Bending is a type of molecular vibration that generally produces a signal in the fingerprint region of an IR spectrum. (IR spectrum can be divided into two regions. The diagnostic region records signals from double bonds, triple bonds, and bonds to X-H. The fingerprint region records signals arising from single bonds.)
Bonds within molecules vibrate at all temperatures greater than absolute zero. Thus, IR radiation also causes vibrational excitation of bonds in a molecule, causing them to vibrate in a number of ways. The most simple to understand are bending and stretching, We can visualize bond vibrations by thinking about a bond as a spring between two atoms and then imagine trying to stretch or bend the spring. Mostly, we find that bending is easier than strectching. Therefore, back in context, bending vibrations are of lower energy and absorptions due to bending tend to occure at lower wavelenghts.
Example – Bending vibration of a water molecule. Notice: Bending vibration is characterized by the change in bond angle