1D and 2D NMR techniques: COSY, NOESY, HECTOR, INADEQUATE MCQs With Answer

Introduction

This quiz set covers advanced 1D and 2D NMR techniques — COSY, NOESY, HECTOR, and INADEQUATE — tailored for M.Pharm students studying MPC 201T Advanced Spectral Analysis. It emphasizes understanding of through-bond and through-space correlations, practical choices between experiments, interpretation of cross-peaks, limitations such as sensitivity and artefacts, and how parameters like mixing time influence results. Questions range from basic principles to application-focused scenarios encountered in structural elucidation of organic and pharmaceutical compounds. Use these MCQs to test and deepen your knowledge of when and how to apply each 2D method and to improve spectral analysis skills essential for research and formulation studies.

Q1. Which of the following best describes the primary correlation detected in a COSY (Correlation Spectroscopy) experiment?

• Through-space dipolar correlations between protons
• Through-bond scalar (J) couplings between protons
• Direct heteronuclear 1H–13C one-bond couplings
• 13C–13C double-quantum coherences

Correct Answer: Through-bond scalar (J) couplings between protons

Q2. NOESY spectra provide information primarily about which of these structural features?

• Connectivity through covalent bonds
• Relative through-space proximity of nuclei (typically ≤5 Å)
• One-bond heteronuclear couplings
• Scalar couplings across three bonds specifically

Correct Answer: Relative through-space proximity of nuclei (typically ≤5 Å)

Q3. The INADEQUATE experiment is uniquely used to detect which of the following correlations?

• 1H–1H scalar couplings within a spin system
• 1H–13C one-bond correlations with high sensitivity
• 13C–13C through-bond connectivities via double-quantum coherence
• Through-space NOE cross-peaks between carbons

Correct Answer: 13C–13C through-bond connectivities via double-quantum coherence

Q4. Which practical limitation most strongly affects routine application of INADEQUATE at natural isotope abundance?

• Its inability to detect long-range couplings
• Very low sensitivity due to low natural abundance of 13C
• Excessive through-space artefacts like NOE
• Requirement for very long 1H relaxation delays only

Correct Answer: Very low sensitivity due to low natural abundance of 13C

Q5. HECTOR experiments are best described as optimized for which purpose?

• Detecting one-bond 1H–1H couplings exclusively
• Measuring T1 relaxation times of protons
• Enhancing detection of long-range heteronuclear (nJCH) correlations
• Generating 13C–13C NOE interactions

Correct Answer: Enhancing detection of long-range heteronuclear (nJCH) correlations

Q6. In a 2D COSY, a cross-peak between two proton signals indicates:

• That the two protons are chemically equivalent
• That there exists a scalar coupling pathway (usually 2J or 3J) between them
• That the two protons are within 10 Å spatial distance
• Only that both protons share the same chemical shift

Correct Answer: That there exists a scalar coupling pathway (usually 2J or 3J) between them

Q7. Which parameter is most critical to adjust when trying to optimize NOESY cross-peak intensities for small molecules?

• The recycle delay between scans
• The mixing time ™
• The spectral width in F1
• The number of data points in F2

Correct Answer: The mixing time ™

Q8. How can you distinguish a NOESY cross-peak from a COSY cross-peak when both experiments are run on the same sample?

• NOESY cross-peaks always appear on the diagonal; COSY cross-peaks appear off-diagonal
• NOESY cross-peaks reflect through-space proximity and can appear between non-coupled protons, while COSY cross-peaks indicate scalar coupling and appear only between J-coupled protons
• COSY cross-peaks are always negative, NOESY always positive
• NOESY peaks are observed only in samples with 13C enrichment

Correct Answer: NOESY cross-peaks reflect through-space proximity and can appear between non-coupled protons, while COSY cross-peaks indicate scalar coupling and appear only between J-coupled protons

Q9. A DQF-COSY (double-quantum filtered COSY) improves which aspect of the conventional COSY spectrum?

• Direct detection of 13C nuclei
• Suppression of diagonal peak distortions and improved cross-peak phase purity
• Conversion of NOE into J-coupling information
• Enhancing detection of very long-range (4J, 5J) couplings

Correct Answer: Suppression of diagonal peak distortions and improved cross-peak phase purity

Q10. In interpreting an INADEQUATE spectrum, a pair of correlated 13C signals connected by a cross-peak indicates:

• They are spatially close but not necessarily bonded
• They are directly bonded (one-bond 13C–13C connection)
• They share a common attached proton
• They are connected through a three-bond 13C–13C pathway only

Correct Answer: They are directly bonded (one-bond 13C–13C connection)

Q11. Which of the following is a major advantage of performing a HECTOR-like heteronuclear experiment over standard HMBC when seeking weak long-range 1H–13C correlations?

• HECTOR eliminates all couplings, providing pure shift spectra
• HECTOR is optimized to increase sensitivity for nJCH correlations and reduce competing pathways, improving detection of weak long-range peaks
• HECTOR requires no decoupling, making acquisition faster
• HECTOR selectively detects only one-bond couplings

Correct Answer: HECTOR is optimized to increase sensitivity for nJCH correlations and reduce competing pathways, improving detection of weak long-range peaks

Q12. Which sign behavior in NOESY cross-peaks can indicate molecular tumbling regime and influence interpretation?

• Sign of NOESY cross-peaks is always positive for all molecular weights
• Sign (positive or negative) of NOE cross-peaks depends on correlation time; small fast-tumbling molecules often give negative NOE, large slow-tumbling give positive NOE
• NOESY cross-peak sign is independent of tumbling and only depends on field strength
• NOESY cross-peak sign only depends on whether protons are exchangeable

Correct Answer: Sign (positive or negative) of NOE cross-peaks depends on correlation time; small fast-tumbling molecules often give negative NOE, large slow-tumbling give positive NOE

Q13. In a COSY experiment, cross-peaks between geminal protons (2J) and vicinal protons (3J) often differ in intensity primarily because:

• 2J couplings are through-space, while 3J are through-bond
• Different magnitudes of scalar coupling constants (J values) lead to different transfer efficiencies during the pulse sequence
• Geminal protons never show cross-peaks in COSY
• Mixing time only allows 3J transfer, not 2J

Correct Answer: Different magnitudes of scalar coupling constants (J values) lead to different transfer efficiencies during the pulse sequence

Q14. Which experimental choice is most appropriate when you need to map full spin systems (all coupled protons within a network) rather than just pairwise J couplings?

• Run a standard COSY only
• Run a NOESY with very long mixing time
• Run a TOCSY (total correlation spectroscopy) experiment, which transfers magnetization throughout the entire spin system
• Run an INADEQUATE experiment on protons

Correct Answer: Run a TOCSY (total correlation spectroscopy) experiment, which transfers magnetization throughout the entire spin system

Q15. Which statement about phase-sensitive NOESY spectra is correct for rigorous distance interpretation?

• NOESY intensities can be directly converted to absolute distances without calibration
• NOESY peak intensities depend on cross-relaxation rates and require calibration or internal standards to convert to distances reliably
• Mixing time has no impact on NOESY build-up curves
• Phase-sensitive NOESY always eliminates spin diffusion

Correct Answer: NOESY peak intensities depend on cross-relaxation rates and require calibration or internal standards to convert to distances reliably

Q16. When would you choose INADEQUATE over HSQC/HMBC in a carbon skeleton elucidation?

• When you need direct one-bond 1H–13C connectivity only
• When you need unequivocal 13C–13C connectivity information to establish carbon–carbon framework, and sample amount and sensitivity allow it
• When you have extremely low sample concentrations and need high sensitivity
• When you want to measure NOE between carbons

Correct Answer: When you need unequivocal 13C–13C connectivity information to establish carbon–carbon framework, and sample amount and sensitivity allow it

Q17. Which artifact commonly appears in NOESY spectra if mixing times are too long for a medium-sized molecule?

• Appearance of anti-phase COSY peaks
• Excessive spin diffusion creating indirect NOE pathways and misleading long-range peaks
• Complete disappearance of diagonal peaks only
• Creation of 13C–13C cross-peaks

Correct Answer: Excessive spin diffusion creating indirect NOE pathways and misleading long-range peaks

Q18. Which feature distinguishes HECTOR-type experiments from HSQC for heteronuclear correlations?

• HECTOR targets direct one-bond 1H–13C only, while HSQC targets long-range couplings
• HECTOR is optimized for detecting long-range (nJ) heteronuclear correlations that HMBC may miss, while HSQC focuses on one-bond correlations with high sensitivity
• HECTOR produces 13C–13C double quantum signals, HSQC produces NOE-based correlations
• HECTOR requires isotopic enrichment to work, HSQC does not

Correct Answer: HECTOR is optimized for detecting long-range (nJ) heteronuclear correlations that HMBC may miss, while HSQC focuses on one-bond correlations with high sensitivity

Q19. In planning a combined strategy to solve an unknown organic structure, which set of experiments is most complementary for establishing both proton connectivities and carbon skeleton?

• Only 1D 1H and 13C experiments
• COSY/TOCSY for proton networks, HSQC for one-bond 1H–13C assignments, and HMBC or HECTOR/INADEQUATE for longer-range carbon connectivity
• Only NOESY and INADEQUATE
• Only HECTOR and NOESY without any 1D spectra

Correct Answer: COSY/TOCSY for proton networks, HSQC for one-bond 1H–13C assignments, and HMBC or HECTOR/INADEQUATE for longer-range carbon connectivity

Q20. For an organic molecule with flexible conformations in solution, which caution is most relevant when interpreting NOESY data?

• NOESY distances reflect only a single static conformation and cannot be averaged
• Observed NOE intensities represent time-averaged distances influenced by conformational ensemble and spin diffusion, so care is needed when deriving static structures
• NOESY is unaffected by molecular motion and gives precise internuclear distances
• INOADEQUATE should be used instead of NOESY for distance measurements

Correct Answer: Observed NOE intensities represent time-averaged distances influenced by conformational ensemble and spin diffusion, so care is needed when deriving static structures

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