IR Introduction

Typically when a molecule is exposed to infra-red (IR) radiation, it absorbs specific frequencies of radiation. The frequencies which are absorbed are dependent upon the functional groups within the molecule and the symmetry of the molecule. IR radiation can only be absorbed by bonds within a molecule, if the radiation has exactly the right energy to induce a vibration of the bond. This is the reason only specific frequencies are absorbed.

Infrared spectroscopy focuses on electromagnetic radiation in the frequency range 400-4000cm-1, where cm-1 is known as wavenumber (1/wavelength), which is equivalent to frequency. To generate the infrared spectrum, radiation containing all frequencies in the IR region is passed through the sample. Those frequencies which are absorbed appear as a decrease in the detected signal. This information is displayed as a spectrum of % transmitted radiation plotted against wavenumber:

The spectrum here is plotted backwards. Right click the spectrum and select reverse plot.

Infrared spectroscopy is very useful for qualitative analysis (identification) of organic compounds because a unique spectrum is produced by every organic substance with peaks corresponding to distinct structural features. Also, each functional group absorbs infrared light at a unique frequency. For example, a carbonyl group, C=O, always absorbs infrared light at 1670-1780 cm-1, which causes the carbonyl bond to stretch.

A carbonyl group always absorbs infrared radiation in this frequency range because the bond between the carbon atoms is constantly stretching and contracting within a range of bond lengths. This "vibration" occurs as if the bond was a spring connecting the two atoms and it always occurs within a certain frequency range, 1670-1780 cm-1. When a molecule is irradiated with infrared radiation, a vibrating bond will absorb energy of the same frequency as its vibration, making the bond vibrate even faster.

In addition to identifying the molecule using IR spectroscopy, often the strength of the bonds within the molecule can be estimated by comparing their stretching ferquency. Just as with a spring, a large vibration frequency corresponds to a long bond, ie a weak bond and a small frequency corresponds to a short bond, ie a strong bond.

The C-H stretch shown appears as a peak at around 3078 cm-1 in the spectrum above.