2-D NMR techniques MCQs With Answer

Introduction: This quiz collection on 2-D NMR techniques is designed for M.Pharm students preparing for Advanced Instrumental Analysis (MPA 201T). It emphasizes conceptual understanding and practical application of common two-dimensional NMR experiments used in structural elucidation of pharmaceuticals and biomolecules. Topics covered include correlation spectroscopy (COSY), heteronuclear single quantum coherence (HSQC), heteronuclear multiple bond correlation (HMBC), NOESY/ROESY, TOCSY, J-resolved, DOSY and advanced considerations such as mixing times, coherence transfer pathways, sensitivity enhancement and artifact suppression. Each question tests interpretation, pulse sequence principles, coupling pathways and experimental choices that influence resolution and assignment accuracy in drug-related NMR studies.

Q1. In a 1H-1H COSY experiment, the cross peaks primarily indicate which of the following?

• Through-space dipolar interactions between protons
• Scalar (J) coupling between protons through bonds
• Correlation between 1H and 13C chemical shifts
• Diffusion differences of molecules in solution

Correct Answer: Scalar (J) coupling between protons through bonds

Q2. HSQC is most useful for which primary purpose in small molecule structure determination?

• Measuring diffusion coefficients of compounds
• Correlating proton chemical shifts with directly attached carbons
• Detecting long-range heteronuclear couplings over three or more bonds
• Determining through-space distances between non-bonded protons

Correct Answer: Correlating proton chemical shifts with directly attached carbons

Q3. HMBC differs from HSQC because it primarily shows correlations due to which mechanism?

• Direct one-bond heteronuclear coupling (1J)
• Zero-quantum coherence transfer
• Long-range heteronuclear couplings (2J and 3J)
• Pure dipolar coupling interactions

Correct Answer: Long-range heteronuclear couplings (2J and 3J)

Q4. In NOESY spectra, cross peaks arise from which physical phenomenon and are therefore distance dependent?

• Scalar J-coupling between nuclei
• Cross relaxation (NOE) via dipolar interactions
• Heteronuclear multiple bond correlations
• Magnetization transfer through chemical exchange only

Correct Answer: Cross relaxation (NOE) via dipolar interactions

Q5. ROESY is preferred over NOESY for molecules with intermediate correlation times because:

• ROESY uses heteronuclear editing to remove artifacts
• ROESY produces positive cross peaks for all molecular tumbling rates
• NOESY cannot detect through-bond couplings
• ROESY requires isotopic labeling for sensitivity

Correct Answer: ROESY produces positive cross peaks for all molecular tumbling rates

Q6. TOCSY (Total Correlation Spectroscopy) is most useful for identifying:

• Single spin–spin couplings only between adjacent protons
• Entire spin networks connected by scalar couplings
• Heteronuclear correlations between 1H and 31P
• Longitudinal relaxation times (T1) of individual protons

Correct Answer: Entire spin networks connected by scalar couplings

Q7. A J-resolved 2D experiment separates which two spectral dimensions to simplify multiplet analysis?

• Chemical shift and diffusion coefficient
• Chemical shift and scalar coupling (J) information
• Heteronuclear coupling and NOE intensities
• Relaxation time and chemical exchange rate

Correct Answer: Chemical shift and scalar coupling (J) information

Q8. In DOSY experiments, what property of molecules is primarily extracted to help distinguish mixture components?

• Scalar coupling constants
• Diffusion coefficients related to hydrodynamic radius
• Long-range heteronuclear coupling patterns
• Cross-relaxation rates between protons

Correct Answer: Diffusion coefficients related to hydrodynamic radius

Q9. Which experimental parameter in NOESY or ROESY controls the balance between sensitivity and build-up of NOE/ROE signals?

• Acquisition time
• Mixing time
• Recycle delay only
• Number of scans per increment only

Correct Answer: Mixing time

Q10. Gradient-selected HSQC experiments are advantageous because they:

• Require isotopically enriched samples to function
• Suppress t1 noise and remove undesired coherence pathways
• Eliminate the need for shimming the magnet
• Directly measure internuclear distances

Correct Answer: Suppress t1 noise and remove undesired coherence pathways

Q11. In an HSQC spectrum, a phase-sensitive pulse sequence can provide which additional information over magnitude-mode HSQC?

• Accurate diffusion coefficients for each cross peak
• Sign information that helps distinguish CH, CH2 and CH3 multiplicity
• Direct measurement of long-range C-H couplings
• Elimination of diagonal peaks entirely

Correct Answer: Sign information that helps distinguish CH, CH2 and CH3 multiplicity

Q12. The INADEQUATE experiment is powerful for carbon skeleton determination because it detects which interaction directly?

• One-bond 13C–13C scalar couplings requiring high 13C enrichment or long acquisition
• Through-space dipolar interactions between carbons
• Heteronuclear 13C–1H long-range couplings
• NOE between 13C and 1H nuclei

Correct Answer: One-bond 13C–13C scalar couplings requiring high 13C enrichment or long acquisition

Q13. For detecting long-range scalar couplings in HMBC, which evolution delay is usually optimized to enhance 2–3 bond couplings?

• Delay tuned to 1JCH (approx. 140–250 Hz)
• Delay matched to 2J/3JCH (approx. 5–15 Hz)
• Very short delay near zero to capture direct couplings
• Delay optimized for NOE mixing (seconds)

Correct Answer: Delay matched to 2J/3JCH (approx. 5–15 Hz)

Q14. When assigning stereochemistry using NOESY, which limitation must be considered?

• NOE intensities are strictly proportional to internuclear distances without exceptions
• Spin diffusion and molecular motion can distort distance interpretation
• NOESY cannot detect any cross peaks for molecules larger than 50 Da
• NOESY provides direct measurement of dihedral angles only

Correct Answer: Spin diffusion and molecular motion can distort distance interpretation

Q15. In heteronuclear experiments using natural abundance 13C, sensitivity is commonly enhanced by which method?

• Using inverse detection on abundant 1H with proton decoupling
• Measuring directly on 13C without 1H observation
• Eliminating decoupling to prevent signal loss
• Shortening recycle delay to below T2 values

Correct Answer: Using inverse detection on abundant 1H with proton decoupling

Q16. Which artifact in 2D spectra can be reduced by using pulsed field gradients?

• Baseline roll due to poor shimming only
• t1 noise and undesired coherence transfer pathways
• Intrinsic chemical shift anisotropy artifacts in solution
• Errors in diffusion coefficient extraction in DOSY only

Correct Answer: t1 noise and undesired coherence transfer pathways

Q17. In a 2D experiment, phase-sensitive detection in the indirect dimension (F1) is often achieved using which technique?

• States-TPPI or echo/antiecho methods
• Pure magnitude-mode Fourier transformation only
• Direct real-time acquisition without t1 increments
• Temperature cycling during acquisition

Correct Answer: States-TPPI or echo/antiecho methods

Q18. Which of the following best describes the purpose of using band-selective pulses in 2D NMR experiments?

• To measure diffusion of a single nucleus type exclusively
• To selectively excite or invert a narrow frequency range, simplifying spectra and improving sensitivity for regions of interest
• To remove all heteronuclear couplings from the spectrum permanently
• To convert NOE into scalar coupling information

Correct Answer: To selectively excite or invert a narrow frequency range, simplifying spectra and improving sensitivity for regions of interest

Q19. When building a resonance assignment for a complex molecule, the typical order of 2D experiments performed is:

• DOSY → NOESY → HMBC → HSQC
• HSQC → COSY/TOCSY → HMBC → NOESY/ROESY
• HMBC → INADEQUATE → ROESY → COSY
• J-resolved → DOSY → INADEQUATE → HSQC

Correct Answer: HSQC → COSY/TOCSY → HMBC → NOESY/ROESY

Q20. In practice, increasing the number of increments in the indirect dimension (t1) primarily improves which aspect of a 2D NMR spectrum?

• Signal-to-noise per unit time without changing resolution
• Resolution along the indirect (F1) chemical shift axis
• The diffusion coefficient accuracy in DOSY experiments
• Suppression of all scalar couplings automatically

Correct Answer: Resolution along the indirect (F1) chemical shift axis

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