Ring size effect on internal double bonds

For internal double bonds, absorption frequency decreases as ring size decreases. This reaches a minimum at 90 degrees, then the frequency rises again:

This is because the C=C stretch in rings is strongly coupled to the neighboring C-C stretch, which happens more as their vectors are in line.

As the angle approaches 90 degrees, the vector sum approaches 0, hence the degree of coupling approaches 0 and frequency approaches a minimum. Below 90 degrees, the coupling can once again take place.

If an endo double bond is part of a ring fusion, frequency moves to the right an amount equilivant to one carbon being removed from the ring:

Absorption frequency increases significantly when one or more alkyl groups are attached to the double bond. This effect is more pronounced for smaller rings, especially cyclopropene.

For this to be useful, ring size is typically determined first via another method,otherwise the frequency can represent too many possibilities.