Coupling constant (J value) MCQs With Answer

Coupling constant (J value) MCQs With Answer

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Introduction

The coupling constant (J value) is a cornerstone parameter in 1H and 13C NMR that helps M. Pharm students decode molecular connectivity, conformation, and stereochemistry in pharmaceutical analysis. Understanding how J arises from scalar spin–spin coupling, why it is reported in Hertz (Hz), and how it varies with dihedral angle, hybridization, and ring conformation enables precise structural assignments. This MCQ set focuses on practical skills—estimating J from spectra, distinguishing first- and second-order patterns, interpreting aromatic and alkenic couplings, and leveraging heteronuclear couplings such as 1JCH and 1JPH. Work through these questions to strengthen your command of Modern Pharmaceutical Analytical Techniques and translate NMR theory into confident, real-world structure elucidation.

Q1. In NMR spectroscopy, the coupling constant (J value) is best defined as:

  • The separation in Hz between components of a multiplet caused by scalar spin–spin coupling
  • The difference in ppm between two chemical shifts
  • The longitudinal relaxation rate of a nucleus
  • The intrinsic linewidth at half-height (FWHM)

Correct Answer: The separation in Hz between components of a multiplet caused by scalar spin–spin coupling

Q2. What are the correct units for reporting a coupling constant (J)?

  • Hertz (Hz)
  • Parts per million (ppm)
  • Tesla (T)
  • Gauss (G)

Correct Answer: Hertz (Hz)

Q3. Which statement about J values and magnetic field strength is TRUE?

  • J is independent of spectrometer field strength, while ppm splitting changes with field
  • J increases linearly with the external magnetic field
  • J decreases as the field strength increases
  • J is measured in ppm and therefore changes with field

Correct Answer: J is independent of spectrometer field strength, while ppm splitting changes with field

Q4. On a 400 MHz 1H NMR, two lines of a doublet are separated by 0.02 ppm. What is J?

  • 0.02 Hz
  • 8 Hz
  • 20 Hz
  • 0.8 Hz

Correct Answer: 8 Hz

Q5. A typical vicinal trans 3J(H–H) across an alkene is closest to:

  • 12–18 Hz
  • 2–4 Hz
  • 0–1 Hz
  • 3–5 Hz

Correct Answer: 12–18 Hz

Q6. A typical vicinal cis 3J(H–H) across an alkene is closest to:

  • 6–12 Hz
  • 12–18 Hz
  • 1–3 Hz
  • 20–30 Hz

Correct Answer: 6–12 Hz

Q7. The typical magnitude of a geminal 2J(H–H) in an aliphatic CH2 group is:

  • 12–18 Hz
  • 0–3 Hz
  • 30–50 Hz
  • 3–5 Hz

Correct Answer: 12–18 Hz

Q8. The Karplus relationship describes how 3J(H–H) varies primarily with:

  • The dihedral angle via a cosine-based function (cos θ and cos² θ terms)
  • Interproton distance only
  • The external magnetic field strength
  • The sample concentration

Correct Answer: The dihedral angle via a cosine-based function (cos θ and cos² θ terms)

Q9. In aromatic systems, the typical ortho 3J(H–H) in a monosubstituted benzene ring is:

  • 6–9 Hz
  • 0–1 Hz
  • 1–3 Hz
  • 12–18 Hz

Correct Answer: 6–9 Hz

Q10. Long-range allylic (W) 4J(H–H) couplings are commonly observed with magnitudes around:

  • 0.5–3 Hz
  • 6–12 Hz
  • 12–18 Hz
  • 20–30 Hz

Correct Answer: 0.5–3 Hz

Q11. A doublet of doublets (dd) pattern in 1H NMR most commonly indicates that a proton:

  • Is coupled to two nonequivalent neighbors with different J values
  • Is coupled to two equivalent neighbors with the same J
  • Shows second-order coupling to one neighbor only
  • Is coupled to three equivalent neighbors (n+1 rule)

Correct Answer: Is coupled to two nonequivalent neighbors with different J values

Q12. A typical one-bond heteronuclear coupling 1J(C–H) has what approximate magnitude?

  • ~1–3 Hz
  • ~7 Hz
  • ~125–220 Hz
  • ~700–1000 Hz

Correct Answer: ~125–220 Hz

Q13. Typical one-bond 1J(P–H) couplings in phosphines or PH groups are approximately:

  • ~7 Hz
  • ~125 Hz
  • ~700 Hz
  • ~2 Hz

Correct Answer: ~700 Hz

Q14. Which feature warns that Δν/J is small and first-order extraction of J may be unreliable?

  • “Rooftop” (leaning) multiplets with distorted intensities
  • Field independence of J
  • Broadband decoupling is applied
  • Use of TMS as internal reference

Correct Answer: “Rooftop” (leaning) multiplets with distorted intensities

Q15. Which factor has the least direct effect on the magnitude of a scalar coupling constant (J)?

  • Dihedral angle between coupled nuclei
  • Hybridization of the atoms along the coupling pathway
  • Electron-withdrawing substituents influencing bonding
  • External magnetic field strength

Correct Answer: External magnetic field strength

Q16. For vicinal couplings in cyclohexane, which relationship is typically largest?

  • Axial–axial 3J(H–H)
  • Axial–equatorial 3J(H–H)
  • Equatorial–equatorial 3J(H–H)
  • All are approximately equal

Correct Answer: Axial–axial 3J(H–H)

Q17. On a 600 MHz 1H NMR, the separation within a multiplet is 0.015 ppm. The coupling constant J is:

  • 0.015 Hz
  • 9 Hz
  • 4 Hz
  • 0.9 Hz

Correct Answer: 9 Hz

Q18. If a proton’s coupling partner is selectively decoupled (irradiated), the observed multiplet for that coupling collapses to a:

  • Singlet
  • Doublet
  • Triplet
  • Quartet

Correct Answer: Singlet

Q19. Which statement correctly distinguishes multiplicity from coupling constant?

  • Multiplicity shows the number and equivalence of neighbors; J gives the line separation in Hz
  • Multiplicity depends on field strength; J does not
  • J equals the number of lines in the multiplet
  • J equals the chemical shift difference between two nuclei

Correct Answer: Multiplicity shows the number and equivalence of neighbors; J gives the line separation in Hz

Q20. When two coupled protons form an AB system (Δν ≈ J), what is typically observed?

  • A first-order doublet with exact separation equal to J
  • A second-order pattern with leaning and unequal intensities
  • A single averaged singlet with no coupling
  • No observable coupling due to cancellation

Correct Answer: A second-order pattern with leaning and unequal intensities

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