Some Vibrations Are Infrared Inactive

How could IR spectroscopy be used to distinguish between the following compounds  

For each of the following pairs of compounds, name one absorption band that could be used to distinguish between them. 

In order for the vibration of a bond to absorb IR radiation, the dipole moment of the bond must change when it vibrates. Therefore, not all bond vibrations give rise to an absorption band. 

For example, the C=C bond in 1-butene has a dipole moment because the molecule is not symmetrical. So when the bond stretches, the dipole moment changes and an absorption band is observed. 

In contrast, 2,3-dimethyl-2-butene is symmetrical, so its C = C bond has no dipole moment. When the bond stretches, it still has no dipole moment. Since stretching is not accompanied by a change in dipole moment, no absorption band is observed. The vibration is said to be infrared inactive. 

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