When a molecule absorbs energy in the infrared region (1–300 μm), the σ bonds of the molecule begin to vibrate. For simple diatomic molecules, such as H 2 or HCl, the only possible vibration is a movement of the two atoms away from and back to each other. This mode is referred to as a “bond stretch”. Triatomic molecules such as CO 2 have two distinct stretching modes—an asymmetrical and a symmetrical mode. In the symmetrical stretch, both oxygen atoms move away from the carbon atom at the same time. Conversely, in the asymmetrical stretch, one oxygen atom moves toward the carbon atom while the second oxygen atom moves away from the carbon atom.
Molecules of three or more atoms have continuously changing bond angles. The opening and closing of the bond angles are called bending modes. Some common bending modes are scissoring, rocking, twisting, and wagging. Scissoring and rocking are in‐plane bending while twisting and wagging are out‐of‐plane bending. Figure 1 illustrates these vibrational modes.
In a molecule, each bond and each group of three or more atoms absorbs infrared radiation at certain wave numbers to give quantized excited stretching and bending vibrational states. Only vibrations that cause a change in dipole moment generate an absorption peak. An observed absorption band (peak) at a specific wavelength proves the existence of a particular bond or group of bonds in the molecule. Conversely, the absence of a peak in the spectrum rules out the presence of the bond that would have produced it. The region between 1400–800 cm −1 is called the fingerprint region of the compound. In this region, so many peaks occur that accurately identifying their origin is impossible. However, because so many peaks exist in this region, two compounds whose spectra are identical in this region must be the same.