Infrared spectoscopyMonday 1 May 2023

Irradiation with IR causes molecules to absorb the radiation as the bonds stretch or bend.

The energy needed to stretch or bend a bond depends on – the bond strength

- the bond length - the mass of the atoms in the

bond The frequencies absorbed are called ‘peaks’

although they are really troughs on the graphic printout.

Origin of IR spectra

The parts of a IR spectrometer.

An infrared spectrometer

Only molecules that change their polarity when they vibrate absorb IR.

So molecules such as H2 or Cl2 do not absorb IR but HCl does.

Which molecules absorb IR.

Stronger bonds require more energy to make them stretch of bend so their peaks occur at higher wavenumber (cm-1) values.

Hydrogen bonding results in a major widening of the O-H peak.

Position of the peaks

Molecules having more than two atoms can vibrate in different ways – this results in a variety of peaks being produced.

More complex shapes

Main regions of absorbtions

Main regions of absorbtions

IR spectrum of ethanol

IR spectrum of propanoic acid

IR spectrum of ethanal

IR spectrum of ethanoic acid