Infrared Spectroscopy
Definition and meaning of Infrared Spectroscopy in chemistry.
Infrared spectroscopy is an analytical technique used to identify specific functional groups within a molecule by measuring how the substance absorbs infrared light. The absorbed energy causes the chemical bonds within the molecule to vibrate and bend.
In more detail
Infrared spectroscopy (IR spectroscopy) is an indispensable analytical technique that chemists use to discover the structural secrets of organic molecules. The process relies on shining a beam of invisible infrared light, which carries heat energy, directly through a chemical sample. When the frequency of the incoming infrared light perfectly matches the natural vibrational frequency of a specific chemical bond in the molecule, the bond absorbs that energy.
This absorption causes the bond to stretch, contract, or bend vigorously, much like a spring absorbing the energy of a bouncing weight. Because different types of chemical bonds (such as single, double, or triple bonds) and different pairings of atoms (such as carbon-oxygen versus carbon-hydrogen) have unique strengths and masses, they each vibrate at very specific, signature frequencies.
An infrared spectrometer measures exactly which frequencies of light are absorbed by the sample and which pass through untouched. The resulting graph, known as an IR spectrum, plots the amount of light transmitted against the frequency of the light. The sharp dips on the graph represent the specific frequencies where light was strongly absorbed by vibrating bonds.
Chemists read an IR spectrum by looking for these signature dips, which act as clear fingerprints for specific functional groups. For example, a massive, broad dip on the left side of the graph almost always indicates the presence of a hydroxyl (OH) group, revealing the molecule is an alcohol.
A sharp, distinct dip in the middle often points to a carbonyl (C=O) double bond. By piecing these clues together, scientists can confidently identify the types of functional groups present in a completely unknown sample.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Energy Source | Infrared radiation |
| Physical Effect | Causes chemical bonds to stretch, bend, and vibrate |
| Primary Use | Identifying specific functional groups in organic molecules |
| Graph Output | An IR spectrum plotting percent transmittance against wavenumber |
| Fingerprint Region | A complex section of the graph unique to every single molecule |
If an unknown clear liquid produces an IR spectrum with a broad absorption dip around 3300 wavenumbers, the chemist can immediately conclude the liquid contains an alcohol functional group.
Frequently asked questions
What is a wavenumber?
A wavenumber is the standard unit used on the x-axis of an IR spectrum; it represents the number of waves that fit into exactly one centimeter.
Can infrared spectroscopy tell you the exact molecular formula?
No, it can only tell you what "pieces" or functional groups are present, not how many there are or exactly how they are connected.
Why do some bonds not show up on the IR spectrum?
For a bond to absorb infrared light, its vibration must cause a change in the molecule's overall dipole moment. Symmetrical bonds often remain invisible.