Principles of Carbon-13 (13C) Nuclear Magnetic Resonance spectroscopy MCQs With Answer

Principles of Carbon-13 (13C) Nuclear Magnetic Resonance spectroscopy MCQs With Answer

Carbon-13 NMR is an essential analytical tool for B.Pharm students to identify carbon environments, determine molecular frameworks and analyze functional groups in organic and pharmaceutical compounds. This introduction focuses on 13C NMR fundamentals: chemical shift ranges (ppm), shielding/deshielding, 1JCH and long-range coupling, proton decoupling, DEPT experiments, HSQC/HMBC 2D techniques, sensitivity issues due to low natural abundance (~1.1%), relaxation (T1), and practical aspects like solvents (CDCl3) and referencing (TMS). Mastering these keywords—13C NMR, chemical shift, DEPT, HSQC, HMBC, proton decoupling, NOE—builds confidence for spectral interpretation. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which compound is commonly used as the reference standard for reporting 13C chemical shifts?

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

Correct Answer: Tetramethylsilane (TMS)

Q2. What is the natural isotopic abundance of carbon-13 in organic samples?

• 0.01%
• 1.1%
• 10.5%
• 50%

Correct Answer: 1.1%

Q3. In 13C NMR, which approximate chemical shift range corresponds to aromatic and alkene carbons?

• 0–50 ppm
• 50–90 ppm
• 100–160 ppm
• 160–220 ppm

Correct Answer: 100–160 ppm

Q4. Typical one-bond 13C–1H coupling constants (1JCH) fall in which range?

• 0–5 Hz
• 10–50 Hz
• 125–250 Hz
• 400–600 Hz

Correct Answer: 125–250 Hz

Q5. Which DEPT experiment displays signals only for CH carbons?

• DEPT-45
• DEPT-90
• DEPT-135
• DEPT-270

Correct Answer: DEPT-90

Q6. What is the primary purpose of broadband proton decoupling in routine 13C NMR?

• To increase the number of resonance lines
• To split 13C peaks into multiplets
• To collapse carbon–hydrogen coupling and simplify spectra
• To measure 13C–13C coupling constants precisely

Correct Answer: To collapse carbon–hydrogen coupling and simplify spectra

Q7. Why is direct integration of peak areas in routine 13C NMR generally not quantitative?

• Because 13C peaks are always identical in intensity
• Due to varying relaxation times and Nuclear Overhauser Effects (NOE)
• Because chemical shifts change peak areas
• Because 13C has no magnetic moment

Correct Answer: Due to varying relaxation times and Nuclear Overhauser Effects (NOE)

Q8. Which technique prevents NOE build-up during acquisition to allow more quantitative 13C measurements?

• Broadband proton decoupling
• Inverse gated decoupling
• DEPT-135
• CP-MAS

Correct Answer: Inverse gated decoupling

Q9. Which 2D heteronuclear experiment correlates carbons with protons over two and three bonds (long-range correlations)?

• HSQC
• NOESY
• HMBC
• TOCSY

Correct Answer: HMBC

Q10. What solid-state technique is commonly used to obtain high-resolution 13C spectra of powders and solids?

• DEPT-90
• CP-MAS (cross polarization magic angle spinning)
• INEPT
• HSQC

Correct Answer: CP-MAS (cross polarization magic angle spinning)

Q11. Which of the following is a principal advantage of 13C NMR in structural elucidation?

• It gives high natural sensitivity without enhancement
• It directly counts hydrogen atoms accurately
• It reveals the number of distinct carbon environments in a molecule
• It replaces the need for mass spectrometry

Correct Answer: It reveals the number of distinct carbon environments in a molecule

Q12. In proton (1H) NMR, small satellite peaks often appear due to coupling with which isotope?

• 12C
• 13C
• 14N
• 2H (deuterium)

Correct Answer: 13C

Q13. Compared to 1H nuclei, 13C relaxation times (T1) are generally:

• Much shorter
• Similar in magnitude
• Longer
• Zero

Correct Answer: Longer

Q14. Low natural abundance of 13C primarily results in which practical issue for NMR experiments?

• Excessive signal-to-noise ratio
• Poor spectral resolution
• Poor sensitivity requiring more scans or enhancements
• Inability to observe any carbon signals

Correct Answer: Poor sensitivity requiring more scans or enhancements

Q15. Chemical shift anisotropy (CSA) affects 13C spectra most strongly in which situation?

• Fast-tumbling small molecules in solution
• High-symmetry molecules with identical carbons
• Solid-state samples and slow tumbling systems
• When using TMS as solvent

Correct Answer: Solid-state samples and slow tumbling systems

Q16. Which polarization transfer method enhances sensitivity of low-γ nuclei like 13C in solution by using 1H magnetization?

• CP-MAS
• INEPT
• DEPT-135
• NOESY

Correct Answer: INEPT

Q17. In 13C NMR the term “downfield” corresponds to which change in chemical shift?

• Lower ppm values and more shielding
• Higher ppm values and more deshielding
• No change in ppm
• Negative ppm values only

Correct Answer: Higher ppm values and more deshielding

Q18. Carbonyl carbons (e.g., ketones, aldehydes) typically appear in which 13C chemical shift region?

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

Correct Answer: 160–220 ppm

Q19. In a DEPT-135 spectrum which type of carbon gives a negative-phase signal?

• Quaternary carbon (C without H)
• CH (methine)
• CH2 (methylene)
• CH3 (methyl)

Correct Answer: CH2 (methylene)

Q20. Which deuterated solvent is most commonly used for routine 13C NMR of organic molecules?

• D2O
• CDCl3
• DMSO (non-deuterated)
• Acetone

Correct Answer: CDCl3

Q21. Chemical shifts in NMR are reported in which units?

• Hertz (Hz)
• Parts per million (ppm)
• Tesla (T)
• Degrees Celsius (°C)

Correct Answer: Parts per million (ppm)

Q22. Which experiment provides one-bond 1H–13C correlations and is especially useful for assigning CH, CH2, CH3 groups?

• HMBC
• HSQC
• DEPT-90
• NOESY

Correct Answer: HSQC

Q23. Long-range heteronuclear 2–3 bond correlations are most effectively observed using which technique?

• HSQC (Heteronuclear Single Quantum Coherence)
• HMBC (Heteronuclear Multiple Bond Correlation)
• DEPT-45
• CP-MAS

Correct Answer: HMBC (Heteronuclear Multiple Bond Correlation)

Q24. Why do quaternary carbons (no attached protons) often produce weaker signals than protonated carbons in 13C NMR?

• They have larger NOE enhancements
• They relax faster and thus appear stronger
• They receive less NOE and often have longer T1 values, reducing observed intensity
• Quaternary carbons do not resonate in 13C NMR

Correct Answer: They receive less NOE and often have longer T1 values, reducing observed intensity

Q25. What is the effect of applying broadband proton decoupling during 13C acquisition?

• Carbon signals split into multiplets by 1H coupling
• Carbon signals collapse to singlets and signal intensity often increases
• All 13C signals disappear
• Only quaternary carbons are observed

Correct Answer: Carbon signals collapse to singlets and signal intensity often increases

Q26. Which DEPT experiment differentiates CH3 and CH signals (positive phase) from CH2 signals (negative phase)?

• DEPT-45 only
• DEPT-90 only
• DEPT-135
• DEPT-270 only

Correct Answer: DEPT-135

Q27. Why is direct observation of 13C–13C coupling uncommon in natural-abundance samples?

• Because 13C–13C coupling constants are zero
• Because 13C has no spin
• Because low 13C natural abundance (~1.1%) makes adjacent 13C–13C pairs rare
• Because coupling only occurs with hydrogen

Correct Answer: Because low 13C natural abundance (~1.1%) makes adjacent 13C–13C pairs rare

Q28. Which single practical change most directly improves signal-to-noise ratio in a 13C NMR experiment?

• Decreasing the number of scans
• Increasing the number of scans
• Using non-deuterated solvents
• Shortening relaxation delay to zero

Correct Answer: Increasing the number of scans

Q29. For accurate quantitative 13C intensities, which combination of settings is recommended?

• Short relaxation delay and broadband decoupling during acquisition
• Long relaxation delay and inverse gated decoupling
• DEPT-90 with no decoupling
• CP-MAS with continuous decoupling

Correct Answer: Long relaxation delay and inverse gated decoupling

Q30. What is the typical overall chemical shift window encountered in routine 13C NMR spectra of organic compounds?

• -10 to +10 ppm
• 0 to 50 ppm
• 0 to 220 ppm
• 500 to 1000 ppm

Correct Answer: 0 to 220 ppm

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com