A coloured molecule absorbs visible light whose energy matches a jump its electrons can make. Infrared light, beyond the red end of the rainbow, carries less energy, and it sets the bonds themselves moving.
Picture a bond as a spring between two balls. The atoms are always on the move: the spring stretches and squeezes, and the angle between two bonds opens and closes. Chemists call these movements vibrations. A stretch changes a bond's length, and a bend changes the angle between two bonds.
Each bond has its own bounce
Two balls on a spring bounce at their own natural speed. A stiffer spring bounces faster, and so do lighter balls. Bonds work the same way:
- a double bond is a stiffer spring than a single bond, so C=O vibrates faster than C–O
- hydrogen is the lightest atom, so bonds to hydrogen, such as C–H and O–H, vibrate fastest of all
A bond absorbs infrared light whose waves match its own vibration, and then it vibrates harder. A C=O bond stretches and squeezes about 50 trillion times a second, and it absorbs infrared light whose waves rise and fall at that same rate.
Chemists label infrared light by its wavenumber: the number of waves that fit into one centimetre. About 1,700 waves of the light C=O absorbs fit into a centimetre, so chemists say C=O absorbs "at 1700". A bigger wavenumber means shorter waves, more energy and a faster vibration.
| Bond | Found in | Wavenumber |
|---|---|---|
| O–H | alcohols and carboxylic acids | 2500 to 3550 |
| C–H | nearly every organic molecule | 2850 to 3000 |
| C=O | acids, ketones, aldehydes and esters | 1680 to 1750 |
| C=C | alkenes | 1620 to 1680 |
| C–O | alcohols, acids and esters | 1000 to 1300 |
Moving charges absorb
A vibration absorbs infrared light when it moves electric charge about. A polar bond has a δ+ end and a δ− end, and as it stretches, those charges move apart and back. Polar bonds, such as C=O and O–H, absorb strongly. C–H bonds share their electrons almost evenly, so they absorb less.
A bond between two identical atoms shares its electrons evenly however far it stretches, so its stretch absorbs no infrared light. Nitrogen, N≡N, and oxygen, O=O, make up 99% of the air, and infrared light passes straight through them. Lesson 29 shows what this means for the carbon dioxide and methane in the air.