Infrared (IR) Spectroscopy

The identification, or determination, of organic structures is one of the most important tasks in organic chemistry. Generally, it is possible to perform this task by comparing the physical properties of a compound to a known standard (e.g., melting point, boiling point, etc.) or by using chemical tests to determine the presence of specific functional groups. Unfortunately, these methods are not useful for compounds that have not been synthesized or fully characterized before or that are very complex. Additionally, relatively large samples are needed, as these methods tend to be destructive to the sample. Most spectroscopic techniques (such as IR spectroscopy), on the other hand, tend to be nondestructive.

Spectroscopy is the study of how light interacts with matter. Spectroscopy techniques are used to determine the constituents of a sample of unknown chemical composition. In addition, the typical analysis only requires a few milligrams of material, and most spectroscopy techniques are nondestructive.

Absorption spectroscopy is the measurement of the amount of light absorbed by a compound as a function of the wavelength of light. Both infrared spectroscopy and ultraviolet spectroscopy are types of absorption spectroscopy. Infrared spectroscopy (IR spectroscopy) is a method that observes the vibrations of bonds and provides evidence of the functional groups present. In comparison, ultraviolet spectroscopy (UV spectroscopy) is a method that observes electronic transitions and provides information on the electronic bonding in the sample.

In general, the most useful information regarding the identity of a molecule will be what functional groups are present, and this can be found in the functional group region. A functional group is a group of atoms with specific physical, chemical, and reactivity properties. Additionally, some functional groups can be considered combinations of different bond types. An ester is a common example of this; the CO₂R contains both ${\rm{C{=}O}}$ and ${\rm{C{-}O}}$ bonds, and both bond types are functional groups that are seen in the functional group region. Because molecules absorb infrared light only at specific frequencies and wavelengths and their vibrational transitions correspond to distinct energies, functional groups have characteristic absorptions.

In infrared spectroscopy, wave numbers (cm^–1) are used to measure the location of stretches. Wave numbers (cm^–1) are the inverse of wavelength (cm), which means the wave number is proportional to energy/frequency, while wavelength is inversely proportional to energy/frequency. The infrared region of the electromagnetic spectrum is 4,000–400 cm^–1, and on an IR spectrum this region can be further subdivided into the functional group section (4,000–1,500 cm^–1) and the fingerprint section (${<}$ 1,500 cm^–1).

Please note that the IR spectrum shows the stretch, but the stretch is often (and incorrectly) referred to as peaks. This has become common usage, so it is important to understand that the word peaks (while incorrect) is often used. The vibrational bending and stretching of various types of bond groups and the frequencies at which these vibrations occur can be found in IR spectra data tables or tables of characteristic absorptions for functional groups. Additionally, these tables will often indicate what type of stretch each functional group will give. For example, an alcohol (OH) will give a very intense rounded stretch between 3,600 and 3,200 cm^–1, while a secondary amine (NH) gives one sharp stretch and a primary amine (NH₂) gives two sharp stretches between 3,500 and 3,300 cm^–1.

In general, carbon to hydrogen bonds (${\rm{C{-}H}}$) give stretches that appear above 2,800 cm^–1. However, the hybridization of the carbon can affect absorbance, with ${\rm{C(}}{sp}^3{\rm{){-}H}}$ appearing around 2,900 cm^–1, ${\rm{C(}}{sp}^2{\rm{){-}H}}$ around 3,100 cm^–1, and ${\rm{C(}}{sp}{\rm{){-}H}}$ around 3,300 cm^–1. Carbonyl groups (${\rm{C{=}O}}$) will produce very strong stretches that appear between 1,780 and 1,630 cm^–1. The different types of carbonyls also produce stretches at specific absorptions; for example, aldehydes give a stretch at 1,740–1,690 cm^–1, while amides give a stretch at 1,690–1,630 cm^–1. Providing there is no overlap between stretches, it is possible to use this information to determine both the number of carbonyls and the type present in a molecule.

Most of the spectrum is considered the functional group region (4,000–1,500 cm^–1), and most functional groups can be found in this region (alkanes, alkenes, alkynes, carbonyls, amines, alcohols, nitriles, etc.). The section below 1,500 cm^–1 is called the fingerprint section. This small section contains the stretches from alkyl halide bonds (${\rm{C{-}X}}$) and the stretches from a variety of bending vibrations (i.e., ${\rm{C{-}H}}$ bending and ${\rm{C{-}C}}$ bending). Identifying individual bonds in this region is much more difficult than identifying individual bonds in the functional group region. This is because there tend to be a lot of stretches and a lot of overlap between stretches. It is more useful to compare the fingerprint region of a sample to that of a known compound to confirm identity.