Infrared spectroscopy

Infrared radiation and covalent bonds

  • Atoms in molecules are joined by covalent bonds which possess energy and vibrate naturally around a certain point. 
  • The amount of bvibration increases with increasing tempertaure.
  • The bonds can absorb IR radiation which makes them bend or stretch more. 
  • The amount that a bond stretches or bends depends upon two factors.
  • The mass of the atoms - heavier atoms vibrate more slowly than lighter ones.
  • The strength of the bond - stronger bonds vibrate faster than weaker ones. 
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IR radiation and atmospheric gases

  • IR radiation passes through the atmosphere to the Earth's surface where most of it is absorbed. However, some is re-emitted from the Earth's surface in the form of longer wavelength IR radiation. 
  • Water vapour, carbon dioxide and methane absorb this longer wavelength IR radiation because it has the same frequency as the ntural frequency of their bonds. 
  • The vibrating bonds in these molecules re-emit this energy as radiation that increases the temperature of the atmosphere close to the Earth's surface, leading to global warming. 
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IR spectroscopy and organic molecules

  • The smaple under investigation is placed inside an IR spectrometer. 
  • A beam of IR radiation in the range 200-4000 cm-1 is passed through the sample. 
  • the molecule absorbs some of the IR frequencies, and the emerging beam of radiation is analysed to identify the frequencies that have been absorbed by the sample. 
  • The IR spectrometer is usually connected to a computer that plots a graph of tranmsmittance against wavenumber. 
  • The dips in the graph are called peaks. Each peak is observed at a wavenumber that can be related to a particular bond in the molecule. 
  • Below 1500cm-1 there are a number of peaks in what is known as the fingerprint region of the spectrum. The fingerprint contain unique peaks which can be used to identify the particular molecule under investigation, either using computer software or by physically comparing the spectrum to booklets of published spectra. 
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Applications of IR spectroscopy

  • Many pollutants can be identified from their IR spectral fingerprints. 
  • Remote sensors analyse the IR spectra of vehicle emissions to detect and measure carbon monoxide, carbon dioxide, and hydrocarbons in busy town centres or by motorways to monitor localised pollution. 
  • IR based breathalysers pass a beam of IR radiation through the captured breath in the sample chamber and detect the IR absorbance of the compounds in the breath. 
  • The characteristic bonds present in ethanol are detected. The more IR radioation absorbed, the higher the reading, and the more ethanol in the breath. 
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