Introduction to 13C NMR MCQs With Answer

Introduction to 13C NMR MCQs With Answer

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Introduction to 13C NMR MCQs With Answer

13C NMR spectroscopy is a cornerstone of structural elucidation in modern pharmaceutical analytical techniques. For M. Pharm students, mastering 13C NMR means understanding not just chemical shift ranges, but also decoupling strategies, relaxation behavior, NOE effects, and multiplicity-editing experiments like DEPT and APT. These concepts are crucial for identifying carbon environments in APIs, impurities, and excipients with confidence. This quiz set focuses on practical and interpretive aspects of 13C NMR—signal patterns, quantitative acquisition, solvent selection, referencing, and common pulse programs—tailored to real analytical challenges in pharmaceutical R&D and QC. Work through the questions to consolidate your understanding and improve data interpretation speed and accuracy.

Q1. What is the natural abundance of 13C in nature?

  • 0.01%
  • 1.1%
  • 10%
  • 50%

Correct Answer: 1.1%

Q2. The most important reason 13C NMR is less sensitive than 1H NMR is:

  • Lower gyromagnetic ratio and low natural abundance of 13C
  • Faster T1 relaxation of 13C
  • High sample viscosity
  • Excessive line broadening from 13C–13C coupling

Correct Answer: Lower gyromagnetic ratio and low natural abundance of 13C

Q3. In broadband proton-decoupled 13C spectra, most carbon signals appear as:

  • Multiplets due to 1JCH coupling
  • Singlets often enhanced by NOE
  • Triplets due to deuterium coupling
  • Doublets only for CH carbons

Correct Answer: Singlets often enhanced by NOE

Q4. The primary purpose of inverse-gated (gated) decoupling in 13C NMR is to:

  • Maximize NOE for higher sensitivity
  • Suppress NOE to obtain accurate quantitative integrals
  • Resolve long-range 13C–13C couplings
  • Shorten acquisition time by reducing T1

Correct Answer: Suppress NOE to obtain accurate quantitative integrals

Q5. In a DEPT-135 experiment, which carbons typically appear with negative phase?

  • Quaternary carbons
  • CH2 carbons
  • CH carbons
  • CH3 carbons

Correct Answer: CH2 carbons

Q6. Which statement about DEPT-90 is correct?

  • Shows only CH carbons
  • Shows CH and CH3 carbons
  • Shows CH2 carbons only
  • Shows quaternary carbons with enhanced intensity

Correct Answer: Shows only CH carbons

Q7. In an APT (Attached Proton Test) spectrum, which carbons are typically positive in phase?

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

Correct Answer: CH and CH3

Q8. Which chemical shift range is most typical for aldehyde/ketone carbonyl carbons in 13C NMR?

  • 0–50 ppm
  • 60–90 ppm
  • 100–150 ppm
  • 190–220 ppm

Correct Answer: 190–220 ppm

Q9. The prominent signal around 77 ppm observed in 13C NMR spectra recorded in CDCl3 is due to:

  • TMS internal standard
  • Residual water
  • Solvent CDCl3 carbon (appearing as a triplet)
  • Carbonyl impurity

Correct Answer: Solvent CDCl3 carbon (appearing as a triplet)

Q10. Typical one-bond 13C–1H coupling constants (1JCH) have magnitudes approximately:

  • 5–15 Hz
  • 20–40 Hz
  • 125–200 Hz
  • 350–500 Hz

Correct Answer: 125–200 Hz

Q11. In off-resonance decoupled 13C NMR, a methyl carbon (CH3) typically appears as a:

  • Singlet
  • Doublet
  • Triplet
  • Quartet

Correct Answer: Quartet

Q12. Which carbons benefit most from NOE enhancement in proton-decoupled 13C NMR?

  • Quaternary carbons
  • All carbons equally
  • Protonated carbons (CH, CH2, CH3)
  • Carbons adjacent to heteroatoms only

Correct Answer: Protonated carbons (CH, CH2, CH3)

Q13. For quantitative 13C NMR, the relaxation delay (D1) is ideally set to:

  • ≤ 1 × longest T1
  • About 2 × longest T1
  • ≥ 5 × longest T1
  • Irrelevant; any short delay works with decoupling

Correct Answer: ≥ 5 × longest T1

Q14. The number of distinct 13C signals in a spectrum generally corresponds to:

  • The total number of carbon atoms in the formula
  • The number of symmetry-unique carbon environments
  • The number of protonated carbons only
  • The number of functional groups present

Correct Answer: The number of symmetry-unique carbon environments

Q15. The standard internal reference used to set 0 ppm in 13C NMR is:

  • Sodium 3-(trimethylsilyl)propionate (TSP)
  • Tetramethylsilane (TMS)
  • Adamantane
  • Chloroform-d (CDCl3)

Correct Answer: Tetramethylsilane (TMS)

Q16. Why are deuterated solvents (e.g., CDCl3, DMSO-d6) used in 13C NMR?

  • To speed up 13C relaxation
  • To provide a deuterium lock and reduce proton background signals
  • To enhance NOE on carbonyl carbons
  • To eliminate the need for shimming

Correct Answer: To provide a deuterium lock and reduce proton background signals

Q17. Which experiment distinguishes CH, CH2, and CH3 groups while suppressing quaternary carbons?

  • APT
  • DEPT
  • COSY
  • HSQC-TOCSY

Correct Answer: DEPT

Q18. Which is a commonly used broadband proton decoupling scheme in 13C NMR?

  • WALTZ-16
  • COSY
  • NOESY
  • RDEPT

Correct Answer: WALTZ-16

Q19. Which ordering correctly lists typical 13C chemical shift regions from lowest to highest ppm?

  • sp2 (alkenes/aromatics) < sp3 (alkyl) < carbonyl
  • sp3 (alkyl) < sp2 (alkenes/aromatics) < carbonyl
  • carbonyl < sp3 (alkyl) < sp2 (alkenes/aromatics)
  • sp3 (alkyl) < carbonyl < sp2 (alkenes/aromatics)

Correct Answer: sp3 (alkyl) < sp2 (alkenes/aromatics) < carbonyl

Q20. Why are 13C–13C couplings rarely observed in routine 13C NMR spectra?

  • They are fully removed by proton decoupling
  • 13C–13C couplings are intrinsically zero
  • The probability of adjacent 13C nuclei is very low due to 1.1% abundance
  • Field inhomogeneity masks them

Correct Answer: The probability of adjacent 13C nuclei is very low due to 1.1% abundance

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