Infrared Spectroscopy

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 correlation tables are tabulated in the literature. The electomagnetic spectrum represents all the different types of discovered radiation on and around our planet. Infrared spectroscopy uses radiation from the infrared section of the electromagnetic spectrum. The infrared radiation interacts with molecules vibrational energy levels or dipole moments. Molecules have a fixed vibrational energy level or dipole moments, that are specific to that particular molecule. On interaction with infrared radiation these molecules move to a higher vibrational energy level. In infrared spectroscopy the molecule absorbes a certain quantum of infrared energy according to its molecule mass, and the composition of the atoms within the molecule (such as the bonds in the molecule and the environments of other bonds within the molecule). A detector then detects the amount of IR absored by the sample and a graph is produced by a computer that can then be interpreted and the structure of the compound under observation can be determined.

IR thermal imaging can sometimes be used to detect cancerous tumours as there is an area in a cancerous tumor that emits more IR radiation than the surrounding tissue.

Background and Theory

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. The names and classifications of these subregions are merely conventions. They are neither strict divisions nor based on exact molecular or electromagnetic properties.

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 and antisymmetrical stretching, scissoring, rocking, wagging and twisting.