Infrared Spectroscopy

Specification Reference Organic chemistry, Organic analysis 3.3.6.3

Quick Notes

Bonds in molecules absorb infrared (IR) radiation at characteristic wavenumbers.
Infrared spectra can identify functional groups in a molecule based on their absorption patterns.
The ‘fingerprint region’ (below 1500 cm⁻¹) allows identification of a specific molecule.

AQA A-Level Chemistry infrared spectrum highlighting the fingerprint region below 1500 cm−1

Full Notes

IR spectroscopy and how it works has been outlined in more detail here
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How Infrared Spectroscopy Works

IR radiation is passed through a sample.

Bonds in the sample can vibrate in specific ways, absorbing IR radiation at characteristic wavenumbers (measured in cm⁻¹).

AQA A-Level Chemistry diagram of an infrared spectrometer showing IR source, sample and detector

An infrared spectrum is produced, showing absorption peaks.

AQA A-Level Chemistry example of an infrared spectrum with labelled absorption peaks

Identifying Functional Groups Using IR Spectroscopy

Different functional groups absorb IR radiation at specific wavenumbers.

Peaks in IR spectra correspond to bond vibrations in a molecule.

By comparing absorbances in an IR spectra to know data book values, we can determine bond types (and functional groups) in the molecule.

AQA A-Level Chemistry reference table image of common IR absorption ranges for functional groups

For Example: Ethanoic acid (CH₃COOH) has two absorbances in its IR spectra that helps identify if. One for the O–H bond (2500 to 3000) and one for the C=O bond (1680 to 1750).

AQA A-Level Chemistry infrared spectrum of ethanoic acid showing broad O–H and sharp C=O absorptions

The ‘Fingerprint Region’ (Below 1500 cm⁻¹)

The region below approximately 1500cm^-1 on an IR spectra is unique to each molecule and is called the 'fingerprint region'.

AQA A-Level Chemistry zoomed view of the fingerprint region in an IR spectrum

As it is very complicated and associated with molecular vibrations (rather than just individual bonds) it is rarely studied in detail however can be used to identify a compound.

A sample's spectra is compared to to a database spectra for a given molecule to enable identification.

Infrared Absorption and Global Warming

Greenhouse gases (CO₂, CH₄, H₂O) absorb IR radiation, trapping heat in the atmosphere.

AQA A-Level Chemistry Diagram showing the greenhouse effect and greenhouse gases absorbing IR in Earth's atmosphere

IR absorption by these gases contributes to climate change.

Wavenumbers for key greenhouse gases:
CO₂: ~2350 cm⁻¹
CH₄: ~1300 cm⁻¹
H₂O: Broad absorption across multiple regions.

Summary

IR spectroscopy measures absorption of IR by vibrating bonds at characteristic wavenumbers.
Characteristic peaks reveal functional groups; the fingerprint region below 1500 cm⁻¹ confirms a specific molecule.
Interpreting key absorptions (e.g., broad O–H, sharp C=O) allows identification of functional groups such as alcohols, acids and carbonyls.
Greenhouse gases absorb IR strongly, contributing to global warming.