More factors that influence the C=O vibration

Conjugation with a alkene

This increases the single-bond character of the carbonyl, lowering its frequency by about 20-40:

Alkynes and aryl groups have a similar effect.

Conjugation with carbonyl also shifts the C=C stretch, by about 10 lower, and the C=C absorption is greatly intensified.

In a conjugated system sometimes the C=O peak is broadened or even split into a doublet. This results from two possible conformations, s-cis and s-trans. The s-cis absorbs at a higher frequency.

For amides, conjugation actually increases the carbonyl stretching frequency relative to the average amide. This is because the sp² character of the double bond pulls in electrons between the carbonyl atoms, strengthening the bond. While the resonance with the alkene is not enough to overcome to resonance with the nitrogen atom.

Ring size

6-membered rings with carbonyls are unstrained, so they absorb at typical values for the carbonyl, 1715 for cyclohexanone for example. Decreasing ring size increases the C=O stretch, for the same reason exo C=C bond stretches increase with decreasing ring size - more P character in the ring, so more s in the sigma bond, so a stronger bond, so a higher frequency.

For ketones and esters, there is about a 30 increase in frequency for each carbon removed from the ring.

For larger rings there isn't an clear rule, in this case frequency ranges from the typical 1715 to slightly less than that.

Apha-substitution effects

Similar to as described with esters and acid chlorides; attaching an electronegative element like chlorine onto the alpha carbon will strengthen the C=O bond.

Using the example of a subsituted chlorine atom, this can split the carbonyl stretch peak into two, due to two different conformations. One has the chlorine pointing away from carbonyl, and one has the chlorine rotated next to the carbonyl. Having the chlorine next to the carbonyl repels the non-bonding electrons in the oxygen atom, resulting in a stronger bond and higher frequency. This effect can be used to establish the conformation of rigid ring systems:

Hydrogen bonding

This lowers the C=O strength hence lowering the frequency. Examples are lowering of the C=O frequency in an acetic acid dimer, and lowering of the ester C=O frequency in methyl salicylate.