Final answer:
The compound with a conjugated system or significant resonance contribution, like a conjugated unsaturated ketone or an amide, has the lowest wavenumber for its carbonyl absorption in infrared spectroscopy.
Step-by-step explanation:
When evaluating the carbonyl absorption in infrared spectroscopy, compounds with carbonyl groups can exhibit different wavenumbers for this absorption based on their structural characteristics. Aldehydes, ketones, carboxylic acids, and esters show absorption at a higher frequency range (1700-1750 cm-1), while amides and conjugated unsaturated ketones exhibit lower wavenumbers (1650-1700 cm-1), due to conjugation and resonance effects that slightly reduce the carbonyl bond strength, leading to lower frequency vibrations. Therefore, the compound with a conjugated system or significant resonance contribution, like a conjugated unsaturated ketone or an amide, typically has the lowest wavenumber for its carbonyl absorption.