Principle of NMR spectroscopy MCQs With Answer

Introduction

This quiz collection on the Principle of NMR Spectroscopy is designed for M.Pharm students preparing for Advanced Instrumental Analysis (MPA 201T). It focuses on core theoretical concepts and practical aspects of nuclear magnetic resonance—nuclear spin properties, Larmor frequency, chemical shift interpretation, coupling patterns, relaxation mechanisms, pulse sequences and common 1D/2D experiments used in pharmaceutical analysis. Questions progress from fundamental definitions to applied problems relevant to structure elucidation, quantitative NMR and dynamic processes in drug molecules. Each item tests conceptual understanding and problem-solving skills useful for exams and laboratory interpretation of NMR spectra.

Q1. Which of the following is the primary requirement for a nucleus to be observable by NMR?

• High natural abundance in the sample
• Non-zero nuclear spin
• High atomic mass
• Presence of unpaired electrons

Correct Answer: Non-zero nuclear spin

Q2. The Larmor frequency of a nucleus in a magnetic field depends on which two parameters?

• Magnetogyric ratio and static magnetic field strength
• Nuclear mass and temperature
• Sample concentration and magnet homogeneity
• Pulse length and receiver gain

Correct Answer: Magnetogyric ratio and static magnetic field strength

Q3. Chemical shift is reported in ppm to make it independent of:

• Magnetic field strength
• Sample concentration
• Pulse sequence
• Solvent polarity

Correct Answer: Magnetic field strength

Q4. Tetramethylsilane (TMS) is commonly used as a chemical shift reference because it is:

• Highly polar and interacts with solutes
• Volatile and easily removed after measurement
• Shielded and produces a single sharp signal at 0 ppm
• Paramagnetic which broadens solvent peaks

Correct Answer: Shielded and produces a single sharp signal at 0 ppm

Q5. Which relaxation time primarily determines longitudinal recovery of magnetization along B0?

• T2 (spin-spin relaxation time)
• T1 (spin-lattice relaxation time)
• Tm (electronic relaxation time)
• Td (diffusion relaxation time)

Correct Answer: T1 (spin-lattice relaxation time)

Q6. J-coupling (scalar coupling) arises from which interaction?

• Dipolar magnetic interaction between distant nuclei
• Electron-mediated through-bond interaction between spin nuclei
• Coupling to the external magnet field inhomogeneity
• Spin-rotation interaction only in gases

Correct Answer: Electron-mediated through-bond interaction between spin nuclei

Q7. In a simple first-order 1H NMR spectrum, a proton coupled to two equivalent neighboring protons will appear as:

• A singlet
• A doublet
• A triplet
• A quartet

Correct Answer: A triplet

Q8. The Karplus equation relates dihedral angle to which NMR parameter?

• Chemical shift
• Spin-lattice relaxation time (T1)
• Vicinal 3J coupling constant
• Transverse relaxation time (T2)

Correct Answer: Vicinal 3J coupling constant

Q9. Quadrupolar nuclei (spin > 1/2) often give broadened signals because they interact strongly with:

• Zeeman splitting only
• Electric field gradients via the nuclear quadrupole moment
• Scalar coupling to protons exclusively
• Solvent viscosity only

Correct Answer: Electric field gradients via the nuclear quadrupole moment

Q10. Fourier transform (FT) NMR provides improved sensitivity over continuous-wave NMR primarily due to:

• Longer acquisition times
• Higher static magnetic fields only
• The multiplex (Fellgett) advantage by collecting all frequencies simultaneously
• Removal of solvent signals prior to experiment

Correct Answer: The multiplex (Fellgett) advantage by collecting all frequencies simultaneously

Q11. In proton-decoupled 13C NMR, why are carbon signals typically observed as singlets?

• Because 13C has no magnetic moment
• Because protons are irradiated to average JCH coupling to zero during acquisition
• Because carbon-carbon coupling is always negligible
• Because 13C concentration is too low to detect multiplicity

Correct Answer: Because protons are irradiated to average JCH coupling to zero during acquisition

Q12. Nuclear Overhauser Effect (NOE) is most useful to determine:

• Scalar coupling constants between bonded nuclei
• Through-space proximity between nuclei within ~5 Å
• Absolute chemical shift referencing
• Magnetic susceptibility of the solvent

Correct Answer: Through-space proximity between nuclei within ~5 Å

Q13. Which pulse sequence is typically used to obtain correlation between directly bonded 1H and 13C nuclei?

• COSY
• DEPT
• HSQC
• NOESY

Correct Answer: HSQC

Q14. Chemical exchange processes that occur on the NMR timescale most directly affect which spectral features?

• Only baseline noise
• Line shape and coalescence of peaks
• Magnetogyric ratio values
• Receiver sensitivity

Correct Answer: Line shape and coalescence of peaks

Q15. In quantitative NMR (qNMR), the most important requirement for accurate integration is:

• Proton decoupling during acquisition
• Accurate knowledge of T1 relaxation times and sufficiently long recycle delay
• Use of a single scan only
• Using deuterated solvents to shift peaks

Correct Answer: Accurate knowledge of T1 relaxation times and sufficiently long recycle delay

Q16. Shimming in NMR refers to:

• Adjusting pulse power to optimize flip angle
• Optimizing the homogeneity of the static magnetic field
• Calibrating receiver gain for maximum signal
• Adding TMS to the sample

Correct Answer: Optimizing the homogeneity of the static magnetic field

Q17. Which factor increases NMR sensitivity for a given nucleus?

• Lower magnetogyric ratio
• Lower static magnetic field strength
• Higher magnetic field strength and larger magnetogyric ratio
• Reducing sample concentration

Correct Answer: Higher magnetic field strength and larger magnetogyric ratio

Q18. A doublet of doublets in a 1H NMR spectrum usually indicates:

• A proton coupled equally to three equivalent protons
• A proton coupled to two non-equivalent neighboring protons with different J values
• A proton experiencing rapid chemical exchange
• A miscalibrated spectrometer

Correct Answer: A proton coupled to two non-equivalent neighboring protons with different J values

Q19. T2 relaxation (spin-spin) leads mainly to which observable in the NMR spectrum?

• A shift in peak positions (chemical shift changes)
• Broadening of resonance lines and loss of transverse magnetization
• An increase in signal amplitude without change in width
• Creation of new peaks due to scalar coupling

Correct Answer: Broadening of resonance lines and loss of transverse magnetization

Q20. In 2D COSY experiments the cross peaks indicate:

• Through-space NOE interactions exclusively
• Scalar (through-bond) coupling between protons
• Direct 1H–13C correlations only
• Absolute molecular weight of the sample

Correct Answer: Scalar (through-bond) coupling between protons

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