1-D NMR techniques MCQs With Answer

Introduction: This quiz set focuses on 1‑D NMR techniques tailored for M.Pharm students preparing for Advanced Instrumental Analysis (MPA 201T). The questions emphasize practical and theoretical aspects of proton and carbon‑13 one‑dimensional experiments, including chemical shift interpretation, coupling patterns, relaxation phenomena, quantitative NMR strategies, pulse sequences (DEPT, INEPT, inverse‑gated decoupling), solvent suppression and selective decoupling. Each question is designed to deepen understanding of how spectral parameters, instrumentation choices and sample preparation affect structural elucidation, impurity profiling and quality control in pharmaceutical analysis. Use these MCQs to test and reinforce your applied knowledge for research and industry scenarios.

Q1. Which primary factor determines the chemical shift of a proton in 1H NMR?

• Local electronic environment and shielding/deshielding effects
• Molecular weight of the compound
• Overall polarity of the solvent only
• Magnet bore size

Correct Answer: Local electronic environment and shielding/deshielding effects

Q2. In 1H NMR, the typical chemical shift range for an aldehydic proton is:

• 0.5–2.0 ppm
• 3.0–5.0 ppm
• 9.0–10.5 ppm
• 12.0–14.0 ppm

Correct Answer: 9.0–10.5 ppm

Q3. Which 1‑D 13C experiment differentiates CH, CH2 and CH3 carbons by phase (positive/negative peaks)?

• Broadband proton decoupled 13C
• DEPT‑135
• 13C APT only
• NOESY

Correct Answer: DEPT‑135

Q4. For quantitative 13C NMR (accurate integrals), which decoupling strategy is preferred to minimize NOE effects?

• Continuous broadband proton decoupling during acquisition
• Inverse gated (gated) proton decoupling
• DEPT128 polarization transfer
• Selective water presaturation

Correct Answer: Inverse gated (gated) proton decoupling

Q5. Which physical property has the strongest influence on longitudinal relaxation time (T1) for nuclei in solution?

• Molecular tumbling / rotational correlation time
• Optical rotation of the molecule
• Sample pH only
• Magnet cryogen temperature

Correct Answer: Molecular tumbling / rotational correlation time

Q6. Second‑order effects (non first‑order multiplets) in proton NMR are most likely when:

• Coupling constants (J, Hz) are much larger than chemical shift separations (Hz)
• Chemical shift differences in Hz are comparable to coupling constants
• All protons are chemically equivalent
• The solvent is deuterated

Correct Answer: Chemical shift differences in Hz are comparable to coupling constants

Q7. The Nuclear Overhauser Effect (NOE) enhancement observed in proton‑decoupled 13C spectra arises primarily from:

• Cross‑relaxation through dipolar interactions between nearby protons and carbons
• Scalar (J) coupling transfer during decoupling
• Chemical exchange processes only
• Magnet shim adjustments

Correct Answer: Cross‑relaxation through dipolar interactions between nearby protons and carbons

Q8. For accurate quantitative 1H NMR, which acquisition parameter is critical regarding relaxation delays?

• Use the shortest possible relaxation delay to save time
• Relaxation delay (D1) should be at least 5× the longest proton T1
• Set D1 equal to the pulse width only
• Relaxation delay is irrelevant for proton NMR quantitation

Correct Answer: Relaxation delay (D1) should be at least 5× the longest proton T1

Q9. Which solvent suppression technique uses a continuous low‑power irradiation at the solvent resonance during the relaxation delay or acquisition?

• Presaturation
• WATERGATE
• DEPT‑90
• NOESY presaturation

Correct Answer: Presaturation

Q10. In a first‑order system, a proton coupled to two equivalent protons (n = 2) will show which multiplicity and intensity pattern?

• Quartet with 1:3:3:1
• Triplet with 1:2:1
• Doublet of doublets
• Broad singlet only

Correct Answer: Triplet with 1:2:1

Q11. Which internal reference standard is most commonly used for chemical shift calibration in both 1H and 13C NMR of organic solvents?

• Chloroform (CHCl3)
• Tetramethylsilane (TMS)
• Dimethyl sulfoxide (DMSO)
• Sodium 3‑trimethylsilylpropionate (TSP) in nonpolar solvents

Correct Answer: Tetramethylsilane (TMS)

Q12. Typical magnitude of long‑range (4J) aromatic proton–proton scalar couplings is approximately:

• 0–3 Hz
• 6–9 Hz
• 12–18 Hz
• 20–30 Hz

Correct Answer: 0–3 Hz

Q13. The observed linewidth at half height in an NMR peak is primarily governed by which parameter?

• Longitudinal relaxation time T1 only
• Transverse relaxation time T2 and magnetic field homogeneity
• Sample concentration only
• Pulse width exclusively

Correct Answer: Transverse relaxation time T2 and magnetic field homogeneity

Q14. 1‑D selective proton decoupling (irradiation of a single resonance) is mainly used to:

• Measure T1 relaxation of that proton only
• Simplify multiplets and assign coupling partners by irradiating one resonance
• Suppress the entire solvent signal
• Estimate molecular weight from splitting

Correct Answer: Simplify multiplets and assign coupling partners by irradiating one resonance

Q15. Which pulse sequence or technique boosts 13C signal sensitivity by transferring polarization from attached protons?

• Inverse gated decoupling
• INEPT (Insensitive Nuclei Enhanced by Polarization Transfer)
• Presaturation
• TRNOE experiment

Correct Answer: INEPT (Insensitive Nuclei Enhanced by Polarization Transfer)

Q16. In heteronuclear 1‑D experiments where detection is performed on the proton channel instead of the X nucleus (e.g., 1H‑detected 13C editing), the main advantage is:

• Lower sensitivity but better resolution
• Greater sensitivity due to detection on the higher‑gamma nucleus (1H)
• Avoiding the need for deuterated solvent
• Complete removal of coupling constants

Correct Answer: Greater sensitivity due to detection on the higher‑gamma nucleus (1H)

Q17. In NMR acquisition, spectral width (SW) in Hz must be set properly to:

• Ensure the magnet stays at room temperature
• Avoid aliasing (folding) by covering the full frequency range of resonances
• Control only the pulse flip angle
• Determine the T1 relaxation time directly

Correct Answer: Avoid aliasing (folding) by covering the full frequency range of resonances

Q18. For quantitative 13C NMR of pharmaceuticals, which approach yields the most reliable integrals?

• DEPT‑135 with short recycle delay
• Inverse gated decoupling with sufficiently long relaxation delay and external or internal quantitation standard
• Broadband decoupling with no delay and NOE enhancement
• Only using proton spectra for carbon quantitation

Correct Answer: Inverse gated decoupling with sufficiently long relaxation delay and external or internal quantitation standard

Q19. When a dynamic chemical exchange process is slow on the NMR timescale, increasing temperature typically causes:

• A splitting of each peak into multiple unresolved components
• Coalescence of exchanging signals into a single averaged resonance as exchange becomes fast
• No change to the spectrum at any temperature
• Immediate disappearance of all couplings

Correct Answer: Coalescence of exchanging signals into a single averaged resonance as exchange becomes fast

Q20. To improve digital resolution (Hz per point) in a 1‑D spectrum without changing spectral width, you should:

• Decrease the number of complex points (TD)
• Increase the number of complex points (acquire more data points) or increase acquisition time (AQ)
• Decrease relaxation delay to zero
• Change solvent to non‑deuterated solvent

Correct Answer: Increase the number of complex points (acquire more data points) or increase acquisition time (AQ)

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