Introduction: Coupling constant and spin-spin coupling are core concepts in proton NMR that help B. Pharm students interpret molecular structure and stereochemistry. The coupling constant (J, in Hz) quantifies scalar or spin-spin coupling between nuclei and produces multiplet splitting patterns that reveal the number and relationship of neighboring protons. Understanding vicinal (3J), geminal (2J), long-range (4J+) and heteronuclear couplings (e.g., 1JCH, 2JHP) alongside chemical shift enables accurate peak assignment and conformational analysis using the Karplus relationship. Mastery of coupling constants, multiplicity, and first- versus second-order effects is essential for drug analysis and structure elucidation. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the coupling constant (J) in NMR?
- The frequency difference between split peaks measured in Hertz
- The chemical shift of a nucleus measured in ppm
- The intensity ratio of multiplet peaks
- The relaxation time of the nucleus
Correct Answer: The frequency difference between split peaks measured in Hertz
Q2. Which type of coupling is usually measured as 3J in 1H NMR?
- Geminal coupling
- Vicinal coupling
- Long-range coupling
- Direct one-bond coupling
Correct Answer: Vicinal coupling
Q3. According to the n+1 rule for first-order spectra, a proton with two equivalent neighboring protons appears as:
- A singlet
- A doublet
- A triplet
- A quartet
Correct Answer: A triplet
Q4. Which unit is used for reporting coupling constants?
- ppm
- Hz
- mT
- kDa
Correct Answer: Hz
Q5. Geminal coupling between two hydrogens on the same carbon is denoted as:
- 1J
- 2J
- 3J
- 4J
Correct Answer: 2J
Q6. The Karplus equation relates vicinal 3J coupling to which structural parameter?
- Bond length between the nuclei
- Dihedral angle between coupled protons
- Electronegativity of substituents
- Solvent dielectric constant
Correct Answer: Dihedral angle between coupled protons
Q7. Typical trans-vicinal 1H-1H coupling constants across a double bond are approximately:
- 0–3 Hz
- 6–12 Hz
- 12–18 Hz
- 20–30 Hz
Correct Answer: 12–18 Hz
Q8. Which phenomenon causes deviation from simple n+1 splitting, producing multiplets like doublet of doublets?
- Equivalent neighboring protons only
- Different coupling constants to two non-equivalent neighbors
- Complete signal overlap with solvent peaks
- Absence of spin-spin coupling
Correct Answer: Different coupling constants to two non-equivalent neighbors
Q9. In aromatic systems, ortho 3J coupling values are typically around:
- 0–1 Hz
- 1–3 Hz
- 6–10 Hz
- 15–20 Hz
Correct Answer: 6–10 Hz
Q10. Long-range 4J or higher couplings are usually:
- Larger than 20 Hz
- Negligible and always zero
- Small but sometimes observable (0–3 Hz)
- Equal to chemical shift differences
Correct Answer: Small but sometimes observable (0–3 Hz)
Q11. What does a doublet of doublets indicate about a proton’s couplings?
- It couples to two equivalent protons with same J
- It couples to two non-equivalent protons with different J values
- It is coupled only to heteronuclei
- It is not coupled to any protons
Correct Answer: It couples to two non-equivalent protons with different J values
Q12. Which experiment removes proton-proton coupling to simplify 1H spectra?
- DEPT
- 13C broadband decoupling
- Proton decoupling
- Selective homonuclear decoupling
Correct Answer: Selective homonuclear decoupling
Q13. One-bond heteronuclear coupling between 1H and 13C (1JCH) is typically:
- Very small (~0–2 Hz)
- Moderate (~5–10 Hz)
- Large (~120–250 Hz)
- Unmeasurable in NMR
Correct Answer: Large (~120–250 Hz)
Q14. Which statement about first-order coupling is true?
- Peak intensities follow simple Pascal’s triangle ratios
- All multiplets are always second-order
- Coupling constants cannot be measured
- Chemical shift differences are zero
Correct Answer: Peak intensities follow simple Pascal’s triangle ratios
Q15. Second-order effects occur when:
- The coupling constants are much larger than chemical shift differences
- Chemical shift differences are comparable to coupling constants
- There is no coupling
- Only heteronuclei are observed
Correct Answer: Chemical shift differences are comparable to coupling constants
Q16. How can J-coupling help determine stereochemistry of vicinal protons in an alkane chain?
- J values are independent of stereochemistry
- Via Karplus relationship connecting J to dihedral angle
- By measuring chemical shifts only
- By integration of peaks
Correct Answer: Via Karplus relationship connecting J to dihedral angle
Q17. In a 1H NMR, two equivalent protons give:
- Different chemical shifts but same coupling
- A single combined signal with the same splitting pattern as one proton
- Two separate multiplets always
- No observable signal
Correct Answer: A single combined signal with the same splitting pattern as one proton
Q18. Which coupling is observed between a proton and a fluorine atom (1H-19F)?
- Homonuclear coupling
- Scalar heteronuclear coupling that can be large
- No coupling due to different nuclei
- Only dipolar coupling in solution
Correct Answer: Scalar heteronuclear coupling that can be large
Q19. If a proton shows a quartet with coupling constant 7 Hz, how many equivalent neighbors does it have?
- 0
- 1
- 2
- 3
Correct Answer: 3
Q20. Which of the following affects the magnitude of 3JHH coupling?
- Dihedral angle, hybridization, and electronegativity of substituents
- Only the boiling point of the solvent
- Only the temperature of the NMR probe
- Crystal lattice parameters
Correct Answer: Dihedral angle, hybridization, and electronegativity of substituents
Q21. Ortho coupling in monosubstituted benzene is mainly which J type?
- 2J (geminal)
- 3J (vicinal, ortho)
- 4J (meta)
- 1J (direct bond)
Correct Answer: 3J (vicinal, ortho)
Q22. What is indicated by a multiplet centered at a chemical shift with very small splitting (<1 Hz)?
- Strong spin-spin coupling with large J
- Possible long-range coupling or second-order effect
- No coupling at all
- Instrument malfunction
Correct Answer: Possible long-range coupling or second-order effect
Q23. Which method gives coupling constant values most directly from the spectrum?
- Measuring distance between peak maxima in Hz
- Comparing integrals only
- Estimating using molecular weight
- Using UV-Vis spectra
Correct Answer: Measuring distance between peak maxima in Hz
Q24. A doublet with J = 7 Hz coupling to another doublet with J = 7 Hz—what pattern arises if the two protons are mutually coupled?
- Two singlets
- Two triplets of equal J
- A doublet of doublets with unequal J
- A multiplet of unresolved shape unrelated to J
Correct Answer: Two triplets of equal J
Q25. Which coupling is most useful for distinguishing diastereotopic protons?
- 1JCH only
- 2J (geminal) and different 3J values to neighbors
- No coupling can distinguish them
- IR spectroscopy peaks
Correct Answer: 2J (geminal) and different 3J values to neighbors
Q26. In a first-order spectrum, peak separations within a multiplet are equal to:
- The chemical shift difference in ppm
- The coupling constant(s) in Hz
- The integral value
- The magnetic field strength in Tesla
Correct Answer: The coupling constant(s) in Hz
Q27. Which statement about scalar (J) coupling is correct?
- It is transmitted through space only
- It is mediated through chemical bonds (scalar coupling)
- It does not depend on molecular conformation
- It is independent of isotopic composition
Correct Answer: It is mediated through chemical bonds (scalar coupling)
Q28. How does heteronuclear decoupling (e.g., 13C{1H}) affect the 13C spectrum?
- It leaves coupling multiplets unchanged
- It collapses 13C–1H multiplets to singlets
- It increases the number of peaks
- It only affects 1H spectra
Correct Answer: It collapses 13C–1H multiplets to singlets
Q29. Which observation suggests two protons are chemically equivalent?
- They have identical chemical shifts and identical coupling patterns to other nuclei
- They show different integrals
- They display different J values to the same neighbors
- They appear at widely separated chemical shifts
Correct Answer: They have identical chemical shifts and identical coupling patterns to other nuclei
Q30. In stereochemical analysis, a large vicinal 3JHH (approx. 10–14 Hz) usually indicates which dihedral angle range per Karplus?
- ~0° (syn-periplanar)
- ~60° (gauche)
- ~120° (gauche/antiperiplanar mix)
- ~180° (anti-periplanar)
Correct Answer: ~180° (anti-periplanar)
