Spin–spin coupling MCQs With Answer

Spin–spin coupling MCQs With Answer

by

Spin–spin coupling MCQs With Answer in Modern Pharmaceutical Analytical Techniques are designed to strengthen your interpretation of NMR spectra—an essential skill for M.Pharm students engaged in structure elucidation, impurity profiling, and metabolite identification. This quiz focuses on how neighboring nuclear spins influence signal splitting, the meaning and measurement of coupling constants (J), and how stereochemistry, bond connectivity, and dynamics shape multiplet patterns. You will encounter practical scenarios—ethyl group patterns, aromatic couplings, diastereotopic protons, heteronuclear couplings, and long‑range interactions—along with conceptual depth on first- versus second-order behavior, Karplus relationships, and decoupling strategies. Work through these questions to sharpen your analytical reasoning for pharmaceutical research and advanced spectral problem-solving.

Q1. What is the fundamental origin of spin–spin coupling that produces multiplets in 1H NMR spectra?

  • A dominant through-space dipolar interaction in rapidly tumbling solutions
  • A through-bond interaction (Fermi contact) between nuclear spins transmitted via bonding electrons
  • A consequence of chemical exchange leading to averaged signals
  • An artifact of poor shimming producing line broadening

Correct Answer: A through-bond interaction (Fermi contact) between nuclear spins transmitted via bonding electrons

Q2. Under which condition does the n+1 rule reliably describe multiplicity for a proton coupled to n neighbors?

  • When the spectrometer operates at ≥ 600 MHz
  • When the coupled protons are chemically and magnetically equivalent and Δν/J ≫ 10
  • When 13C satellites are visible
  • When there is fast chemical exchange between sites

Correct Answer: When the coupled protons are chemically and magnetically equivalent and Δν/J ≫ 10

Q3. Which statement about the coupling constant (J) is most accurate?

  • It is measured in ppm and scales with spectrometer frequency
  • It is measured in Hz and is largely independent of spectrometer frequency
  • It is measured in Gauss and depends on the magnetic field drift
  • It is determined only by the solvent viscosity

Correct Answer: It is measured in Hz and is largely independent of spectrometer frequency

Q4. In flexible alkanes at room temperature, a typical vicinal 3J(H,H) value is:

  • 0.5–1.0 Hz
  • 6–8 Hz
  • 12–16 Hz
  • 25–40 Hz

Correct Answer: 6–8 Hz

Q5. In monosubstituted aromatics, which is a typical range for ortho 3J(H,H) coupling?

  • 0–0.5 Hz
  • 1–3 Hz
  • 6–9 Hz
  • 12–16 Hz

Correct Answer: 6–9 Hz

Q6. In aromatic systems, meta 4J(H,H) coupling is generally observed in which range?

  • ~0 Hz; essentially always absent
  • ~0.5–1.0 Hz only at low temperature
  • ~1–3 Hz and often visible in good-resolution spectra
  • ~8–12 Hz, larger than ortho coupling

Correct Answer: ~1–3 Hz and often visible in good-resolution spectra

Q7. The Karplus relationship correlates 3J(H,H) with which structural parameter?

  • Interproton distance only
  • Dihedral angle, with maxima near 0°/180° and a minimum near ~90°
  • Absolute configuration (R/S) independent of geometry
  • Molecular weight

Correct Answer: Dihedral angle, with maxima near 0°/180° and a minimum near ~90°

Q8. What is the typical magnitude (|J|) of geminal 2J(H,H) couplings in simple aliphatic systems?

  • ~0.5–1.5 Hz
  • ~3–5 Hz
  • ~12–16 Hz (sign often not determined in routine 1D spectra)
  • ~30–50 Hz

Correct Answer: ~12–16 Hz (sign often not determined in routine 1D spectra)

Q9. When do first-order (simple) splitting patterns and the n+1 rule generally apply between coupled protons?

  • When Δν (in Hz) ≫ J, typically Δν/J ≥ 10
  • When Δν/J ≤ 1
  • Only in deuterated chloroform
  • Only for isolated spin systems with no long-range couplings

Correct Answer: When Δν (in Hz) ≫ J, typically Δν/J ≥ 10

Q10. A doublet of doublets (dd) arises when a proton:

  • Couples to two equivalent protons with equal J values
  • Couples to two non-equivalent protons with different J values
  • Experiences rapid exchange with solvent
  • Is decoupled from all neighbors

Correct Answer: Couples to two non-equivalent protons with different J values

Q11. The classic ethyl pattern in 1H NMR typically shows:

  • CH3 singlet and CH2 singlet (both decoupled)
  • CH3 triplet and CH2 quartet with the same 3J(H,H) ≈ 7 Hz
  • CH3 quartet and CH2 triplet with different J values
  • CH3 doublet and CH2 doublet due to 13C coupling

Correct Answer: CH3 triplet and CH2 quartet with the same 3J(H,H) ≈ 7 Hz

Q12. In a –CH2– group adjacent to a stereogenic center, the two methylene protons often appear as:

  • Equivalent protons that do not couple to each other
  • Diastereotopic protons (AB) that couple to each other and to neighbors, often forming ABX patterns
  • Averaged singlet due to rapid conformational averaging
  • Quartet due to spin–spin self-coupling

Correct Answer: Diastereotopic protons (AB) that couple to each other and to neighbors, often forming ABX patterns

Q13. Exchangeable OH/NH protons in 1H NMR typically show:

  • Strong, well-resolved couplings in all solvents
  • No coupling because 1H nuclei cannot couple with heteroatoms
  • Broad signals that may not show coupling unless exchange is slowed (dry solvent, low temperature, no acid/base)
  • Invariably a singlet at 1.0 ppm

Correct Answer: Broad signals that may not show coupling unless exchange is slowed (dry solvent, low temperature, no acid/base)

Q14. How does direct 1H–13C coupling typically appear in a standard proton NMR spectrum (without 13C decoupling)?

  • As intense multiplets for every proton due to 100% 13C abundance
  • As weak satellite doublets flanking the main 1H signal with 1J(CH) ~ 120–220 Hz
  • As disappearance of all proton signals
  • As a triplet centered on each 1H resonance

Correct Answer: As weak satellite doublets flanking the main 1H signal with 1J(CH) ~ 120–220 Hz

Q15. A proton directly bonded to 31P commonly shows which feature in 1H NMR?

  • A singlet due to rapid P–H exchange
  • A doublet with a large 1J(PH), often hundreds of Hz
  • A quartet with J ≈ 7 Hz
  • No signal due to decoupling by default

Correct Answer: A doublet with a large 1J(PH), often hundreds of Hz

Q16. In 13C NMR with off-resonance proton decoupling, multiplicities generally appear as:

  • All singlets regardless of attached hydrogens
  • Singlet (quaternary), doublet (CH), triplet (CH2), quartet (CH3)
  • Only triplets for all carbons
  • Unchanged full proton-coupled patterns

Correct Answer: Singlet (quaternary), doublet (CH), triplet (CH2), quartet (CH3)

Q17. Selective proton decoupling (irradiation) during a 1H NMR experiment typically:

  • Collapses only heteronuclear couplings
  • Collapses couplings to the irradiated proton, simplifying multiplets and aiding assignment
  • Reduces T1 relaxation times
  • Increases J values by a constant factor

Correct Answer: Collapses couplings to the irradiated proton, simplifying multiplets and aiding assignment

Q18. Which statement about long-range (4J) allylic coupling in alkenes is most accurate?

  • It is never observed in 1H NMR
  • It often has a small magnitude (~1–3 Hz) and can produce additional fine splitting
  • It is larger than vicinal couplings (~20–30 Hz)
  • It appears only at spectrometer frequencies ≥ 800 MHz

Correct Answer: It often has a small magnitude (~1–3 Hz) and can produce additional fine splitting

Q19. Which observation most strongly indicates second-order (non-first-order) coupling in a pair of interacting protons?

  • Perfectly symmetric doublets with equal intensities
  • “Roofing” (leaning) of lines in an AB pattern as Δν becomes comparable to J
  • Total collapse to a singlet
  • Appearance of 13C satellites

Correct Answer: “Roofing” (leaning) of lines in an AB pattern as Δν becomes comparable to J

Q20. In spin-system notation, an AX system is characterized by:

  • Two protons with Δν ≈ J, producing severe second-order effects
  • Two magnetically equivalent protons with no coupling
  • Two coupled protons where Δν (in Hz) ≫ J, yielding first-order patterns
  • A three-spin system requiring 2D NMR

Correct Answer: Two coupled protons where Δν (in Hz) ≫ J, yielding first-order patterns

Leave a Comment