Relaxation processes in NMR MCQs With Answer

Relaxation processes in NMR MCQs With Answer

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Relaxation processes in NMR MCQs With Answer

Nuclear Magnetic Resonance (NMR) relaxation is central to understanding line shapes, sensitivity, molecular dynamics, and quantitative analyses in Modern Pharmaceutical Analytical Techniques. For M. Pharm students, mastering T1 (spin-lattice) and T2 (spin-spin) relaxation, T2*, NOE, quadrupolar effects, paramagnetic relaxation, and the role of correlation time (τc) helps interpret spectra, optimize experiments, and evaluate drug–protein interactions. This quiz focuses on mechanisms, mathematical relationships, experimental measurements (inversion-recovery, spin-echo/CPMG), field and temperature effects, and applications like MRI contrast and MAS in solids. Each question is designed to test conceptual clarity and practical insight, preparing you for advanced analytical problem-solving in pharmaceutical research.

Q1. Which statement best describes T1 (spin-lattice) relaxation in NMR?

  • Recovery of longitudinal magnetization via energy exchange with the surroundings
  • Decay of transverse magnetization due to spin-spin interactions and field inhomogeneity
  • Broadening of resonances due to chemical exchange between two sites
  • Mixing of different coherence orders during a pulse sequence

Correct Answer: Recovery of longitudinal magnetization via energy exchange with the surroundings

Q2. Which parameter predominantly determines the observed linewidth at half-height in a solution NMR spectrum?

  • T1
  • T2
  • T2*
  • Rotational correlation time (τc)

Correct Answer: T2*

Q3. For small molecules in the fast-tumbling (extreme narrowing) regime, how does increasing viscosity (i.e., increasing τc) affect relaxation?

  • T1 increases and T2 decreases
  • Both T1 and T2 increase
  • Both T1 and T2 decrease
  • T1 decreases and T2 increases

Correct Answer: Both T1 and T2 decrease

Q4. Which experiment is most commonly used to measure T1 in solution NMR?

  • Inversion-recovery (180°–τ–90°)
  • Hahn spin-echo (90°–τ–180°–τ)
  • CPMG multi-echo train
  • COSY

Correct Answer: Inversion-recovery (180°–τ–90°)

Q5. Which nucleus is most likely to exhibit strong quadrupolar relaxation in NMR?

  • 14N
  • 13C
  • 1H
  • 19F

Correct Answer: 14N

Q6. What primarily causes T2 (spin-spin) relaxation?

  • Energy exchange with the lattice resulting in longitudinal recovery
  • Dephasing from local field fluctuations and spin-spin interactions without net energy exchange
  • Resonance offset induced by RF miscalibration
  • Population inversion between Zeeman levels

Correct Answer: Dephasing from local field fluctuations and spin-spin interactions without net energy exchange

Q7. Which relationship between T1 and T2 is generally valid in liquids?

  • T1 = T2 for all nuclei
  • T1 ≤ T2
  • T2 ≤ T1
  • T1 × T2 is constant

Correct Answer: T2 ≤ T1

Q8. Which expression correctly relates T2* to T2 and static field inhomogeneity?

  • 1/T2* = 1/T2 + 1/Tinhom
  • T2* = T2 + Tinhom
  • 1/T2* = 1/T2 − 1/Tinhom
  • T2* = T2 × Tinhom

Correct Answer: 1/T2* = 1/T2 + 1/Tinhom

Q9. What is the typical effect of paramagnetic ions on NMR signals?

  • Increase T1 and sharpen lines
  • Decrease T1 and T2, causing line broadening
  • Decrease chemical shift dispersion
  • Increase T2 without affecting T1

Correct Answer: Decrease T1 and T2, causing line broadening

Q10. The Nuclear Overhauser Effect (NOE) primarily arises from which mechanism?

  • Scalar J-coupling modulation during RF irradiation
  • Cross-relaxation via dipole-dipole interactions that transfers polarization
  • Quadrupolar coupling with the electric field gradient
  • Second-order chemical shift anisotropy averaging

Correct Answer: Cross-relaxation via dipole-dipole interactions that transfers polarization

Q11. For small molecules in the fast-motion regime, how does increasing magnetic field strength (B0) typically affect T1?

  • T1 decreases significantly
  • T1 increases
  • T1 remains unchanged
  • T1 becomes equal to T2

Correct Answer: T1 increases

Q12. For dipolar relaxation of spin-1/2 nuclei, which spectral density combination commonly appears in 1/T1?

  • J(0) − J(ω0)
  • J(ω0) + 4J(2ω0)
  • 3J(0) + 5J(ω0) + 2J(2ω0)
  • J(2ω0) − 4J(ω0)

Correct Answer: J(ω0) + 4J(2ω0)

Q13. Which sequence is most suitable for measuring T2 while mitigating B0 inhomogeneity effects?

  • Inversion-recovery
  • Hahn spin-echo
  • CPMG (Carr-Purcell-Meiboom-Gill)
  • NOESY

Correct Answer: CPMG (Carr-Purcell-Meiboom-Gill)

Q14. What is the principal effect of magic-angle spinning (MAS) on relaxation and line shapes in solid-state NMR?

  • Shortens T1 and broadens lines
  • Averages anisotropic interactions, lengthening T2* and narrowing lines
  • Eliminates all dipolar couplings, making T2 infinite
  • Increases chemical shift anisotropy to improve resolution

Correct Answer: Averages anisotropic interactions, lengthening T2* and narrowing lines

Q15. Which statement best describes relaxation behavior in rigid solids compared to liquids?

  • T2 is typically much shorter in solids due to strong, unaveraged anisotropic interactions
  • T1 is always shorter in solids than in liquids
  • T2 is longer in solids because spins are immobilized
  • Relaxation times are identical in solids and liquids

Correct Answer: T2 is typically much shorter in solids due to strong, unaveraged anisotropic interactions

Q16. Exchange broadening that elevates R2 (1/T2) on the micro- to millisecond timescale is best characterized by which experiment?

  • DEPT
  • HSQC
  • CPMG relaxation dispersion
  • TOCSY

Correct Answer: CPMG relaxation dispersion

Q17. Which parameter most directly governs the efficiency of dipolar relaxation in isotropic liquids?

  • RF pulse flip angle
  • Motional correlation time (τc)
  • Receiver gain
  • Sample pH

Correct Answer: Motional correlation time (τc)

Q18. Which ion is widely used as a T1 MRI contrast agent due to strong paramagnetic relaxation enhancement?

  • Gd3+
  • Zn2+
  • Na+
  • K+

Correct Answer: Gd3+

Q19. What is the approximate relationship between the linewidth at half-height (Δν1/2) and T2*?

  • Δν1/2 ≈ 1/(π T2*)
  • Δν1/2 ≈ π T2*
  • Δν1/2 ≈ 1/(2π T1)
  • Δν1/2 ≈ π/T1

Correct Answer: Δν1/2 ≈ 1/(π T2*)

Q20. For small molecules in solution, how does increasing temperature typically influence relaxation?

  • τc increases, making T1 and T2 shorter
  • τc decreases, making T1 and often T2 longer
  • τc remains constant, but T1 decreases
  • T2 always becomes shorter while T1 is unaffected

Correct Answer: τc decreases, making T1 and often T2 longer

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