Chemical shift and influencing factors MCQs With Answer

Chemical shift and influencing factors MCQs With Answer

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Chemical shift and influencing factors MCQs With Answer is designed to reinforce core NMR concepts in the M. Pharm syllabus of Modern Pharmaceutical Analytical Techniques. Chemical shift is central to structure elucidation, stereochemical analysis, impurity profiling, and formulation compatibility studies. These practice questions go beyond definitions to probe how shielding, deshielding, magnetic anisotropy, hybridization, solvent, temperature, concentration, pH, and paramagnetic impurities modulate 1H and 13C chemical shifts. You will also test your grasp of reference standards (TMS, DSS/TSP), δ versus τ scales, field strength effects, and diagnostic shift ranges (alkyl, vinylic, aromatic, carbonyl, heteroatom-adjacent). Each MCQ includes a concise answer to cement mechanistic understanding that directly supports research and analytical method development.

Q1. In high-resolution NMR, the chemical shift δ is defined as which of the following?

  • (νreference − νsample)/ν0 × 10^6 (ppm)
  • (νsample − νreference)/ν0 × 10^6 (ppm)
  • (ν0 − νsample)/νreference × 10^6 (ppm)
  • (νsample/ν0) × 10^6 (ppm)

Correct Answer: (νsample − νreference)/ν0 × 10^6 (ppm)

Q2. Which statement about shielding and chemical shift is most accurate?

  • More shielding causes downfield shift (higher δ)
  • More shielding causes upfield shift (lower δ)
  • Shielding does not affect δ, only J-coupling
  • Shielding affects δ only at low magnetic fields

Correct Answer: More shielding causes upfield shift (lower δ)

Q3. Arrange the 1H chemical shifts of methyl protons in CH3F, CH3Cl, CH3Br, and CH3I from highest δ to lowest δ.

  • CH3I > CH3Br > CH3Cl > CH3F
  • CH3F > CH3Cl > CH3Br > CH3I
  • CH3Cl > CH3F > CH3I > CH3Br
  • CH3Br > CH3I > CH3Cl > CH3F

Correct Answer: CH3F > CH3Cl > CH3Br > CH3I

Q4. For 1H NMR, which hybridization order typically gives increasing chemical shift (δ) for attached protons?

  • sp3 < sp < sp2
  • sp < sp2 < sp3
  • sp2 < sp < sp3
  • sp3 < sp2 < sp

Correct Answer: sp3 < sp < sp2

Q5. The aldehydic proton (~9–10 ppm) appears far downfield primarily due to which combination of effects?

  • Electron donation by carbonyl and aromatic ring current
  • Magnetic anisotropy of C=O and inductive electron withdrawal
  • Scalar coupling to carbonyl and strong hydrogen bonding
  • Spin saturation and exchange with water

Correct Answer: Magnetic anisotropy of C=O and inductive electron withdrawal

Q6. Which statement best describes aromatic ring-current effects on 1H chemical shifts?

  • Protons outside the ring are shielded; inside the ring are deshielded
  • Protons outside the ring are deshielded; inside the ring are shielded
  • Ring current affects only 13C shifts, not 1H
  • Ring current shifts all aromatic protons upfield to ~2–3 ppm

Correct Answer: Protons outside the ring are deshielded; inside the ring are shielded

Q7. Which is the best rationale for a more downfield OH chemical shift in DMSO-d6 compared with CDCl3?

  • Stronger hydrogen bonding to DMSO deshields the OH proton
  • DMSO-d6 has a stronger external magnetic field
  • Lock signal in DMSO-d6 lowers δ values
  • DMSO-d6 eliminates exchange broadening

Correct Answer: Stronger hydrogen bonding to DMSO deshields the OH proton

Q8. Increasing temperature most commonly shifts exchangeable OH/NH protons in protic systems in which direction, and why?

  • Downfield, due to stronger hydrogen bonding at higher T
  • Upfield, due to reduced hydrogen bonding at higher T
  • No change in δ; only line width changes
  • Random drift, due to field-frequency lock instability

Correct Answer: Upfield, due to reduced hydrogen bonding at higher T

Q9. Moving from a 400 MHz to an 800 MHz spectrometer, which statement is correct?

  • Chemical shift in ppm doubles
  • Chemical shift in Hz doubles, ppm remains essentially constant
  • J-couplings double in Hz, ppm unchanged
  • Both δ (ppm) and J (Hz) remain unchanged

Correct Answer: Chemical shift in Hz doubles, ppm remains essentially constant

Q10. Tetramethylsilane (TMS) is used as a reference in organic solvents because it is:

  • Highly deshielded, giving a large positive δ
  • Highly shielded, inert, volatile, and gives a single sharp signal
  • Paramagnetic and enhances relaxation
  • Insoluble and removed before acquisition

Correct Answer: Highly shielded, inert, volatile, and gives a single sharp signal

Q11. In aqueous 1H NMR, which internal reference is commonly preferred over TMS?

  • DSS (4,4-dimethyl-4-silapentane-1-sulfonate) or TSP
  • Acetone-d6
  • CDCl3
  • Toluene

Correct Answer: DSS (4,4-dimethyl-4-silapentane-1-sulfonate) or TSP

Q12. Which 13C resonance range correctly matches the functional group?

  • Alkyl C: 110–160 ppm
  • Alkyne sp C: 0–40 ppm
  • Aldehyde/ketone C=O: ~190–220 ppm
  • Carboxylate/ester C=O: ~0–20 ppm

Correct Answer: Aldehyde/ketone C=O: ~190–220 ppm

Q13. Which 1H environment is typically most downfield?

  • Benzylic CH2 (~2.2–2.9 ppm)
  • Vinylic CH (~4.5–6.5 ppm)
  • Aldehydic CH (~9–10 ppm)
  • Acetylenic ≡CH (~2.0–3.0 ppm)

Correct Answer: Aldehydic CH (~9–10 ppm)

Q14. The residual proton signal of CDCl3 appears at approximately which δ in 1H NMR?

  • 7.26 ppm
  • 4.79 ppm
  • 2.50 ppm
  • 0.00 ppm

Correct Answer: 7.26 ppm

Q15. Which statement best describes the distance dependence of deshielding by an electronegative substituent (e.g., Cl) on 1H chemical shift?

  • Effect is similar for α, β, and γ positions
  • Strongest at α, weaker at β, minimal at γ
  • Strongest at γ due to hyperconjugation
  • Independent of distance; only orientation matters

Correct Answer: Strongest at α, weaker at β, minimal at γ

Q16. Why do terminal alkyne protons (≡C–H) resonate upfield (~2–3 ppm) compared to vinylic protons?

  • Stronger s-character increases deshielding
  • Alkyne anisotropy shields protons along the molecular axis
  • J-coupling reduces δ for sp carbons
  • Paramagnetic shift from π electrons in alkynes

Correct Answer: Alkyne anisotropy shields protons along the molecular axis

Q17. Which effect do trace paramagnetic impurities (e.g., Fe3+) most likely have on NMR spectra?

  • Sharpen peaks and shift them upfield
  • Broaden peaks and cause unpredictable shift changes
  • Increase chemical shift in ppm uniformly
  • Eliminate solvent residual peaks

Correct Answer: Broaden peaks and cause unpredictable shift changes

Q18. For carboxylic acids, how does increasing sample concentration typically affect the OH 1H chemical shift?

  • Shifts upfield due to monomer formation
  • Shifts downfield due to increased dimerization and H-bonding
  • Unaffected by concentration
  • Shifts to exactly 12.00 ppm regardless of structure

Correct Answer: Shifts downfield due to increased dimerization and H-bonding

Q19. Which statement about δ (ppm) versus τ (tau) scale is correct?

  • τ = δ + 10; higher τ means downfield
  • τ = 10 − δ; higher τ means upfield
  • τ = 10 × δ; scales are identical in direction
  • τ is obsolete and unrelated to δ

Correct Answer: τ = 10 − δ; higher τ means upfield

Q20. Which set correctly matches approximate 1H δ ranges for common environments?

  • Alkyl CH3: 0.8–1.5; Allylic CH: 1.7–2.3; O–CH: 3.0–4.5; Aromatic: 6.0–8.5
  • Alkyl CH3: 2.5–4.0; Allylic CH: 6.0–8.0; O–CH: 0.5–1.5; Aromatic: 9.0–10.0
  • Alkyl CH3: 6.0–8.0; Allylic CH: 0.8–1.5; O–CH: 9.0–10.0; Aromatic: 3.0–4.5
  • Alkyl CH3: 9.0–10.0; Allylic CH: 4.5–6.5; O–CH: 1.7–2.3; Aromatic: 0.8–1.5

Correct Answer: Alkyl CH3: 0.8–1.5; Allylic CH: 1.7–2.3; O–CH: 3.0–4.5; Aromatic: 6.0–8.5

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