Infrared Spectroscopy

Infrared (IR) spectroscopy is used to identify functional groups in organic molecules. It works by measuring which frequencies of infrared radiation are absorbed by a compound, corresponding to specific bond vibrations. This lesson covers the principles of IR spectroscopy, key absorption frequencies, how to interpret IR spectra, and the link to greenhouse gases.

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How IR Spectroscopy Works

The Principle

Covalent bonds in molecules are not rigid — they vibrate. These vibrations include stretching (change in bond length) and bending (change in bond angle). When the frequency of infrared radiation matches the natural vibrational frequency of a bond, the radiation is absorbed and the amplitude of the vibration increases.

For a bond to absorb IR radiation, the vibration must cause a change in dipole moment. This is why symmetrical molecules like N₂, O₂, and H₂ do not absorb IR radiation (they have no dipole moment and their vibrations do not create one), whereas CO₂ and H₂O do absorb (their asymmetric stretches and bending modes change the dipole moment).

The IR Spectrum

An IR spectrum plots transmittance (%) on the y-axis against wavenumber (cm⁻¹) on the x-axis. Absorptions appear as dips (troughs) pointing downward.

• Wavenumber = 1/wavelength (in cm). Typical range: 4000–400 cm⁻¹.
• The region 1500–400 cm⁻¹ is the fingerprint region — a complex pattern of absorptions unique to each compound. It is difficult to assign individual peaks here, but the overall pattern can be used like a fingerprint to identify a compound by comparison with a database.

Key Definition: The fingerprint region (below ~1500 cm⁻¹) contains many overlapping absorptions from C–C, C–O, C–N stretches and various bending vibrations. No two compounds have the same fingerprint region, making it useful for identification.

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Key IR Absorptions

The following table summarises the characteristic absorptions you must know for A-Level Chemistry. These must be memorised or will be provided in the data booklet — check your specification.

Bond	Type of Compound	Wavenumber Range (cm⁻¹)	Appearance
O–H	Alcohols, phenols	3200–3600	Broad
O–H	Carboxylic acids	2500–3300	Very broad, overlaps C–H region
N–H	Amines, amides	3300–3500	Medium, may show two peaks (primary amine)
C–H	Alkyl groups	2850–2960	Medium to strong
C=O	Aldehydes, ketones, carboxylic acids, esters, amides	1680–1750	Strong, sharp
C=C	Alkenes	1620–1680	Medium
C–O	Alcohols, esters, ethers	1000–1300	Strong
C≡N	Nitriles	2200–2260	Medium, sharp
C≡C	Alkynes	2100–2260	Variable

C=O Absorption Detail

The exact position of the C=O absorption varies with the type of carbonyl compound:

Compound Type	Typical C=O Wavenumber (cm⁻¹)
Acid anhydride	1800–1850 (two bands)
Acid chloride	1770–1815
Ester	1735–1750
Aldehyde	1720–1740
Ketone	1705–1725
Carboxylic acid	1700–1725
Amide	1630–1690

Exam Tip: The C=O absorption is often the most useful single peak in an IR spectrum. It is strong, sharp, and its exact position narrows down the type of carbonyl compound. Always look for C=O first when interpreting an IR spectrum.

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Interpreting IR Spectra: A Systematic Approach

1. Look at the region 2500–3600 cm⁻¹ first:
   • Broad absorption around 3200–3600 cm⁻¹ → O–H (alcohol) or N–H (amine).
   • Very broad absorption from ~2500 to 3300 cm⁻¹ (often overlapping C–H peaks) → O–H of carboxylic acid.