Infrared Spectroscopy

• • In infrared (IR) spectroscopy, a beam of IR radiation is passed through a sample of a chemical.
  • The IR radiation is absorbed by the covalent bonds in the molecules, increasing their vibrational energy (they vibrate more).
  • Bonds between different atoms absorb different frequencies of IR radiation.
  • Bonds in different places in a molecule absorb different frequencies too - so the O-H bond in an alcohol and the O-H in a carboxylic acid absorb different frequencies.
  • Wavenumber is the measure used for the frequency (it's just 1/wavelength).
• • An infrared spectrometer produces a graph, known as a spectrum, that shows you what frequencies of radiation the bonds in a molecule are absorbing.
  • The peaks show you where radiation is being absorbed — the 'peaks' on IR spectra are upside down.
  • Before 1500cm^-1 is the fingerprinting region which is unique for each molecule. To use this region to identify a molecule it should be compared to a known spectra.
• Interpreting IR spectra

  
• Predicting infrared spectra

  
• Applications of IR spectroscopy

  • Breathalysers
  • Monitoring pollutants
• Breathalysers

  • The amount of ethanol vapour in the driver's breath is found by measuring the intensity of the peak corresponding to the C-H bond in the IR spectrum.
  • It's chosen because it's not affected by any water vapour in the breath. The IR spectrum from the breathalyser test is kept and can be used as evidence in court during the offender's trial.

  
• Monitoring pollutants

  • Infrared spectroscopy is used to monitor the concentrations of polluting gases in the atmosphere. These include carbon monoxide (CO) and nitrogen monoxide (NO), which are both present in car emissions.
  • The intensity of the peaks corresponding to the C…O or N=O bonds can be studied to monitor their levels.