Infrared spectroscopy

• Covalent bonds possess energy and vibrate naturally about a central point, the amount of vibration increasing with increasing temperature. The atoms in molecules are therefore in constant motion.

  The bonds can absorb infrared radiation, which makes them bend or stretch more.
• One type of vibration, a stretch, is a rhythmic movement along the line between the atoms so that the distance between the two atomic centres increases and decreases.

  The second type of vibration, a bend, results in a change in bond angle.

  The amount that a bond stretches or bends depends on:

  • the mass of the atoms in the bond - heavier atoms vibrate more slowly than lighter atoms
  • The strength of the bond - stronger bonds vibrate faster than weaker bonds
• Any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond.

  The frequency values are very large so chemists use a more convenient scale called wavenumber, which is proportional to frequency.

  The vibrations of most bonds are observed in the IR wavenumber range of 200cm-1 to 4000cm-1
• Infrared spectroscopy is used to identify the functional groups present in organic molecules:

  1. The sample under investigation is placed inside an IR spectrometer
  2. A beam of IR radiation in the range 200-4000cm-1 is passed through the sample
  3. 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
  4. The IR spectrometer is usually connected to a computer that plots a graph of transmittance against wavenumber
• The dips in the IR spectrum are still 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 peaks can be difficult to identify in the fingerprint region
• Bonds with their corresponding wavenumber can be found on the data sheet
• Many pollutants can be identified by their IR spectral fingerprints. Remote sensors analyse the IR spectra of vehicle emissions to detect and measure carbon monoxide, carbon dioxide, and hydrocarbons in 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 radiation absorbed, the higher the reading, and the more ethanol in the breath
• A typical sequence for identification would include:

  • Elemental analysis - use of the percentage composition data to determine the empirical formula
  • Mass spectrometry - use of the molecular ion peak from a mass spectrum to determine the molecular mass; use of fragment ions to identify sections of a molecule
  • Infrared spectroscopy - use of absorption peaks from an infrared spectrum to identify bonds and functional groups present in the molecule
• Once you have both the empirical formula and the molecular mass of a compound, you can determine the molecular formula of your unknown compound. By then using evidence from the infrared spectrum, it may be possible to identify an unknown compound.