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).
Each bond has a natural frequency of vibration. When infrared radiation of exactly that frequency passes through the sample, the bond absorbs the energy and vibrates more. Radiation of other frequencies passes straight through. This is why each type of bond absorbs at a characteristic wavenumber.
The frequency at which a bond vibrates depends on:
Bond strength: stronger bonds vibrate at higher frequencies, so they absorb at higher wavenumbers. C=O (1680 to 1750 cm−1) absorbs at a higher wavenumber than C-O (1000 to 1300 cm−1).
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More in Organic Analysis
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- High Resolution Mass Spectrometry
- Precise Relative Molecular Mass
- Infrared Spectroscopy
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- Interpreting IR Spectra
- Infrared Radiation and the Greenhouse Effect
All 9 lessons in Organic Analysis · All AQA AS-level Chemistry topics