Nuclear magnetic double resonance MCQs With Answer

This blog presents a focused set of Nuclear Magnetic Double Resonance MCQs with answers tailored for M.Pharm students studying Advanced Instrumental Analysis (MPA 201T). The questions probe core principles — two-frequency irradiation, heteronuclear decoupling, polarization transfer (INEPT, DEPT, CP), Hartmann–Hahn matching, and modern double-resonance experiments used in solution and solid-state NMR (REDOR, CPMAS, STD). Emphasis is placed on practical interpretation of spectra, experimental parameters (B1 fields, contact times, decoupling schemes) and applications in structural and interaction studies. Use these MCQs to test conceptual understanding, experimental design decisions, and the rationale behind sensitivity and resolution enhancements in double-resonance NMR.

Q1. What best defines nuclear magnetic double resonance?

• The simultaneous application of two RF fields to manipulate interactions between two different nuclear spin species
• Using a single RF pulse to excite all spins in a single nucleus
• The measurement of chemical shift anisotropy under static conditions
• Applying high-power decoupling only during sample spinning

Correct Answer: The simultaneous application of two RF fields to manipulate interactions between two different nuclear spin species

Q2. Heteronuclear decoupling in NMR is primarily achieved by which mechanism?

• Continuous or composite RF irradiation of the coupling nucleus to remove scalar (J) coupling effects in the observed nucleus
• Increasing the static magnetic field strength to separate spin transitions
• Cooling the sample to reduce molecular motion and spin interactions
• Applying gradient pulses during acquisition to dephase coupled spins

Correct Answer: Continuous or composite RF irradiation of the coupling nucleus to remove scalar (J) coupling effects in the observed nucleus

Q3. The Hartmann–Hahn condition required for efficient cross polarization is best expressed as:

• The matching of the nutation (spin-lock) frequencies: γI·B1,I = γS·B1,S
• The equality of Larmor frequencies for both nuclei: ωI = ωS
• The product of gyromagnetic ratios equals the static field: γI·γS = B0
• Setting the magic-angle spinning rate equal to the difference in chemical shifts

Correct Answer: The matching of the nutation (spin-lock) frequencies: γI·B1,I = γS·B1,S

Q4. The principal advantage of cross polarization (CP) in solid‑state NMR is:

• Transfer of polarization from abundant high-γ spins (e.g., 1H) to dilute low-γ nuclei (e.g., 13C) to enhance sensitivity
• Direct observation of quadrupolar nuclei without decoupling
• Measuring longitudinal relaxation times (T1) with higher accuracy
• Eliminating chemical shift anisotropy entirely

Correct Answer: Transfer of polarization from abundant high-γ spins (e.g., 1H) to dilute low-γ nuclei (e.g., 13C) to enhance sensitivity

Q5. INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) is primarily used in solution NMR to:

• Enhance signal of low-γ nuclei by polarization transfer via scalar (J) coupling using pulsed sequences
• Suppress solvent peaks by continuous-wave irradiation
• Determine internuclear distances through dipolar coupling in solids
• Measure chemical exchange rates using saturation transfer

Correct Answer: Enhance signal of low-γ nuclei by polarization transfer via scalar (J) coupling using pulsed sequences

Q6. DEPT experiments in NMR provide which specific information alongside sensitivity enhancement?

• Multiplicity classification (CH, CH2, CH3) of 13C resonances while enhancing signal
• Absolute internuclear distances in solid samples
• Direct observation of quadrupolar satellites
• Suppression of residual water signal only

Correct Answer: Multiplicity classification (CH, CH2, CH3) of 13C resonances while enhancing signal

Q7. Nuclear Overhauser Effect (NOE) experiments often implement double resonance by:

• Saturating one nucleus and observing intensity changes in another due to cross-relaxation
• Using two simultaneous pulses to produce scalar coupling artifacts
• Spinning the sample at two different MAS rates simultaneously
• Applying selective inversion pulses to measure T2 only

Correct Answer: Saturating one nucleus and observing intensity changes in another due to cross-relaxation

Q8. Saturation Transfer Difference (STD) NMR, a double-resonance approach, is mainly applied to:

• Detect and map ligand binding to large receptor/protein by saturation transfer from receptor to ligand signals
• Enhance 13C signals in small organic molecules in solution
• Measure dipolar couplings in rigid solids
• Calibrate RF field strength across different spectrometers

Correct Answer: Detect and map ligand binding to large receptor/protein by saturation transfer from receptor to ligand signals

Q9. Composite decoupling sequences such as WALTZ and MLEV are used because they:

• Provide broadband heteronuclear decoupling with improved homogeneity and reduced artifacts compared to simple continuous-wave decoupling
• Increase MAS rates without changing rotor hardware
• Directly measure quadrupolar coupling constants in half-integer nuclei
• Eliminate the need for Hartmann–Hahn matching in CP

Correct Answer: Provide broadband heteronuclear decoupling with improved homogeneity and reduced artifacts compared to simple continuous-wave decoupling

Q10. Which experimental parameter most directly determines the Hartmann–Hahn match during a CP experiment?

• Amplitude of the spin-lock RF fields applied to both nuclei (B1 amplitudes)
• Static magnetic field strength (B0) only
• Magic-angle spinning speed exclusively
• Receiver gain during acquisition

Correct Answer: Amplitude of the spin-lock RF fields applied to both nuclei (B1 amplitudes)

Q11. The typical shape of a CP signal buildup curve as a function of contact time is:

• An initial exponential growth toward a maximum followed by potential decay due to spin‑lattice relaxation in the rotating frame (T1ρ)
• A linear increase indefinitely with contact time
• An immediate maximum with no dependence on contact time
• A sinusoidal oscillation independent of relaxation

Correct Answer: An initial exponential growth toward a maximum followed by potential decay due to spin‑lattice relaxation in the rotating frame (T1ρ)

Q12. Applying continuous heteronuclear decoupling on the 1H channel during 13C acquisition typically results in:

• Collapse of 13C–1H multiplets to singlets and improved 13C sensitivity
• Broadening of 13C lines due to increased J coupling
• Complete loss of all 13C signal due to saturation
• Observation of only spinning sidebands without main peaks

Correct Answer: Collapse of 13C–1H multiplets to singlets and improved 13C sensitivity

Q13. In solid-state NMR, which nuclei are most commonly targeted for enhancement via double‑resonance CP techniques?

• Low-γ, low‑natural‑abundance nuclei such as 13C and 15N
• High-γ abundant nuclei like 1H only
• Only quadrupolar nuclei with I > 1/2
• Nuclei with no J coupling to protons

Correct Answer: Low-γ, low‑natural‑abundance nuclei such as 13C and 15N

Q14. Selective decoupling experiments help in spectral assignment by:

• Saturating or decoupling a specific coupled nucleus to reveal which resonances are coupled partners
• Increasing the global resolution of the entire spectrum without selectivity
• Measuring absolute chemical shifts without referencing
• Replacing MAS to average anisotropic interactions

Correct Answer: Saturating or decoupling a specific coupled nucleus to reveal which resonances are coupled partners

Q15. When using cross polarization, a large offset between the rf carrier and the nucleus’s resonance frequency will typically:

• Reduce CP efficiency because the effective B1 (nutation) is reduced off-resonance
• Enhance CP efficiency by providing broader matching conditions
• Have no effect; offsets do not influence CP
• Only affect T1 relaxation but not CP transfer

Correct Answer: Reduce CP efficiency because the effective B1 (nutation) is reduced off-resonance

Q16. Effective spin‑locking for double resonance experiments requires:

• Stable and well-calibrated RF amplitudes to maintain steady nutation frequencies during the contact period
• Rapid sweeping of RF amplitude throughout the contact time
• Turning off all decoupling sequences during the contact time
• Using the weakest possible RF to avoid heating

Correct Answer: Stable and well-calibrated RF amplitudes to maintain steady nutation frequencies during the contact period

Q17. INEPT sequences are less effective if the scalar coupling constant (J) between donor and acceptor nuclei is:

• Very small relative to relaxation rates, because transfer efficiency drops
• Very large, which always prevents transfer
• Zero, which increases transfer efficiency
• Negative, making transfer impossible

Correct Answer: Very small relative to relaxation rates, because transfer efficiency drops

Q18. REDOR (Rotational-Echo Double Resonance) experiments are primarily used to:

• Measure heteronuclear dipolar couplings and estimate internuclear distances in solids
• Suppress solvent in solution NMR spectra
• Enhance signal of 1H by transferring from 13C
• Determine chemical exchange rates in fast-exchanging systems

Correct Answer: Measure heteronuclear dipolar couplings and estimate internuclear distances in solids

Q19. Insufficient decoupling power during acquisition typically causes:

• Residual multiplet structure, decreased signal-to-noise and line broadening
• Complete elimination of all couplings and artificially sharpened peaks
• Improved sensitivity due to reduced heating
• Faster T1 relaxation only, with no spectral effects

Correct Answer: Residual multiplet structure, decreased signal-to-noise and line broadening

Q20. A major advantage of combining MAS with cross polarization (CPMAS) in solid-state NMR is:

• Simultaneous enhancement of low-γ nuclei sensitivity and narrowing of anisotropic line broadening for improved resolution
• Avoiding the need for any decoupling on the proton channel
• Measuring chemical shifts without referencing standards
• Monitoring rapid chemical exchange processes in liquids

Correct Answer: Simultaneous enhancement of low-γ nuclei sensitivity and narrowing of anisotropic line broadening for improved resolution

Download