Infrared Spectroscopy

How Does It Work?

When infrared radiation is shone on a molecule:

• The covalent bonds (the connections between atoms) absorb some of this radiation
• This absorbed energy makes the bonds vibrate in specific ways
• Different types of bonds absorb different amounts of radiation at different frequencies

A machine called a spectrophotometer does the following:

1. It shines infrared waves at a sample (the substance being tested)
2. It measures how much radiation is absorbed at each frequency
3. It produces a graph called an IR spectrum showing which bonds are present

Types of Vibration

There are different modes (ways) that bonds can vibrate:

1. Symmetric Stretching

• Both ends of the bond stretch out and compress together at the same time
• Imagine pulling both ends of a spring outwards, then pushing them inwards together

2. Asymmetric Stretching

• One end stretches out while the other compresses
• Like pulling one end of a spring while pushing the other end

3. Symmetric Bending

• The atoms move toward or away from each other in a scissoring motion
• Like opening and closing a pair of scissors

4. Asymmetric Bending

• The atoms rock or sway from side to side

Each of these vibration modes happens at a specific frequency in the infrared region of the electromagnetic spectrum.