Infra-red Spectroscopy

Chemistry· Unit 16: Introduction to analytical chemistry, Learning Objective 2· 25 min read

1. Principles of IR Absorption★★☆☆☆⏱ 8 min

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📘 Definition

Infra-red Spectroscopy

An analytical technique that uses IR radiation (wavenumber $400 - 4000 c m^{- 1}$ ) to identify covalent bonds. Only bonds that undergo a change in dipole moment when vibrating absorb IR radiation.

Example:

Polar bonds like O-H and C=O absorb IR strongly, while non-polar bonds like O=O do not absorb at all.

When IR radiation passes through a sample, bonds absorb energy at specific wavenumbers matching their natural vibrational frequency. A spectrum plots % transmittance of radiation (y-axis) against wavenumber (x-axis, decreasing from left to right).

\tilde{\nu} = \frac{1}{\lambda}

Where $\tilde{ν}$ = wavenumber ( $c m^{- 1}$ ), $λ$ = wavelength of radiation (cm).

📐 Worked Example

Explain why N₂ gas does not absorb IR radiation, but HBr does.

1
1. IR absorption only occurs when bond vibration causes a change in the molecule's net dipole moment.
2
1. N₂ has a non-polar N≡N bond; symmetric stretching does not change the dipole moment (which remains zero at all times).
3
1. HBr has a polar H-Br bond; stretching changes the distance between the partial charges, so the net dipole moment changes during vibration.
4
Therefore, only HBr absorbs IR radiation, while N₂ does not.

Exam tip:

Highly polar bonds produce strong, clear absorption peaks, while non-polar bonds produce weak or no peaks.

2. Characteristic Absorptions and Functional Groups★★★☆☆⏱ 10 min

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Each type of covalent bond absorbs IR within a narrow, characteristic wavenumber range that is mostly unaffected by the rest of the molecule. This makes it possible to identify functional groups directly from IR spectra.

📘 Definition

Fingerprint Region

The region of the IR spectrum between 400 $c m^{- 1}$ and 1500 $c m^{- 1}$ that contains unique absorptions from bending vibrations of the entire molecule. No two different compounds have the same fingerprint region.

Example:

Used to confirm an unknown compound matches a known reference sample.

📐 Worked Example

An unknown compound has a strong peak at 1720 $c m^{- 1}$ and a broad peak between 3200 – 3600 $c m^{- 1}$ . No peak is seen between 2500 – 3300 $c m^{- 1}$ . What functional groups are present?

1
1. A strong peak around 1700 – 1750 $c m^{- 1}$ is characteristic of a C=O (carbonyl) bond.
2
1. A broad peak between 3200 – 3600 $c m^{- 1}$ corresponds to an O-H bond in an alcohol. Carboxylic acid O-H would appear as a very broad peak between 2500 – 3300 $c m^{- 1}$ , which is not observed here.
3
1. The combination of a carbonyl peak and alcohol O-H peak means the compound contains both a hydroxyl group and a carbonyl group, for example a hydroxyketone or hydroxycarboxylic acid (ruling out a simple carboxylic acid due to the missing broad O-H peak at lower wavenumber).
4
Final answer: The compound contains an alcohol hydroxyl group and a carbonyl group.

3. Confirming Organic Structures with IR Spectra★★★☆☆⏱ 7 min

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In most CIE exam questions, you will be given a proposed structure for an unknown compound and asked to use the IR spectrum to confirm if the structure is correct. You will rarely need to determine a full unknown structure from the IR spectrum alone.

📐 Worked Example

A student proposes that an unknown compound is ethyl ethanoate (an ester, formula $C H_{3} C O O C H_{2} C H_{3}$ ). The IR spectrum has a strong peak at 1735 $c m^{- 1}$ and a strong peak between 1000 – 1300 $c m^{- 1}$ . There is no broad peak between 2500 – 3600 $c m^{- 1}$ . Does this support the student's proposal?

1
1. Ethyl ethanoate is an ester, so it contains a C=O bond and a C-O bond, but no O-H or N-H bonds.
2
1. Ester C=O has a characteristic absorption at 1730 – 1750 $c m^{- 1}$ , which matches the observed peak at 1735 $c m^{- 1}$ .
3
1. C-O bonds have a characteristic strong absorption between 1000 – 1300 $c m^{- 1}$ , which matches the observed peak in this range.
4
1. The absence of a broad O-H peak between 2500 – 3600 $c m^{- 1}$ is consistent with ethyl ethanoate, which has no O-H bond.
5
All observed absorptions match the proposed structure, so the student's proposal is supported.

✓ Quick check

Test your understanding of key peaks:

Which peak would you expect to see in an amine but not an alkane?
- Peak at 1700 $c m^{- 1}$
- Peak at 3400 $c m^{- 1}$
- Peak at 1200 $c m^{- 1}$
- No peak difference
Reveal answer
Peak at 3400 $cm^{-1}$ —
Correct! Amines have an N-H bond that absorbs at 3300-3500 $c m^{- 1}$ , which alkanes do not have.

4. Common Pitfalls

Wrong move:

Confusing the O-H peak positions of carboxylic acids and alcohols

Why:

Both have broad O-H peaks, but they appear in very different wavenumber ranges

Correct move:

Alcohol O-H = broad at 3200-3600 $c m^{- 1}$ ; Carboxylic acid O-H = very broad at 2500-3300 $c m^{- 1}$

Wrong move:

Claiming all covalent bonds absorb IR radiation

Why:

Only bonds that produce a change in dipole moment when vibrating absorb IR

Correct move:

Non-polar bonds like O=O and N≡N do not absorb IR radiation, as their vibration does not change dipole moment

Wrong move:

Trying to identify functional groups from the fingerprint region first

Why:

The fingerprint region has many overlapping peaks from whole-molecule vibrations, making functional group identification difficult

Correct move:

Always analyze the region above 1500 $c m^{- 1}$ first to find key functional group peaks

Wrong move:

Assuming a peak at ~1700 $c m^{- 1}$ always means a carboxylic acid

Why:

All carbonyl groups (ketones, aldehydes, esters, carboxylic acids) have a C=O peak around 1700 $c m^{- 1}$

Correct move:

Check for other peaks (O-H for carboxylic acid, C-O for ester) to distinguish between carbonyl functional groups

5. Quick Reference Cheatsheet

Bond/Functional Group	Wavenumber Range ( $c m^{- 1}$ )	Peak Characteristics
O-H (alcohol, H-bonded)	3200 – 3600	Broad
O-H (carboxylic acid)	2500 – 3300	Very broad
C=O (all carbonyls)	1640 – 1750	Strong
C-O	1000 – 1300	Strong
N-H (amine/amides)	3300 – 3500	Medium, sharp
C=C (alkene)	1620 – 1680	Weak

6. Frequently Asked

Do I need to memorize all wavenumber ranges for CIE 9701 exams?

Most ranges are provided in the CIE data booklet, but you should memorize key ranges for O-H, C=O and C-O to quickly answer time-limited questions.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 22
Identify ester from IR spectrum
2023 · 13
Explain IR absorption for O-H bond
2024 · 21
Match spectrum to alcohol structure

Going deeper

official data bookletCIE 9701 IR Absorption TableWavenumber ranges are provided here for exams

What's Next

Infra-red spectroscopy is a core technique for confirming organic functional groups, and it is almost always used alongside other analytical methods to determine the full structure of unknown compounds. Mastery of IR peak interpretation is essential for both practical assessments and written exam questions in CIE A-Level Chemistry. Next, you can explore other key analytical techniques covered in the syllabus to build a full understanding of structure determination.