AQA Chem4: Infra-red Spectroscopy

• A pair of atoms joined by a chemical bond are always vibrating
• Stronger bonds vibrate faster at a higher frequency and heavier atoms make the bond vibrate more slowly at a lower frequency
• When we shine infra-red radiation through a sample, the bonds absorb energy from the radiation and vibrate more
• Particular bonds can only absorb radiation that has the same frequency as the natural frequency of the bond
• Therefore the radiation that emerges from the sample will be missing the frequencies that correspond to the bonds in the sample

• the peaks in an infra-red spectrum represent particular bonds
• you identify the type of bond using the infra-red absorption data
• in infra-red spectrums the region below 1500cm-1 is known as the fingerprint region, it is due to the vibrations of the whole molecule
• some bonds have broad peaks for example, the O-H alcohol group with an absorption of 3220-3550cm-1 while some bonds such as C=O have sharp peaks at 1680-1750cm-1

Ketones and Aldehydes:

• the peak produced by ketones and aldehydes is usually strong and sharp
• in aldehydes it is between 1740 and 1720 cm-1
• in ketones it is between 1725 and 1700 cm-1

Carboxylic Acids:

• have two important peaks
• one at around 3100 cm-1 due to an O-H peak
• one at around 1700 cm-1 due to a C=O peak

Esters:

• a strong C=O peak at around 1750 cm-1
• two peaks for the C-O bonds as the stretching frequency of each C-O bonds is different due to the different atoms around them