Infrared Spectroscopy: How Molecules Vibrate

Chemical bonds aren’t rigid sticks. They behave more like springs, constantly stretching, compressing and bending. Each type of bond vibrates at its own natural frequencies, and those frequencies happen to fall in the infrared region of the spectrum. Shine infrared radiation through a sample, see which frequencies it absorbs, and you learn which bonds, and so which functional groups, it contains.

Infrared (IR) spectroscopy is one of the fastest ways to answer the question “what kind of molecule is this?”, and it’s used in everything from organic chemistry labs to roadside breathalysers and satellite climate monitoring.

Bonds as springs

Imagine two balls joined by a spring. Pull them apart and let go, and they oscillate at a natural frequency that depends on:

• the stiffness of the spring: stronger bonds vibrate at higher frequencies. Triple bonds > double bonds > single bonds.
• the masses of the balls: lighter atoms vibrate at higher frequencies. Bonds to hydrogen (O–H, N–H, C–H) vibrate at high frequencies because hydrogen is so light.

A molecule absorbs infrared radiation when the radiation’s frequency matches one of its vibrational frequencies and the vibration changes the molecule’s dipole moment. That second condition matters: symmetric molecules like N₂ and O₂ don’t absorb IR, but CO₂, H₂O and CH₄ do. That’s why nitrogen and oxygen aren’t greenhouse gases, but carbon dioxide, water vapour and methane are.

Types of vibration

• Stretching: the bond length changes. Can be symmetric or asymmetric.
• Bending: the angle between bonds changes (scissoring, rocking, wagging, twisting).

Stretching vibrations usually occur at higher frequencies than bending vibrations.

Wavenumbers

IR spectra are plotted using wavenumber, measured in cm⁻¹:

wavenumber = 1 ÷ wavelength (in cm)

Wavenumber is proportional to frequency and energy, so higher wavenumbers mean higher-energy vibrations. A typical IR spectrum runs from 4000 cm⁻¹ (left) to about 400–500 cm⁻¹ (right).

Reading an IR spectrum

An IR spectrum plots % transmittance (y-axis) against wavenumber (x-axis).

• Absorptions appear as dips (troughs), pointing downwards, because less light is transmitted at those wavenumbers.
• The x-axis runs from high to low wavenumber, left to right.
• Peaks are described by position (wavenumber), intensity (strong, medium, weak) and shape (sharp or broad).

Key absorptions to learn

Bond	Found in	Wavenumber (cm⁻¹)	Appearance
O–H	alcohols	3200–3550	strong, broad (hydrogen bonding)
O–H	carboxylic acids	2500–3300	very broad, often overlapping C–H
N–H	amines, amides	3300–3500	medium; primary amines often show two peaks
C–H	alkanes, alkenes, arenes	2850–3100	medium to strong, sharp
C≡N	nitriles	2220–2260	medium, sharp
C≡C	alkynes	2100–2260	weak to medium
C=O	aldehydes, ketones, acids, esters, amides	1650–1750	strong, sharp: often the most obvious peak
C=C	alkenes	1620–1680	medium to weak
C=C (aromatic)	benzene rings	about 1450–1600	several peaks
C–O	alcohols, esters, ethers, acids	1000–1300	strong
C–Cl, C–Br	haloalkanes	500–800	in the fingerprint region

Exam data sheets usually provide a similar table; learn to use it quickly.

The fingerprint region

Below about 1500 cm⁻¹, spectra contain many overlapping absorptions from complex vibrations of the whole molecule. This fingerprint region is unique to each compound, like a human fingerprint.

It’s hard to interpret peak by peak, but it’s very useful for identification: if the fingerprint region of an unknown matches that of a known compound in a database, they’re almost certainly the same substance.

A step-by-step method

1. Look above 3000 cm⁻¹. A broad peak around 3200–3550 suggests O–H in an alcohol. A very broad one from 2500–3300 suggests a carboxylic acid. Sharper peaks at 3300–3500 suggest N–H.
2. Look around 1700 cm⁻¹. A strong, sharp peak means C=O is present (aldehyde, ketone, acid, ester or amide).
3. Combine the clues. O–H (very broad) + C=O → carboxylic acid. C=O but no O–H → ketone, aldehyde or ester. O–H (broad) but no C=O → alcohol.
4. Check the 1000–1300 region for C–O.
5. Use other techniques (mass spectrometry, NMR) to confirm the full structure.

Worked examples

Example 1: ethanol, CH₃CH₂OH

• Broad absorption at about 3300 cm⁻¹: O–H (alcohol)
• Absorptions around 2900 cm⁻¹: C–H
• Strong absorption around 1050 cm⁻¹: C–O
• No peak around 1700 cm⁻¹: no C=O.

Example 2: propanone (acetone), CH₃COCH₃

• Strong, sharp absorption at about 1715 cm⁻¹: C=O
• C–H around 2900–3000 cm⁻¹
• No broad O–H.

Example 3: ethanoic acid, CH₃COOH

• Very broad absorption from about 2500 to 3300 cm⁻¹: O–H (carboxylic acid)
• Strong absorption around 1710 cm⁻¹: C=O
• Together, these two strongly indicate a carboxylic acid.

Example 4: following an oxidation

When ethanol is oxidised to ethanoic acid, the IR spectrum changes: the alcohol O–H band is replaced by the very broad acid O–H band, and a strong new C=O absorption appears near 1710 cm⁻¹. If the product is only partly oxidised to ethanal (an aldehyde), there’s a C=O peak but no broad O–H. IR is an easy way to check how far an oxidation has gone.

The spectrometer

Modern instruments are Fourier transform infrared (FTIR) spectrometers. They pass all infrared frequencies through the sample at once, using an interferometer, then use a mathematical technique (the Fourier transform) to convert the signal into a spectrum. FTIR is fast and sensitive.

Many labs use ATR (attenuated total reflectance) accessories: the sample is simply pressed onto a diamond crystal, with no preparation needed. Older methods involved grinding solids with potassium bromide into discs, because KBr is transparent to IR.

Real-world uses

• Breathalysers: some evidential breathalysers measure the absorption of IR by ethanol vapour in breath, often at C–H or C–O absorption bands.
• Climate science: CO₂, CH₄, N₂O and H₂O absorb infrared radiation emitted by the Earth; this is the greenhouse effect. Satellites measure these gases by their IR absorption.
• Pharmaceutical quality control: checking the identity of raw materials by matching their fingerprint regions.
• Forensics: identifying drugs, paints and fibres.
• Polymer industry: identifying plastics for recycling.
• Gas monitoring: carbon monoxide and carbon dioxide detectors often use IR absorption.

Hydrogen bonding broadens peaks

The O–H peak of an alcohol is broad because hydrogen bonds between molecules slightly weaken and lengthen the O–H bonds by different amounts, so there’s a spread of vibration frequencies. In a very dilute solution in a non-polar solvent, where molecules can’t hydrogen bond to each other, the same O–H appears as a sharp peak near 3600 cm⁻¹.

Limitations

• IR identifies functional groups but usually not the full structure; it’s combined with mass spectrometry and NMR.
• Water absorbs strongly in the IR, which complicates measurements of aqueous samples.
• Symmetric bonds (such as the C=C in a symmetric alkene) may absorb only weakly.

Key takeaways

• IR radiation makes bonds vibrate; each bond type absorbs at characteristic wavenumbers (cm⁻¹).
• Stronger bonds and lighter atoms vibrate at higher wavenumbers.
• A vibration must change the dipole moment to absorb IR, which is why CO₂ and CH₄ are greenhouse gases but N₂ and O₂ aren’t.
• Key peaks: broad O–H (3200–3550 alcohol; 2500–3300 acid), sharp C=O (1650–1750), C–H (2850–3100).
• The fingerprint region (below 1500 cm⁻¹) identifies compounds by comparison with databases.