13C NMR basics MCQs With Answer

13C NMR basics MCQs With Answer is designed for M.Pharm students preparing for advanced instrumental analysis exams. This short quiz set focuses on core concepts of carbon-13 nuclear magnetic resonance spectroscopy: isotope properties, chemical shift ranges, decoupling techniques, sensitivity and relaxation behavior, spectral editing (DEPT), and common 2D correlations (HSQC/HMBC). Questions emphasize practical interpretation of spectra for pharmaceutical compounds, experimental parameters affecting signal intensity and quantitation, and typical chemical shift regions for functional groups. These MCQs will help reinforce theory, improve problem-solving ability in structure elucidation, and consolidate knowledge relevant to analytical characterization of drug molecules.

Q1. Which of the following is the natural abundance of the 13C isotope relevant for NMR?

• 0.011% (0.00011)
• 1.1%
• 11%
• 27%

Correct Answer: 1.1%

Q2. The reference compound commonly used to set the 13C chemical shift scale to 0 ppm is:

• Chloroform (CHCl3)
• Tetramethylsilane (TMS)
• Deuterated water (D2O)
• Acetone-d6

Correct Answer: Tetramethylsilane (TMS)

Q3. In a broadband proton-decoupled 13C NMR spectrum, a carbon directly bonded to protons typically appears as:

• A multiplet due to 13C-1H coupling
• A singlet because 1H coupling is removed
• A doublet of doublets from long-range couplings
• An unresolved baseline hump

Correct Answer: A singlet because 1H coupling is removed

Q4. Which chemical shift range is most typical for sp3 aliphatic carbons in 13C NMR?

• 0–50 ppm
• 50–100 ppm
• 100–160 ppm
• 160–220 ppm

Correct Answer: 0–50 ppm

Q5. In a DEPT-135 experiment, which signal phases are observed for CH, CH2 and CH3 carbons respectively?

• CH positive, CH2 positive, CH3 negative
• CH positive, CH2 negative, CH3 positive
• CH negative, CH2 positive, CH3 positive
• All signals are suppressed

Correct Answer: CH positive, CH2 negative, CH3 positive

Q6. Quaternary carbons (no attached protons) in a 13C spectrum typically show:

• Very strong signals due to high NOE
• Weaker signals or may be missing in DEPT due to no attached protons
• Characteristic doublet splitting from 13C-13C coupling
• Large chemical shifts near 0 ppm

Correct Answer: Weaker signals or may be missing in DEPT due to no attached protons

Q7. Which technique is most suitable to correlate directly bonded 1H and 13C nuclei in a 2D NMR experiment?

• NOESY
• HSQC (Heteronuclear Single Quantum Coherence)
• HMBC (Heteronuclear Multiple Bond Correlation)
• TOCSY

Correct Answer: HSQC (Heteronuclear Single Quantum Coherence)

Q8. Typical one-bond 1JCH coupling constants observed in 13C NMR are in the range of:

• 0.5–5 Hz
• 5–20 Hz
• 100–280 Hz
• 500–1000 Hz

Correct Answer: 100–280 Hz

Q9. The CDCl3 solvent produces a characteristic 13C triplet near which ppm value?

• 7 ppm
• 77 ppm
• 118 ppm
• 225 ppm

Correct Answer: 77 ppm

Q10. Why are 13C NMR signals generally much weaker than 1H signals?

• Because 13C has a lower gyromagnetic ratio and lower natural abundance
• Because carbon nuclei do not interact with radiofrequency fields
• Because 13C spectra are always recorded at lower magnetic fields
• Because 13C has a larger T1 leading to signal cancellation

Correct Answer: Because 13C has a lower gyromagnetic ratio and lower natural abundance

Q11. The nuclear Overhauser effect (NOE) in proton-decoupled 13C experiments typically:

• Decreases signal intensity of protonated carbons
• Increases intensity of protonated carbons relative to quaternary carbons
• Has no effect on intensities
• Only affects 13C-13C coupling patterns

Correct Answer: Increases intensity of protonated carbons relative to quaternary carbons

Q12. For quantitative 13C NMR (accurate integrals), which practice is recommended?

• Use short delay times to maximize throughput
• Use inverse-gated decoupling and long relaxation delays (≥5×T1)
• Always use DEPT-135 for integration
• Record only a few scans to avoid NOE

Correct Answer: Use inverse-gated decoupling and long relaxation delays (≥5×T1)

Q13. Which carbon chemical shift region is most indicative of carbonyl carbons (ketones, aldehydes, esters, amides) in 13C NMR?

• 0–50 ppm
• 50–100 ppm
• 100–160 ppm
• 160–220 ppm

Correct Answer: 160–220 ppm

Q14. Satellite peaks in a 1H NMR spectrum caused by 13C coupling are typically observed because:

• 13C is highly abundant and dominates the spectrum
• Natural abundance 13C couples to adjacent protons producing small satellites
• They arise from impurities only
• They indicate decomposition of the sample

Correct Answer: Natural abundance 13C couples to adjacent protons producing small satellites

Q15. Which of the following statements about DEPT-90 is correct?

• DEPT-90 shows only CH2 carbons
• DEPT-90 shows only CH carbons
• DEPT-90 shows CH and CH3 but not CH2
• DEPT-90 shows quaternary carbons only

Correct Answer: DEPT-90 shows only CH carbons

Q16. Which experimental add-in is commonly used to shorten 13C T1 relaxation times and speed up acquisition?

• Adding residual water
• Adding a paramagnetic relaxation agent such as Cr(acac)3
• Lowering the magnetic field strength
• Using non-deuterated solvent

Correct Answer: Adding a paramagnetic relaxation agent such as Cr(acac)3

Q17. In an HMBC experiment, the cross-peaks show correlations between:

• Directly bonded 1H and 13C only (one bond)
• 1H and 13C separated by two and three bonds (long-range)
• Only 13C-13C couplings
• Only through-space 1H-1H NOE contacts

Correct Answer: 1H and 13C separated by two and three bonds (long-range)

Q18. Which statement best explains why integration of 13C NMR signals is generally not reliable without special care?

• Because 13C resonances all have identical relaxation times
• Because NOE, different T1 relaxation times and decoupling conditions affect signal intensities
• Because 13C atoms do not produce measurable magnetization
• Because spectrometers do not record signal amplitudes for 13C

Correct Answer: Because NOE, different T1 relaxation times and decoupling conditions affect signal intensities

Q19. Which type of 13C-13C coupling is usually observable in natural abundance samples without isotopic enrichment?

• Strong one-bond 13C-13C coupling across all carbons
• No observable 13C-13C coupling except very weak satellites due to low abundance
• Large long-range 13C-13C couplings up to 100 Hz
• 13C-13C couplings that collapse upon proton decoupling

Correct Answer: No observable 13C-13C coupling except very weak satellites due to low abundance

Q20. A quaternary aromatic carbon in a proton-decoupled 13C spectrum will most likely:

• Appear with the highest intensity due to proton enhancement
• Be absent in a standard proton-decoupled 13C spectrum
• Appear but often with lower intensity and slower relaxation than protonated aromatic carbons
• Always appear at exactly 0 ppm

Correct Answer: Appear but often with lower intensity and slower relaxation than protonated aromatic carbons

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