Questions: ¹³C NMR and IR Spectroscopy for Structure Determination

Question 1

A compound with molecular formula C₈H₈ gives only 4 peaks in its ¹³C NMR spectrum. What is the most chemically reasonable interpretation?

• A. The instrument malfunctioned — a compound with 8 carbons must produce 8 peaks
• B. The compound has 4 unique carbon environments; the other 4 carbons are symmetry-equivalent to existing peaks
• C. Only carbons bearing hydrogen atoms appear in standard ¹³C NMR spectra
• D. The compound contains 4 carbon-13 atoms and 4 carbon-12 atoms; only ¹³C is detected

Answer: (B)

In ¹³C NMR, each chemically distinct carbon environment produces exactly one peak, but symmetry-equivalent carbons give only one signal. A compound with high symmetry (like styrene or cyclooctatetraene derivatives) can have far fewer peaks than total carbons. The number of peaks tells you the number of unique carbon environments, not the total carbon count. Option C is wrong because quaternary carbons (no attached H) appear in standard ¹³C NMR, though not in DEPT.

Question 2

An IR spectrum shows a broad absorption from 2500–3300 cm⁻¹ and a strong carbonyl peak at 1710 cm⁻¹. What functional group is most consistent with both absorptions together?

• A. Ketone — ketone C=O stretches appear near 1715 cm⁻¹ and no other absorptions are needed
• B. Aldehyde — the broad absorption is the characteristic aldehyde C-H stretch
• C. Carboxylic acid — the very broad O-H stretch (2500–3300 cm⁻¹) combined with a carbonyl near 1710 cm⁻¹ is diagnostic for -COOH
• D. Ester — esters show strong carbonyl peaks near 1735 cm⁻¹ and a broad O-H

Answer: (C)

The combination of a very broad O-H stretch from 2500–3300 cm⁻¹ (caused by strong hydrogen bonding in the carboxylic acid dimer) and a C=O stretch near 1710 cm⁻¹ is characteristic of a carboxylic acid. A ketone would lack the broad O-H absorption; an aldehyde would show a distinctive pair of weak C-H stretches near 2720 and 2820 cm⁻¹; an ester C=O would be higher (1735 cm⁻¹) and lacks the O-H stretch.

Question 3

Fewer peaks in a ¹³C NMR spectrum generally indicate that a molecule has fewer total carbon atoms.

• T. True
• F. False

Answer: False

The number of ¹³C peaks equals the number of *chemically distinct* carbon environments, not the total carbon count. A highly symmetric molecule like benzene (C₆H₆) gives only one ¹³C peak because all 6 carbons are equivalent. Para-xylene (C₈H₁₀) gives only 4 peaks despite 8 carbons. Fewer peaks mean higher molecular symmetry, not fewer atoms.

Question 4

Calculating the degree of unsaturation from the molecular formula before interpreting spectral data is good practice because it constrains what structural features are possible.

• T. True
• F. False

Answer: True

Degree of unsaturation (= rings + double bonds + 2× triple bonds, calculated from the molecular formula) provides an independent structural constraint before any spectral interpretation. If DoU = 4, you know the structure must contain exactly 4 degrees of unsaturation in total — for example, one benzene ring (DoU = 4) accounts for all of them, ruling out additional carbonyl groups or rings. This prevents proposing structures that contradict the molecular formula.

Question 5

What additional structural information does DEPT provide that a standard ¹³C NMR spectrum alone cannot, and why is this important for structure determination?

Model answer: Standard ¹³C NMR shows the chemical shift and number of unique carbon environments but cannot distinguish how many hydrogens are attached to each carbon. DEPT uses different pulse angles to create subspectra where CH₃ and CH point upward, CH₂ points downward, and quaternary carbons (C with no H) disappear entirely. This tells you the substitution pattern of each carbon — methyl, methylene, methine, or quaternary — which is essential for assembling the connectivity of the molecule and distinguishing, for example, a CH₂ at 40 ppm from a quaternary C at the same shift.

Without DEPT, you might mistake a quaternary carbon (e.g., a carbon bonded to four other carbons in a ring junction) for a CH group with an unusual chemical shift. This ambiguity frequently leads to incorrect structure proposals. DEPT resolves it directly, making it a standard companion to ¹³C NMR in structure determination.