Spin-spin coupling and coupling constant MCQs With Answer

Spin-spin coupling and coupling constant MCQs With Answer

Introduction: This quiz set focuses on spin-spin coupling and coupling constants — core concepts in high-resolution NMR important for M.Pharm students studying Advanced Instrumental Analysis. You will find clear, targeted multiple-choice questions that probe theory and practical interpretation: through-bond interactions, geminal/vicinal/long-range couplings, the Karplus relationship, first- versus second-order spectra, stereochemical information from J values, and practical measurement/simulation issues used in pharmaceutical structural analysis. These questions emphasize deeper understanding needed to interpret complex multiplets in real drug molecules and spectra, and to use coupling constants reliably for conformation and stereochemistry assignments.

Q1. What is spin-spin coupling in NMR?

• A through-bond magnetic interaction between non-equivalent nuclear spins that causes peak splitting
• A through-space dipolar interaction observable in liquid-state 1H NMR
• A change in T1 relaxation time due to solvent effects
• The broadening of peaks caused by magnetic field inhomogeneity

Correct Answer: A through-bond magnetic interaction between non-equivalent nuclear spins that causes peak splitting

Q2. What are the units of a coupling constant (J) reported in NMR?

• Parts per million (ppm)
• Hertz (Hz)
• Seconds (s)
• Degrees (°)

Correct Answer: Hertz (Hz)

Q3. Which range best describes typical vicinal (3J_HH) coupling constants in freely rotating aliphatic systems?

• 0–1 Hz
• 3–12 Hz
• 30–50 Hz
• 100–200 Hz

Correct Answer: 3–12 Hz

Q4. What does the Karplus equation correlate in organic molecules?

• The magnitude of 3J coupling constants with dihedral angle between coupled nuclei
• The chemical shift with solvent polarity
• The intensity of peaks with concentration
• The coupling constant with temperature only

Correct Answer: The magnitude of 3J coupling constants with dihedral angle between coupled nuclei

Q5. Typical 3J coupling constant for trans-alkene vicinal protons is approximately:

• 0–2 Hz
• 3–6 Hz
• 12–18 Hz
• 40–60 Hz

Correct Answer: 12–18 Hz

Q6. What splitting pattern is observed for a proton coupled to two equivalent neighbouring protons (with equal J)?

• Singlet
• Doublet
• Triplet
• Quartet of doublets

Correct Answer: Triplet

Q7. Which statement about the sign of coupling constants is correct?

• Coupling constants are always positive and their sign is experimentally reported directly
• Coupling constants can be positive or negative, but routine 1D NMR displays only magnitudes
• Coupling constants are always negative for vicinal couplings
• Coupling constants do not have a sign; they are dimensionless

Correct Answer: Coupling constants can be positive or negative, but routine 1D NMR displays only magnitudes

Q8. What condition commonly leads to second-order (non-first-order) multiplet patterns?

• When chemical shift differences (Δν in Hz) between coupled nuclei are comparable to the coupling constant J
• When sample concentration is extremely low
• When the spectrometer frequency is above 1 GHz only
• When relaxation times are extremely long

Correct Answer: When chemical shift differences (Δν in Hz) between coupled nuclei are comparable to the coupling constant J

Q9. Long-range coupling (4J, 5J) is most commonly observed in which type of system?

• Isolated saturated alkanes without conjugation
• Conjugated or aromatic systems where through-bond transmission is facilitated
• Purely through-space separated methylene groups (>5 Å apart)
• Inert gases dissolved in solvent

Correct Answer: Conjugated or aromatic systems where through-bond transmission is facilitated

Q10. What does a “doublet of doublets” (dd) indicate for a proton resonance?

• The proton is coupled to two equivalent protons with identical J values
• The proton is coupled to two non-equivalent protons with two different J values
• The proton is not coupled to any other nucleus
• The proton is coupled to three identical protons

Correct Answer: The proton is coupled to two non-equivalent protons with two different J values

Q11. What is the effect of broadband proton decoupling on a 13C NMR spectrum?

• It increases the number of carbon resonances by splitting each into multiplets
• It removes 1H–13C coupling and collapses multiplets to singlets
• It has no effect because 13C is insensitive to proton coupling
• It inverts carbon peak phases only

Correct Answer: It removes 1H–13C coupling and collapses multiplets to singlets

Q12. When is spectral simulation most useful for determining J values?

• For perfectly resolved, isolated first-order doublets only
• When multiplets overlap and second-order effects or multiple couplings complicate direct reading
• Only for solid-state NMR spectra
• When no coupling is present

Correct Answer: When multiplets overlap and second-order effects or multiple couplings complicate direct reading

Q13. Which factor most strongly influences the magnitude of vicinal (3J) coupling constants?

• The dihedral angle between the coupled nuclei (Karplus dependence)
• The absolute molecular mass of the compound
• The color of the solvent
• The total number of protons in the molecule

Correct Answer: The dihedral angle between the coupled nuclei (Karplus dependence)

Q14. Typical ortho (3J) coupling constant in a substituted benzene ring is approximately:

• 0.5–1 Hz
• 2–4 Hz
• 6–10 Hz
• 20–30 Hz

Correct Answer: 6–10 Hz

Q15. Geminal (2J) coupling between diastereotopic methylene hydrogens commonly falls in which range?

• 0–2 Hz
• 12–18 Hz
• 50–80 Hz
• 200–300 Hz

Correct Answer: 12–18 Hz

Q16. Which statement about the use of J coupling in stereochemical assignment is correct?

• J coupling provides no stereochemical information and is ignored in structure elucidation
• Large vicinal J values can indicate anti or trans relationships, while small values indicate gauche or cis geometries
• Stereochemistry can be assigned only by mass spectrometry, not J values
• J values are identical for enantiomers and therefore useless

Correct Answer: Large vicinal J values can indicate anti or trans relationships, while small values indicate gauche or cis geometries

Q17. What multiplicity results from a proton coupled to three non-equivalent neighbouring protons with three different J values?

• Quartet
• Triplet of doublets
• Doublet of doublet of doublets (ddd)
• Multiplet with no discernible pattern only

Correct Answer: Doublet of doublet of doublets (ddd)

Q18. What is “virtual coupling” in NMR spectra?

• An observed small splitting between two nuclei that are not directly coupled, caused by second-order mixing of spins
• Coupling that occurs only at very high temperature
• Coupling observed only in heteronuclear decoupled spectra
• A synonym for through-space dipolar coupling in solids

Correct Answer: An observed small splitting between two nuclei that are not directly coupled, caused by second-order mixing of spins

Q19. Typical one-bond 1J_CH coupling constants measured in high-resolution NMR fall approximately in which range?

• 1–5 Hz
• 10–40 Hz
• 125–250 Hz
• 500–1000 Hz

Correct Answer: 125–250 Hz

Q20. Which parameter measured from high-resolution proton NMR is most directly used to assign relative stereochemistry in drug molecules?

• Peak integration only
• Coupling constants (J values) and their patterns
• Absolute signal intensity in arbitrary units
• Solvent residual peak position

Correct Answer: Coupling constants (J values) and their patterns

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