Chemistry
IR Spectroscopy
Quick fact
IR spectroscopy can distinguish between two molecules that have the same atoms but different shapes (isomers) because their vibrational patterns differ.
Why this is interesting
You’ve seen how visible light reveals colors, but what if you could 'see' the invisible bonds that hold a molecule together? That’s exactly what IR spectroscopy does—it lets us identify molecules by their unique vibrational 'fingerprints'.
Read the full explanation
Understanding IR Spectroscopy
Molecules are not static; their atoms constantly vibrate like balls connected by springs. Each type of bond (C-H, O-H, C=O, etc.) has a natural vibration frequency. Infrared light carries energy that matches these frequencies. When IR light of a specific frequency hits a molecule, it gets absorbed if the bond's vibration can be excited. A spectrometer scans through a range of IR frequencies and records which ones are absorbed. The result is an IR spectrum—a graph of absorption vs. frequency. Each distinct molecule produces a unique pattern, much like a fingerprint, allowing chemists to identify unknown substances or confirm expected structures.
A deeper explanation
The underlying mechanism involves changes in the molecule's dipole moment during vibration. For a vibration to absorb IR light, the bond must undergo a change in dipole moment; symmetrical vibrations like the stretch of a homonuclear diatomic molecule (e.g., N≡N) do not absorb IR. The frequency of absorption is determined by the bond strength and the masses of the atoms (Hooke's law). Peaks are typically reported in wavenumbers (cm⁻¹). Functional groups give characteristic peaks: O-H appears around 3300 cm⁻¹, C=O around 1700 cm⁻¹. This makes IR spectroscopy invaluable in organic chemistry for identifying functional groups, monitoring reactions, and analyzing complex samples in pharmaceuticals, forensics, and environmental science.