Bonds in a molecule absorb infrared radiation at characteristic wavenumbers.

The idea

A covalent bond behaves like two masses joined by a spring, vibrating at a frequency fixed by the strength of the bond and the masses of the atoms it joins. When infrared radiation of exactly that frequency passes through the sample it is absorbed, the vibration becomes larger, and a trough appears on the spectrum. Because each type of bond has its own natural frequency, absorptions appear at characteristic wavenumbers, measured in cm⁻¹, which you look up on the data sheet rather than recalling. AQA credits any value inside the tabulated range, so the broad alcohol O–H is accepted anywhere from about 3230 to 3550 cm⁻¹ and the carbonyl C=O from about 1680 to 1750 cm⁻¹.

What earns the marks

• Quoting a wavenumber lying inside the range given on the data sheet, such as 1720 cm⁻¹ for C=O.
• Naming the bond responsible for that absorption.
• Describing the shape of the peak where it distinguishes, such as the broad alcohol O–H.
• Linking the absorption to vibration of the bond at a characteristic frequency.

Common mistakes and how to put them right

• Quoting a remembered wavenumber that falls outside the range on the data sheet.

• Reading the horizontal axis the wrong way, since wavenumber decreases from left to right.

• Giving the unit as cm or nm rather than cm⁻¹.

• Naming a functional group when the question asked for the bond.

Exam tips

• Always open the data sheet before quoting a value; examiners accept the tabulated range, not your memory.
• Write the unit cm⁻¹ every time, including inside explanations.
• Describe the peak as broad or sharp where the data sheet distinguishes them; the shape can carry a mark.