Structural Characterization: IR, 1H NMR, 13C NMR and MS of selected natural products MCQs With Answer

Structural Characterization: IR, 1H NMR, 13C NMR and MS of selected natural products is an essential revision resource for M.Pharm students preparing for MPC 104T. This quiz collection focuses on interpreting spectral data of common natural product classes—alkaloids, terpenes, phenolics and glycosides—using infrared, proton and carbon NMR, DEPT experiments and mass spectrometry. Questions emphasize practical pattern recognition (chemical shifts, coupling constants, multiplicities, isotopic patterns and characteristic fragmentations) and use of 2D correlations (HSQC/HMBC) to assign skeletons and functional groups. The MCQs are crafted to test both conceptual understanding and applied problem-solving needed for structure elucidation in pharmaceutical research and quality control.

Q1. Which functional group is most likely indicated by a sharp, strong IR absorption near 1715 cm−1 in a natural product?

• Ether
• Ketone
• Aldehyde
• Carboxylic acid

Correct Answer: Ketone

Q2. A 1H NMR signal at δ 9.8 ppm (1H, d, J = 2.0 Hz) in a natural product most likely corresponds to which proton type?

• Phenolic OH
• Aromatic proton
• Aldehydic proton
• Vinylic proton

Correct Answer: Aldehydic proton

Q3. In a DEPT-135 13C NMR experiment which carbon type typically appears with an inverted (negative) phase?

• CH3
• CH2
• Quaternary carbon (C)
• CH

Correct Answer: CH2

Q4. In electron ionization (EI) mass spectrometry, the base peak is defined as:

• The molecular ion peak (M+)
• The most abundant fragment ion (highest intensity)
• The highest m/z peak observed
• The [M+1] isotopic peak

Correct Answer: The most abundant fragment ion (highest intensity)

Q5. The McLafferty rearrangement observed in EI-MS commonly requires which structural feature?

• An aromatic ring with a methoxy substituent
• A carbonyl group with a γ-hydrogen (six-membered cyclic transition)
• A tertiary alcohol adjacent to an aromatic ring
• A conjugated diene system

Correct Answer: A carbonyl group with a γ-hydrogen (six-membered cyclic transition)

Q6. A para-disubstituted benzene ring in 1H NMR typically gives which aromatic proton pattern?

• A single multiplet integrating to 4H
• Two doublets each integrating to 2H (AA’BB’ pattern)
• A complex multiplet of 5H
• A singlet integrating to 4H

Correct Answer: Two doublets each integrating to 2H (AA’BB’ pattern)

Q7. When a 1H NMR spectrum is recorded before and after addition of D2O, which change indicates the presence of an exchangeable proton (OH or NH)?

• Appearance of new multiplets in aromatic region
• Disappearance of a signal assigned to OH or NH after D2O addition
• Doubling of integrals for all aliphatic signals
• Shift of carbonyl carbon in 13C NMR

Correct Answer: Disappearance of a signal assigned to OH or NH after D2O addition

Q8. A methoxy group attached to an aromatic ring (anisole type) typically appears in 1H NMR at approximately:

• δ 0.9 ppm
• δ 1.6 ppm
• δ 3.7 ppm
• δ 5.5 ppm

Correct Answer: δ 3.7 ppm

Q9. A 13C NMR resonance observed near δ 200 ppm most likely corresponds to which carbon environment in a natural product?

• Carboxylic acid carbonyl
• Carbonyl carbon of a ketone or aldehyde
• Aromatic quaternary carbon
• Alkene sp2 carbon

Correct Answer: Carbonyl carbon of a ketone or aldehyde

Q10. In mass spectrometry, the presence of chlorine in a molecule is suggested by an M+2 peak whose intensity is approximately:

• Equal to M (1:1)
• About half of M (1:2)
• About one-third of M (1:3)
• Negligible compared to M

Correct Answer: About one-third of M (1:3)

Q11. The principal advantage of high-resolution mass spectrometry (HRMS) in natural product analysis is:

• Higher ionization efficiency for all compounds
• Ability to measure exact mass to high precision for elemental composition determination
• Direct observation of NMR coupling constants
• Determination of stereochemistry without further experiments

Correct Answer: Ability to measure exact mass to high precision for elemental composition determination

Q12. A vicinal coupling constant (3J) of ≈ 15 Hz between two olefinic protons typically indicates what stereochemistry across the double bond?

• Cis (Z) configuration
• Trans (E) configuration
• Geminal coupling
• No stereochemical information

Correct Answer: Trans (E) configuration

Q13. HMBC (Heteronuclear Multiple Bond Correlation) NMR experiments are primarily used to detect which type of correlation?

• One-bond (1J) H–C correlations only
• Long-range (2–3 bond) H–C correlations, useful for connecting quaternary carbons
• Proton–proton coupling networks only
• Direct observation of carbon–carbon connectivities only

Correct Answer: Long-range (2–3 bond) H–C correlations, useful for connecting quaternary carbons

Q14. A multiplet at δ 7.2–7.4 integrating to 5H in a 1H NMR spectrum typically indicates which substitution pattern on a benzene ring?

• Para-disubstituted benzene
• Monosubstituted benzene
• Ortho-disubstituted benzene
• Tetrasubstituted benzene

Correct Answer: Monosubstituted benzene

Q15. In the mass spectrum of a glycoside, a common neutral loss of 162 Da corresponds to loss of which moiety?

• Water (H2O)
• Methyl group (CH4 equivalent)
• Glucose (hexose) unit
• Carbon dioxide (CO2)

Correct Answer: Glucose (hexose) unit

Q16. An IR spectrum showing a very broad band from about 2500–3300 cm−1 together with a strong C=O stretch near 1700 cm−1 is diagnostic for which functional group?

• Primary amine
• Alcohol
• Carboxylic acid
• Aldehyde

Correct Answer: Carboxylic acid

Q17. Which 13C NMR experiment selectively displays only CH carbons (single proton attached)?

• DEPT-135
• DEPT-90
• Broadband 13C with proton decoupling
• HSQC

Correct Answer: DEPT-90

Q18. Which 13C NMR experiment will not give signals for quaternary carbons (carbons with no attached hydrogens)?

• DEPT-135
• Broadband 13C with proton decoupling
• INADEQUATE
• Natural abundance 13C with NOE suppression

Correct Answer: DEPT-135

Q19. A molecular ion region showing two peaks M and M+2 in approximately 1:1 ratio in EI-MS indicates the presence of which halogen?

• Chlorine
• Bromine
• Iodine
• Fluorine

Correct Answer: Bromine

Q20. Benzylic methylene protons (CH2 adjacent to an aromatic ring) in many natural products typically resonate in 1H NMR at approximately:

• δ 0.5–1.5 ppm
• δ 1.8–2.2 ppm
• δ 2.3–3.0 ppm
• δ 4.5–5.5 ppm

Correct Answer: δ 2.3–3.0 ppm

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