Determination of structures of small molecules: IR spectroscopy

The second segment of small molecular structure determination is IR spectroscopy. IR spectroscopy stands for infrared spectroscopy. IR spectroscopy tells us specific characteristics that the molecule has, which helps to narrow down possibilities.

It's like CSI and other detective dramas. Consider a group of people, one of which is the culprit. If I know that the culprit has black hair, I can narrow down the potential suspects. 

What is spectroscopy?
Molecules can absorb or emit light. How they absorb and emit light can tell us about the molecule itself. The science that involves shining light onto a molecule is called spectroscopy. Studying how molecules emit light is also called spectroscopy.

What is IR spectroscopy?
Infrared spectroscopy is where infrared light is shone onto a molecule and seeing what it absorbs. Consider aniline, which was introduced in mass spectrometry.

Aniline has a nitrogen connected to a hydrogen (an N attached to a H).

This causes the molecule to absorb specific types (wavelengths) of IR light. This shows up in the IR spectrum of aniline is shown below and the N-H dip is seen, where IR light is absorbed by aniline.

So if a scientist finds a molecule that he thinks is aniline. It has a mass of 93 Daltons, which is shown by mass spectrometry. It also has N-H which is shown by IR spectroscopy. This helps confirm to the scientist that they are correct.

If infrared spectroscopy is still not enough to work out what molecule the molecule is, we can the ultimate weapon: NMR spectroscopy.