equal integration in the aromatic region
• Four unrelated singlets between 0–3 ppm
• Multiple broad signals disappearing on D2O

Correct Answer: Two doublets of equal integration in the aromatic region

Q18. In a methyl group attached to an sp2 carbon (vinylic CH3), the expected 1H chemical shift is approximately:

• 0.8–1.2 ppm
• 1.5–2.5 ppm
• 3.5–4.5 ppm
• 6.0–7.0 ppm

Correct Answer: 1.5–2.5 ppm

Q19. A proton showing a doublet of doublet of doublets (ddd) indicates:

• It couples equivalently to three identical protons
• It couples to three non-equivalent protons with three different J values
• It is exchangeable with solvent
• It is a singlet overlapped by noise

Correct Answer: It couples to three non-equivalent protons with three different J values

Q20. Which statement about long-range 4JHH couplings is correct?

• 4JHH couplings are always greater than 12 Hz
• They are often small (0–3 Hz) and may or may not be observed depending on geometry
• They cannot be detected in high-resolution NMR
• They are the primary cause of DEPT signal phases

Correct Answer: They are often small (0–3 Hz) and may or may not be observed depending on geometry

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