What is IR Spectroscopy?

Infrared (IR) spectroscopy is one of those beautifully simple yet powerful techniques in chemistry. Instead of looking at what something looks like, we look at how its bonds vibrate when exposed to infrared radiation.

Every bond in a molecule absorbs IR radiation at a specific frequency, giving us a unique fingerprint of the substance.

What Does an IR Spectrum Look Like?

An IR spectrum is a graph of:

Wavenumber (cm⁻¹) on the x-axis
Transmittance (%) on the y-axis

Dips in the graph show where radiation is absorbed.

The Key Idea: Bond Vibrations

When IR radiation hits a molecule:

Bonds stretch and bend
Each type of bond absorbs at a different frequency

Think of bonds like springs:

Stronger bonds → higher frequency
Heavier atoms → lower frequency

The Important Regions to Learn

1. The Functional Group Region (4000–1500 cm⁻¹)

This is where most of your identification marks come from in exams.

Key peaks to remember:

Bond	Wavenumber (cm⁻¹)	Shape
O–H (alcohol)	3200–3600	Broad
N–H	3300	Medium
C–H	2800–3000	Sharp
C=O	~1700	Strong & sharp

👉 If you see a broad peak around 3300 cm⁻¹, think alcohol immediately.

2. The Fingerprint Region (1500–500 cm⁻¹)

Complex and messy

Unique to each compound

Used to confirm identity by comparison with known spectra

👉 In exams, you rarely analyse this in detail — but it’s crucial in real chemistry.

Worked Example

Question: Identify the functional group from this IR data:

Broad peak at 3300 cm⁻¹

No peak at 1700 cm⁻¹

Answer:

Broad 3300 → O–H present

No 1700 → no C=O

✅ Substance is likely an alcohol

Exam Tips (AQA / OCR / Edexcel)

Always quote wavenumbers in your answer

Use key words:

“broad peak”

“sharp peak”

“strong absorption”

Link peak → bond → functional group

👉 A 2–3 mark question is often just:

“Peak at 1700 cm⁻¹ indicates C=O, therefore a carbonyl compound.”

Common Mistakes

❌ Confusing O–H and N–H (O–H is broader)

❌ Forgetting units (cm⁻¹)

❌ Ignoring absence of peaks (just as important!)

Why IR Spectroscopy Matters

In real labs, IR is used to:

Identify unknown compounds

Check purity

Support other techniques like NMR and mass spectrometry

It’s quick, reliable, and a cornerstone of analytical chemistry.

20 March 2026