Infrared spectroscopy

Infrared spectroscopy (IR spectroscopy) is the subset of spectroscopy that deals with the infrared region of the electromagnetic spectrum. It covers a range of techniques, the most common being a form of absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify compounds or investigate sample composition. Infrared spectroscopy.
The infrared portion of the electromagnetic spectrum is divided into three regions;

The near-, mid- and far- infrared, named for their relation to the visible spectrum.

The far-infrared, approximately 400-10 cm-1 (1000–30 μm), lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy.

The mid-infrared, approximately 4000-400 cm-1 (30–1.4 μm) may be used to study the fundamental vibrations and associated rotational-vibrational structure.

The higher energy near-IR, approximately 14000-4000 cm-1 (1.4–0.8 μm) can excite overtone or harmonic vibrations.

Simple diatomic molecules have only one bond, which may stretch. More complex molecules have many bonds, and vibrations can be conjugated, leading to infrared absorptions at characteristic frequencies that may be related to chemical groups.

For example, the atoms in a CH2 group, commonly found in organic compounds can vibrate in six different ways: symmetrical, antisymmetrical, stretching, scissoring, rocking, wagging and twisting.