Infrared Spectroscopy

• Atoms in molecules are joined by covalent bonds. These bonds possess energy and vibrate naturally about a central point, the amount of vibration increases with increasing temperature
• The atoms in molecules are therefore in constant motion. The bonds can absorb infrared (IR) radiation, which makes the 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:

  1. the mass of the atoms in the bond - heavier atoms vibrate more slowly than lighter atoms
  2. the strength on 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 200cm-1 to 4000cm-1
• Much of the sun's visible and IR radiation is relatively unaffected by atmospheric gases. The 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 ('greenhouse gases') absorb this longer-wavelength IR radiation, bc it has the same frequency as the natural frequency of their bonds. Eventually 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
• water vapour, carbon dioxide and methane are the three most abundant greenhouse gases in the atmosphere

  many incentives are available to householders are available to householders for reducing pollution and for converting to renewable energy to renewable sources of energy that do emit carbon dioxide
• Infrared spectroscopy and organic molecules: chemists use infrared spectroscopy as a means of identifying the functional groups present in an organic molecule

  1. the sample under investigation is placed inside an IR spectrometer
  2. a beam of IR radiation in the range 200-400 cm-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
• 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:

  • contains unique peaks which can be used to identify the particular molecule under investigation
  • either using a computer software or by physically comparing the spectrum to booklets of published spectra
• the infrared spectra of common functional groups:

  • difficult to predict with certainty the identity of a functional group from a peak in the fingerprint region, but outside that region, peaks are clearer
• • all organic compounds produce a characteristic peak between 2850 and 3100 cm-1 from the presence of C-H bonds
  • often confused with the O-H peak in alcohols
• alcohol:

  • absorbance peak within the range 3200-3600 cm-1 caused by the O-H bond in an alcohol
  • also a peak between 1000-3000cm-1 caused by C-O - often difficult to see due to many other peaks in fingerprint
• aldehyde or ketone:

  • key absorbance peak within range 1630-1820 cm-2 caused by c=o bond - typically absorbs close to 1700cm-1
• carboxylic acid:

  • key absorbance peak within range 1630-1820cm-1 caused by the c=o bond - typically absorbs around 1700 cm-1
  • broad peak at 2500-3330 cm-2 caused by o-h group
  • also peak at 1000-1300 cm-1 that represents the c-o bond - not always reliable as it is in fingerprint
• Application of IR spec:

  • many pollutants can be identified by their IR spectral fingerprints
  • remote sensors analyse the IR spectra of vehicle emissions to detect and measure CO, CO2 ad 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 radiation absorbed, the higher the reading and the more ethanol in the breath
• putting it all together:

  • typical sequence for identification would include:
• 1. elemental analysis - use of percentage composition data to determine the empirical formula
  2. mass spec - 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
  3. infrared - use of absorption peaks from an infrared spectrum to identify bonds and functional groups present in the molecule
• once have the empirical formula and the molecular mass of a compound - can determine the molecular formula of unknown compound - by using evidence from IR spectrum possible to identify a compound