BRAINCAKEBond Vibrations and IR Absorption
Covalent bonds are not rigid. They behave like springs, constantly vibrating by stretching (the bond length changes) and bending (the bond angle changes). When a molecule absorbs infrared radiation, the energy makes its bonds vibrate more.
Each bond has a natural frequency of vibration. A bond absorbs IR radiation only when the frequency of the radiation matches this natural frequency. The frequency depends on:
• bond strength: stronger bonds vibrate at higher frequency, so they absorb at higher wavenumber.
• mass of the atoms: bonds to light atoms vibrate faster. Hydrogen is the lightest atom, so O-H, N-H and C-H bonds absorb at high wavenumbers (above 2500 cm−1).
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More in Analytical Techniques
- Infrared Spectroscopy
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- Interpreting IR Spectra
- IR in Breathalysers and Pollution Monitoring
- Mass Spectrometry of Organic Compounds
- The Molecular Ion Peak
- The M+1 Peak
- Fragmentation
All 10 lessons in Analytical Techniques · All OCR AS-level Chemistry topics