IR Absorption Frequency: Delocalization

Carbonyl groups in conjugated ketones absorb at lower stretching frequencies than nonconjugated ketones.

Electron delocalization in conjugated ketones induces a partial single-bond character in the carbonyl group, leading to absorptions at lower stretching frequencies.

If substituents are attached to the carbonyl group, then electronic effects influence its stretching absorptions.

For instance, the carbonyl group of an amide stretches at a lower frequency than the carbonyl group of a ketone.

This is attributed to the lone pair of nitrogen that participates in resonance electron donation, leading to a reduced double-bond character. The weaker carbonyl bond, therefore, shows weak IR absorptions.

The carbonyl group of an ester stretches at a higher frequency than the carbonyl group of a ketone.

This is due to the stronger carbonyl bond arising from the electron-withdrawing inductive effect of the oxygen atom.