What are the factors that influence the intensity of an IR absorption band?
The most important factor that influences the intensity of an IR absorption band is the change in dipole moment that occurs during a vibration.
For example, an aldehyde C=O stretch usually occurs near 1730 cm⁻¹. An alkene C=C stretch usually occurs near 1650 cm⁻¹.
The C=O stretch is much more intense than the C=C stretch. The C=O bond is highly polar, so its dipole moment changes considerably as the bond stretches (
Butyraldehyde has an intense C=O absorption at 1731 cm⁻¹.
In contrast, the C=C absorption in 2,3-dimethylbut-2-ene is not detectable.
The C=C bond is nonpolar, and the molecule is symmetrical, so there is no change in dipole moment during the stretch.
You can see the C=C stretch in a terminal alkyne such as oct-1-ene.
The bond is not completely symmetric, so there is a small change in dipole moment. But the C=C peak at 1644 cm⁻¹ is still quite weak.