Chemistry
Infrared Spectroscopy
Quick fact
Infrared spectroscopy can easily tell a diamond from a piece of graphite, even though both are pure carbon, because their atomic bonding patterns absorb infrared light at completely different wavelengths.
Why this is interesting
You know how a fingerprint can uniquely identify a person? Molecules have their own 'fingerprints' too—and infrared spectroscopy is the technique that reads them.
Read the full explanation
Understanding Infrared Spectroscopy
Imagine each molecular bond is like a tiny tuning fork—it vibrates at a specific natural frequency. When you shine infrared light (which carries energy at various frequencies) onto a sample, bonds that match the light's frequency absorb that energy and vibrate more intensely. The instrument measures how much light is absorbed at each frequency, creating a spectrum of peaks. Each peak corresponds to a particular type of bond (like O-H, C=O, or N-H) and its environment. By reading the positions and shapes of these peaks, chemists can identify which functional groups are present in the molecule—much like recognizing a song from its musical notes.
A deeper explanation
Infrared spectroscopy works because infrared photons have energies that match the energy differences between vibrational quantum states of covalent bonds. For a bond to absorb infrared radiation, its vibration must cause a change in the molecule's dipole moment—this is the selection rule. When the incoming light frequency matches the bond's natural vibrational frequency, resonance occurs and energy is transferred, reducing the transmitted light intensity. The resulting absorption bands are recorded as a spectrum (wavenumber vs. transmittance). The region from 4000–1500 cm⁻¹ shows characteristic group frequencies (e.g., O-H stretch around 3300 cm⁻¹), while the 'fingerprint region' (1500–400 cm⁻¹) is unique to the entire molecule. This technique is vital for quality control, drug analysis, and identifying unknown compounds because it provides immediate structural clues without destroying the sample.