Infrared spectroscopy

Q: How do covalent bonds in molecules behave and react when they absorb infrared (IR) radiation?

Bonds act like flexible springs that stretch and bend; they absorb IR radiation matching their natural frequency, which increases their vibration amplitude.

Q: What two factors determine the frequency of infrared radiation absorbed by a bond?

Bond strength (stronger bonds absorb at higher frequencies) and atom mass (bonds with lighter atoms absorb at higher frequencies).

Q: What is a wavenumber and how does it relate to energy and frequency?

A wavenumber (\( \text{cm}^{-1} \)) is the reciprocal of wavelength (\( \frac{1}{\lambda} \)); higher wavenumbers correspond to higher frequencies and higher energies.

Q: Why do greenhouse gases like \( \text{CO}_2 \), \( \text{H}_2\text{O} \), and \( \text{CH}_4 \) absorb infrared radiation while \( \text{N}_2 \) and \( \text{O}_2 \) do not?

Greenhouse gases have polar bonds whose vibrations absorb outgoing IR radiation and change the dipole moment, whereas non-polar diatomic molecules (\( \text{N}_2 \), \( \text{O}_2 \)) cannot absorb IR.

Q: How are the axes arranged and read on a typical infrared spectrum?

The x-axis displays wavenumber (\( \text{cm}^{-1} \)) decreasing from high to low, and the y-axis displays percentage transmittance (\( \% \)), with absorptions appearing as downward troughs.

Q: Where does the carbonyl (\( \text{C=O} \)) absorption appear in an IR spectrum, and what does it look like?

It appears between \( 1650\text{ cm}^{-1} \) and \( 1750\text{ cm}^{-1} \) as a strong, sharp, deep trough.

Q: How can you distinguish an alcohol \( \text{O-H} \) peak from a carboxylic acid \( \text{O-H} \) peak on an IR spectrum?

An alcohol \( \text{O-H} \) is a smooth, broad trough at \( 3200-3600\text{ cm}^{-1} \), while a carboxylic acid \( \text{O-H} \) is an extremely broad, ragged band at \( 2500-3300\text{ cm}^{-1} \) that overlaps \( \text{C-H} \) peaks.

Q: What is the fingerprint region in an IR spectrum, and how is it used to identify a compound?

It is the complex region below \( 1500\text{ cm}^{-1} \) unique to each molecule, which is compared against a database of known spectra to confirm identity.

Q: What are two real-world analytical applications of infrared spectroscopy?

Roadside breathalysers (measuring ethanol \( \text{C-H} \) absorption) and monitoring air pollution (detecting gases like \( \text{CO} \) and \( \text{SO}_2 \)).

Chemistry 1110 · CCEA AS Level · 10 min read · Updated: Aug 31, 2026

Welcome to Infrared (IR) Spectroscopy!

Imagine being a chemical detective. You are given an unlabelled bottle containing a clear liquid. Is it an alcohol, a carboxylic acid, an aldehyde, or something else entirely? Instead of tasting it (which would be very dangerous!), chemists shine invisible infrared radiation through the sample to reveal its identity. In this chapter, you will discover how molecules absorb infrared radiation, how to read an infrared spectrum, and how to identify functional groups with confidence.

Don't worry if this topic looks intimidating at first glance! Reading an IR spectrum is a lot like recognising silhouettes: once you learn a few distinct shapes and numbers, you will be able to crack any structure with ease.

1. How Infrared Spectroscopy Works

Molecules in Motion: Stretching and Bending

In covalent molecules, bonds are not rigid sticks. Instead, think of covalent bonds as flexible springs that are constantly vibrating, stretching, and bending.

• Stretching vibration: The distance between two bonded atoms increases and decreases rhythmically along the bond axis.
• Bending vibration: The bond angle between three atoms changes rhythmically.

Analogy: The Ball-and-Spring Model

Imagine two heavy wooden balls connected by a metal spring. If you pull them apart and let go, they oscillate back and forth at a natural frequency. Heavy balls on a loose spring vibrate slowly, while light balls on a stiff, tight spring vibrate very rapidly. Molecular bonds behave in exactly the same way!

Absorption of Infrared Radiation

Infrared radiation lies just beyond visible red light in the electromagnetic spectrum. Every chemical bond vibrates at its own characteristic, natural frequency. When a molecule is hit with infrared radiation whose frequency matches the bond's natural vibration frequency, the bond absorbs energy and begins to vibrate with a greater amplitude (it stretches and bends more violently).

What determines the frequency absorbed?
• Bond Strength: Stronger bonds (like \( \text{C=O} \)) are stiffer and absorb at higher frequencies than weaker single bonds (like \( \text{C-O} \)).
• Atom Mass: Bonds involving lighter atoms (like \( \text{O-H} \) or \( \text{C-H} \)) vibrate faster and absorb at higher frequencies than bonds between heavier atoms (like \( \text{C-C} \) or \( \text{C-Cl} \)).

Wavenumbers: The Chemist's Unit of Frequency

Instead of using standard frequency in Hertz (which would involve enormous numbers), infrared spectra use a unit called the wavenumber, represented in \( \text{cm}^{-1} \) (reciprocal centimetres).
• Wavenumber is simply the number of waves per centimetre (\( \text{wavenumber} = \frac{1}{\text{wavelength in cm}} \)).
• A higher wavenumber corresponds to a higher frequency and higher energy.

Key Takeaway: Different covalent bonds absorb characteristic frequencies of IR radiation based on bond strength and the masses of the attached atoms. We measure this in wavenumbers (\( \text{cm}^{-1} \)).

2. The Greenhouse Effect and IR Radiation

The Science of Global Warming

The very same principle of molecular vibration explains how our planet stays warm—and why excess greenhouse gas emissions cause global climate change!

• The Sun emits short-wavelength radiation (visible and UV light) that passes through the atmosphere and warms the Earth's surface.
• The warm Earth radiates this energy back out into space as longer-wavelength infrared radiation.
• Greenhouse gases in the atmosphere, such as water vapour (\( \text{H}_2\text{O} \)), carbon dioxide (\( \text{CO}_2 \)), and methane (\( \text{CH}_4 \)), contain polar covalent bonds that absorb this outgoing IR radiation.
• The bonds inside these gas molecules vibrate more vigorously and re-emit the IR radiation in all directions, trapping heat in the lower atmosphere.

Did you know? Symmetrical, non-polar diatomic molecules like nitrogen (\( \text{N}_2 \)) and oxygen (\( \text{O}_2 \)), which make up \( 99\% \) of our air, cannot absorb IR radiation because their vibrations do not change the molecular dipole moment. That is why they are not greenhouse gases!

Key Takeaway: Greenhouse gases (\( \text{CO}_2 \), \( \text{H}_2\text{O} \), \( \text{CH}_4 \)) absorb IR radiation emitted by Earth because their covalent bonds vibrate at infrared frequencies, trapping heat in our atmosphere.

3. Reading an Infrared Spectrum

Understanding the Axes

When you look at an infrared spectrum, you will see a graph that looks upside down compared to regular maths graphs:

• Horizontal Axis (x-axis): Wavenumber in \( \text{cm}^{-1} \), running from high values on the left (typically \( 4000\text{ cm}^{-1} \)) to low values on the right (around \( 400\text{ cm}^{-1} \)).
• Vertical Axis (y-axis): Percentage Transmittance (\( \% \)). If all the radiation passes straight through the sample, transmittance is \( 100\% \) (top of the graph). When a bond absorbs radiation, less light passes through, producing a downward-pointing trough known as an absorption band or peak.

Quick Rule of Thumb: A deep trough pointing downwards means strong absorption at that specific wavenumber!

4. Identifying Key Functional Groups

At AS Level, you are expected to recognise three vital absorption peaks that reveal the presence of particular functional groups. You will always be supplied with a Data Booklet in exams, but learning these visual "shapes" makes identifying them effortless!

1. The Carbonyl Group: \( \text{C=O} \)

• Where it appears: Between \( 1650\text{ cm}^{-1} \) and \( 1750\text{ cm}^{-1} \).
• Appearance: A very strong, sharp, deep "sword-like" spike that points far down towards the bottom of the spectrum.
• Found in: Aldehydes, ketones, carboxylic acids, and esters.
• Memory Trick: Think of the \( \text{C=O} \) peak as a sharp icicle or dagger hanging from the ceiling near \( 1700\text{ cm}^{-1} \).

2. The Alcohol \( \text{O-H} \) Group

• Where it appears: Between \( 3200\text{ cm}^{-1} \) and \( 3600\text{ cm}^{-1} \).
• Appearance: A broad, smooth, rounded "U-shaped" trough.
• Why is it broad? Extensive hydrogen bonding between alcohol molecules weakens and varies the strength of the \( \text{O-H} \) bonds, causing them to absorb over a wider range of frequencies.
• Found in: Alcohols (e.g. ethanol, propan-1-ol).
• Memory Trick: The alcohol \( \text{O-H} \) looks like a smooth tongue or a wide bowl sitting well to the left of the spectrum.

3. The Carboxylic Acid \( \text{O-H} \) Group

• Where it appears: Between \( 2500\text{ cm}^{-1} \) and \( 3300\text{ cm}^{-1} \).
• Appearance: An extremely broad, ragged, uneven trough that overlaps the standard \( \text{C-H} \) stretch (which sits around \( 2850 - 3000\text{ cm}^{-1} \)).
• Why is it so broad? Carboxylic acids form very strong hydrogen-bonded dimers, spreading the absorption over a massive range of wavenumbers.
• Found in: Carboxylic acids (e.g. ethanoic acid).
• Memory Trick: A carboxylic acid spectrum looks like a messy, jagged "beard" spanning all the way from \( 2500 \) to \( 3300\text{ cm}^{-1} \).

Summary Comparison Table of Characteristic Bands

• \( \text{C-H} \) bond: \( 2850 - 3000\text{ cm}^{-1} \) — Sharp/medium peaks (present in almost all organic compounds).
• \( \text{O-H} \) (alcohol): \( 3200 - 3600\text{ cm}^{-1} \) — Broad, smooth, rounded scoop.
• \( \text{O-H} \) (carboxylic acid): \( 2500 - 3300\text{ cm}^{-1} \) — Very broad, ragged band overlapping \( \text{C-H} \).
• \( \text{C=O} \) (carbonyl): \( 1650 - 1750\text{ cm}^{-1} \) — Strong, narrow, deep spike.

Key Takeaway: Look for the presence or absence of two main indicators: the sharp spike of \( \text{C=O} \) (\( \sim 1700\text{ cm}^{-1} \)) and the broad troughs of \( \text{O-H} \) (\( 3200-3600\text{ cm}^{-1} \) for alcohols, \( 2500-3300\text{ cm}^{-1} \) for acids).

5. Step-by-Step Guide: How to Deduce a Compound

When given an unknown spectrum in an exam, follow these simple steps:

Step 1: Check for a \( \text{C=O} \) peak (\( 1650 - 1750\text{ cm}^{-1} \))
• If YES: The molecule contains a carbonyl group (aldehyde, ketone, carboxylic acid, or ester).
• If NO: The molecule is not an aldehyde, ketone, acid, or ester.

Step 2: Check for an \( \text{O-H} \) peak
• If there is a smooth, separate broad band at \( 3200 - 3600\text{ cm}^{-1} \), you have an alcohol.
• If there is an extremely broad, ragged band spanning \( 2500 - 3300\text{ cm}^{-1} \) combined with a \( \text{C=O} \) spike, you have a carboxylic acid.
• If there is no \( \text{O-H} \) peak at all, but a \( \text{C=O} \) is present, the compound is an aldehyde, ketone, or ester.

Step 3: Confirm using the molecular formula (if provided)
• For example, if the formula is \( \text{C}_3\text{H}_6\text{O} \) and you see a \( \text{C=O} \) peak but no \( \text{O-H} \), it must be propanal or propanone!

6. The Fingerprint Region

What is the Fingerprint Region?

The region of the spectrum below \( 1500\text{ cm}^{-1} \) is called the fingerprint region.

• Why is it special? This area contains a complex, crowded pattern of many overlapping bending vibrations of the molecule's carbon skeleton (such as \( \text{C-C} \) and \( \text{C-O} \) vibrations).
• Unique Identity: Just like a human fingerprint is unique to one individual, the exact pattern in this region is entirely unique to one specific compound.
• Database Comparison: Even if two compounds have the exact same functional group (for example, propan-1-ol and propan-2-ol), their spectra will look almost identical above \( 1500\text{ cm}^{-1} \), but their fingerprint regions below \( 1500\text{ cm}^{-1} \) will be noticeably different.
• Chemists identify an unknown substance by comparing its fingerprint region against a computer database of known spectra. An exact match confirms the compound's identity and purity.

Key Takeaway: The region below \( 1500\text{ cm}^{-1} \) is unique to each molecule. It is used to confirm the exact identity of a compound by comparing it with a library of known spectra.

7. Real-World Applications of IR Spectroscopy

Infrared spectroscopy is not just used in university research labs—it plays a vital role in everyday public safety and environmental protection!

• Police Breathalysers: Modern roadside breathalysers measure the concentration of ethanol in a driver's breath. The device shines an IR beam through the breath chamber and measures the absorption by \( \text{C-H} \) bonds in ethanol (\( \text{CH}_3\text{CH}_2\text{OH} \)). The greater the absorption, the higher the blood alcohol concentration.
• Monitoring Air Pollution: IR spectrometers can continuously monitor vehicle exhaust emissions and factory flues to detect harmful gases like carbon monoxide (\( \text{CO} \)), sulfur dioxide (\( \text{SO}_2 \)), and unburnt hydrocarbons in real time.
• Testing Drug Purity: Pharmaceutical companies use the fingerprint region to verify that newly manufactured medicines are pure and free from harmful impurities.

8. Common Mistakes to Avoid

• Mistake 1: Confusing alcohol and acid \( \text{O-H} \) peaks.
Correction: Alcohol \( \text{O-H} \) is a smooth U-shape at \( 3200 - 3600\text{ cm}^{-1} \). Carboxylic acid \( \text{O-H} \) is huge, ragged, covers \( 2500 - 3300\text{ cm}^{-1} \), and swallows the \( \text{C-H} \) peaks.

• Mistake 2: Forgetting that absorption peaks point downwards.
Correction: In IR spectroscopy, high transmittance is at the top (\( 100\% \)), and strong absorptions form deep troughs pointing towards \( 0\% \).

• Mistake 3: Trying to interpret every single peak in the fingerprint region.
Correction: Do not attempt to assign individual bonds below \( 1500\text{ cm}^{-1} \). In exams, state that the fingerprint region is compared with a database of known spectra to confirm identity.

Quick Review Summary

• Covalent bonds absorb specific frequencies of IR radiation, causing them to vibrate (stretch and bend).
• Greenhouse gases (\( \text{CO}_2 \), \( \text{H}_2\text{O} \), \( \text{CH}_4 \)) trap heat because their bonds absorb IR emitted from the Earth's surface.
• \( \text{C=O} \) appears as a strong, sharp peak at \( 1650 - 1750\text{ cm}^{-1} \).
• Alcohol \( \text{O-H} \) appears as a broad, smooth trough at \( 3200 - 3600\text{ cm}^{-1} \).
• Carboxylic acid \( \text{O-H} \) appears as a very broad, ragged trough at \( 2500 - 3300\text{ cm}^{-1} \).
• Fingerprint region (\( < 1500\text{ cm}^{-1} \)) provides a unique spectrum for exact identification via database comparison.
• Real-world uses include breathalysers and monitoring emissions.

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

How do covalent bonds in molecules behave and react when they absorb infrared (IR) radiation?

Bonds act like flexible springs that stretch and bend; they absorb IR radiation matching their natural frequency, which increases their vibration amplitude.

What two factors determine the frequency of infrared radiation absorbed by a bond?

Bond strength (stronger bonds absorb at higher frequencies) and atom mass (bonds with lighter atoms absorb at higher frequencies).

What is a wavenumber and how does it relate to energy and frequency?

A wavenumber (\( \text{cm}^{-1} \)) is the reciprocal of wavelength (\( \frac{1}{\lambda} \)); higher wavenumbers correspond to higher frequencies and higher energies.

Why do greenhouse gases like \( \text{CO}_2 \), \( \text{H}_2\text{O} \), and \( \text{CH}_4 \) absorb infrared radiation while \( \text{N}_2 \) and \( \text{O}_2 \) do not?

Greenhouse gases have polar bonds whose vibrations absorb outgoing IR radiation and change the dipole moment, whereas non-polar diatomic molecules (\( \text{N}_2 \), \( \text{O}_2 \)) cannot absorb IR.

How are the axes arranged and read on a typical infrared spectrum?

The x-axis displays wavenumber (\( \text{cm}^{-1} \)) decreasing from high to low, and the y-axis displays percentage transmittance (\( \% \)), with absorptions appearing as downward troughs.

Where does the carbonyl (\( \text{C=O} \)) absorption appear in an IR spectrum, and what does it look like?

It appears between \( 1650\text{ cm}^{-1} \) and \( 1750\text{ cm}^{-1} \) as a strong, sharp, deep trough.

How can you distinguish an alcohol \( \text{O-H} \) peak from a carboxylic acid \( \text{O-H} \) peak on an IR spectrum?

An alcohol \( \text{O-H} \) is a smooth, broad trough at \( 3200-3600\text{ cm}^{-1} \), while a carboxylic acid \( \text{O-H} \) is an extremely broad, ragged band at \( 2500-3300\text{ cm}^{-1} \) that overlaps \( \text{C-H} \) peaks.

What is the fingerprint region in an IR spectrum, and how is it used to identify a compound?

It is the complex region below \( 1500\text{ cm}^{-1} \) unique to each molecule, which is compared against a database of known spectra to confirm identity.

What are two real-world analytical applications of infrared spectroscopy?

Roadside breathalysers (measuring ethanol \( \text{C-H} \) absorption) and monitoring air pollution (detecting gases like \( \text{CO} \) and \( \text{SO}_2 \)).

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