Small rings introduce strain inside the

Small rings introduce strain inside the ring and higher s character outside it 

The two peaks for anhydrides are the symmetrical and anti symmetrica I stretches for the two C=0 groups see Chapter 3, p. 71, 

Conjugation with tc electrons or lone pairs affects IR C=0 stretches 

We need to see how conjugation works when it is with a tt bond rather than with a lone pair. This will make the concept more general as it will apply to aldehydes and ketones as well as to acid groups. How can we detect if an unsaturated carbonyl compound is conjugated or not Well, compare these two unsaturated aldehydes. 

Because the infrared carbonyl frequencies follow such a predictable pattern, it is possible to make a simple list of correlations using just three factors. Two are the ones we have been discussing—conjugation (frequency-lowering) and the inductive effect (frequency-raising). The third is the effect of small rings and this we next need to consider in a broader context. 

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