IR Spectroscopy in Practice

Introduction

Infrared spectroscopy is not confined to research laboratories. The same bond vibrations you use to identify functional groups are used every day to catch drink-drivers, to understand how the atmosphere traps heat and to follow chemical reactions in industrial plants. This page connects the principles of IR absorption to three important real-world applications.

Core explanation

Breathalysers. Ethanol in the blood passes into the air in the lungs, so the ethanol concentration in exhaled breath is proportional to that in the blood. Evidential breath-testing instruments pass infrared radiation through a sample chamber filled with breath and measure how much is absorbed. They typically monitor the C–H stretching region near 2950 cm⁻¹ (about 3.4 μm) and often a C–O band near 1050 cm⁻¹ (about 9.5 μm). The O–H band is not used because water vapour in breath absorbs strongly in the same region. The amount of absorption increases with ethanol concentration, so the instrument can convert absorbance into a concentration. Measuring at more than one wavelength helps distinguish ethanol from other substances with C–H bonds. Roadside screening devices often use a fuel cell instead, with IR instruments used for evidential measurement at a police station.

Greenhouse gases. The Earth's surface, warmed by sunlight, emits energy as infrared radiation. A molecule can absorb this radiation only if one of its vibrations changes its dipole moment. N₂ and O₂, which make up about 99% of dry air, are symmetrical diatomics with no dipole change on stretching, so they are IR inactive. CO₂, H₂O and CH₄ do have IR-active vibrations. Carbon dioxide absorbs strongly through its bending mode near 667 cm⁻¹ (15 μm) and its asymmetric stretch near 2349 cm⁻¹. Methane absorbs through C–H stretching and bending, and water through O–H stretching and bending. After absorbing, these molecules re-emit infrared radiation in all directions, including back towards the surface. This slows the loss of heat to space: the greenhouse effect. The effectiveness of a gas depends on how strongly it absorbs, on whether it absorbs in wavelength windows not already covered by other gases, and on how long it persists in the atmosphere.

Reaction monitoring. When a reaction converts one functional group into another, a reactant band disappears and a product band appears. Oxidising a primary alcohol to a carboxylic acid, for example, replaces the alcohol O–H band near 3300 cm⁻¹ with a very broad acid O–H band and a new C=O band near 1710 cm⁻¹. Modern probes using ATR can sit inside a reaction vessel and record spectra every few seconds, allowing chemists to see when a reaction is complete, detect intermediates and control industrial processes safely without removing samples.

Step-by-step reasoning

To use IR to monitor a reaction:

1. Identify a functional group lost from the reactant and one gained in the product. 2. Choose bands for each that do not overlap with other species present. 3. Record spectra at regular time intervals. 4. Track the decrease of the reactant band and the growth of the product band. 5. The reaction is complete when the bands stop changing.

Visual explanation

Picture a series of spectra stacked one above another in time order. The reactant's O–H band shrinks from each spectrum to the next, while a sharp C=O band grows beside it, like one candle burning down as another is lit.

Real-world analogy

A greenhouse gas behaves like a blanket. The blanket does not produce heat itself; it absorbs the warmth your body gives off and radiates some of it back, slowing heat loss. Molecules of CO₂ and CH₄ do the same with the infrared radiation emitted by the Earth.

Real-world example

Satellites such as NASA's Orbiting Carbon Observatory measure the absorption of reflected sunlight by CO₂ at specific near-infrared wavelengths. From the depth of these absorption lines they map atmospheric carbon dioxide concentrations around the globe, identifying major sources and sinks of the gas.

Why?

Why are nitrogen and oxygen not greenhouse gases even though they are far more abundant than carbon dioxide? Their only vibration is the stretch of a symmetrical diatomic molecule, which produces no change in dipole moment. They therefore cannot absorb infrared radiation emitted by the Earth.

Common misconception

"Carbon dioxide has no dipole, so it cannot absorb infrared radiation." CO₂ has no permanent dipole, but its asymmetric stretch and bending vibrations create a changing dipole moment. Only its symmetric stretch is IR inactive.

Worked example

Question: A chemist monitors the conversion of propan-2-ol into propanone. Which bands should decrease and which should appear?

Reasoning: Propan-2-ol has an O–H group, giving a broad band near 3300–3400 cm⁻¹ and a C–O band near 1100 cm⁻¹. Propanone has a C=O group but no O–H.

Answer: The broad O–H band and the C–O band near 1100 cm⁻¹ decrease, and a strong C=O band near 1715 cm⁻¹ appears.

Quick check

1. Why does an IR breathalyser avoid measuring the O–H band of ethanol? Answer: Water vapour in exhaled breath also absorbs strongly in the O–H region, so it would interfere.

Exam focus

For breathalysers, name the bond measured (usually C–H, sometimes C–O) and link absorption to concentration. For greenhouse gases, explain absorption in terms of a change in dipole moment and name the IR-active vibrations. In reaction-monitoring questions, identify one band lost and one band gained.

Advanced insight

The global warming potential of a gas compares its warming effect with that of CO₂ over a set period. Methane's value is roughly 80 over 20 years and about 28 over 100 years. It absorbs more strongly per molecule and in a less saturated spectral window than CO₂, but is removed from the atmosphere much faster.

Summary

IR breathalysers measure ethanol by its C–H and C–O absorptions, avoiding the O–H region where water interferes. Greenhouse gases such as CO₂, CH₄ and H₂O absorb Earth's infrared radiation because their vibrations change their dipole moments, whereas N₂ and O₂ cannot. In reaction monitoring, the loss of reactant bands and the growth of product bands track progress in real time.

Practice questions

1. Explain why the concentration of ethanol in breath can be related to the amount of IR radiation absorbed. Answer: More ethanol molecules in the sample chamber absorb more radiation at the chosen C–H or C–O wavenumber, so absorbance increases with concentration. 2. Explain why methane is a greenhouse gas but nitrogen is not. Answer: Methane has vibrations that change its dipole moment and so absorb infrared radiation; nitrogen's only vibration is symmetrical and produces no dipole change. 3. Which vibration of CO₂ is IR inactive, and why? Answer: The symmetric stretch, because both C=O bonds stretch equally and the dipole moment remains zero. 4. Describe how IR could show that the oxidation of ethanol to ethanoic acid is complete. Answer: The alcohol O–H band near 3300 cm⁻¹ is replaced entirely by the very broad acid O–H band and a C=O band near 1710 cm⁻¹, and the spectrum stops changing.