Infrared Spectroscopy

  • Spectroscopy is based upon the idea of measuring the energy needed to produce a change from one energy level to another.
  • The energy possessed by chemical particles is quantised; there can only be a number of definite energy values, rather than a whole range of energy values.
  • Infrared spectroscopy takes advantage of the vibrational energy changes that occur in molecules when they are exposed to infrared radiation (1014 – 1013 Hz).
  • Frequency and Wavelength are related to each other:

    C = v
    (C = Speed of Light)

  • The Wavenumber is the reciprocal of the wavelength (1/), and thus is a direct measure of the frequency.

Interaction between Infrared radiation and molecules

  • The bonds in simple diatomic molecules (HCl, HI, HBr…) can only vibrate by stretching:
  • For these molecules there is only one infrared absorption. The energy of this corresponds to the molecules changing from their lowest vibrational energy level, to the next higher one.
  • The energy needed to excite a vibration is different for each molecule; this is because this energy is related to the bond strength (weaker bonds require less energy to vibrate than stronger ones).
  • Thus the frequencies of infrared absorption differ for each molecule.
  • In more complex molecules, there are more ways in which bond deformations are possible. Most of these more complex molecules involve more than two atoms.
  • Carbon dioxide can vibrate in the following ways:
  • Below 1500cm-1 there is a very detailed “fingerprint” region which can be used with a database of infrared spectra for identification by comparison.
  • Even more complex molecules have more vibrational modes. These are sometimes labelled by the descriptions (rocking, scissoring, twisting and wagging).
  • All these vibrational modes and the fact that different bonds have different energies lead to infrared spectrums being very complicated.
  • Not all of the spectrum has to be identified however, only one or two peaks may need to be identified.
  • The peaks are characteristic of specific bonds.
  • Below is the infrared spectrum of ethanol with some identified fingerprints:

How the Infrared Spectrometer works

  • Infrared radiation from a heat source is split into two beams.
  • One of the beams goes through the sample, and the other goes through a reference chamber, ensuring that absorptions from water and carbon dioxide in the air are cancelled out.
  • The beams are then sent along the same path, in separate alternating pulses, achieved using a beam chopper (rotating disc with segment cut out of it).
  • The beams are then analysed by passing them through a sample of sodium chloride or potassium chloride (both invisible to IR radiation) or through a diffraction grating.
  • Only light of one particular frequency is allowed to focus onto the detector at one time.
  • The spectrum is achieved by scanning the frequencies and recording the associated light intensities.
  • When the sample is not absorbing, there will be no difference between the alternating pulses reaching the detector, so no signal is recorded.
  • When a vibration is being excited, the sample beam intensity will be reduced and a signal generated.
  • The record of an IR spectrum seems to be upside down (the baseline is at the top); however it is transmittance that is being recorded and this is at a maximum when no light is being absorbed.

Interpreting the Spectra

  • Below is the infrared spectra of oct-1-ene, which shows most of the characteristic absorptions of a hydrocarbon.
  • The peaks on the spectrum can be identified using the absorption energies in the data book.
  • The C-H absorption arises at around 3000 cm-1 (the shoulder of the stronger and broader absorption of the OH bond (see ethanol’s spectrum))
  • The OH absorption is more intense in the ethanol spectrum even though there are five times as many C-H bonds as OH bonds. Likewise, in the spectrum of propanone the C=O bond (1720 cm-1) is very intense compared with the C=C bond in octene.
  • These differences in intensities are because the strongest absorptions arise when there is a large change in bond polarity associated with the vibration.
  • Hence polar bonds such as O-H, C-O and C=O give more intense absorptions than non-polar C-H, C-C and C=C bonds.

Example analysis of a compound

The infrared spectrum of benzoic acid is shown below:

  1. An O-H group is present in the molecule (identified by the region between 2500-3300 cm-1)
  2. Absorption between 1680-1750 shows that a C=O is present.
  3. Absorption around 1300 cm-1 again shows the presence of an OH group.
  4. Absorption around 900-1100 cm-1 could be due to the benzene ring or to the carbon-oxygen bond of the acid grouping.

    Useful books for revision

    Revise A2 Chemistry for Salters (OCR A Level Chemistry B)
    Salters (OCR) Revise A2 Chemistry