IR Activity and Changing Dipole Moments

Introduction

Nitrogen and oxygen make up about 99% of dry air, yet they are almost transparent to infrared radiation. Carbon dioxide, present at only about 0.04%, absorbs strongly and is a major greenhouse gas. The difference has nothing to do with how much gas is present and everything to do with a simple rule: a vibration can only absorb infrared radiation if it changes the molecule's dipole moment . This page explains that rule and how to apply it.

Core explanation

How IR radiation interacts with a molecule. Infrared radiation is an oscillating electric field. To transfer energy to a vibrating molecule, the field must "grip" an oscillating electric charge distribution within it. This happens only if the vibration makes the dipole moment change back and forth at the same frequency as the radiation.

The selection rule. A vibration is IR active only if it causes a change in dipole moment. The molecule need not have a permanent dipole; what matters is that the dipole changes during the vibration.

Homonuclear diatomic molecules. In N₂, O₂, H₂ and Cl₂, both atoms are identical, so the electrons are shared equally. Stretching the bond keeps the molecule perfectly symmetrical and non-polar at every moment. The dipole moment is zero throughout, so these molecules are IR inactive .

Heteronuclear diatomic molecules. In HCl or CO, the atoms have different electronegativities, so the bond is polar. Stretching changes the separation of the partial charges, so the dipole moment changes and the vibration is IR active. HCl absorbs at about 2886 cm⁻¹.

Carbon dioxide. CO₂ is linear and has no overall dipole, but it still absorbs IR:

Mode Change in dipole? IR activity --- --- --- Symmetric stretch (about 1340 cm⁻¹) No — the two bond dipoles stay equal and opposite Inactive Asymmetric stretch (2349 cm⁻¹) Yes — one bond dipole grows as the other shrinks Strongly active Bend (667 cm⁻¹) Yes — the molecule becomes bent, creating a net dipole Active

Water. H₂O is bent and polar. All three of its modes change the dipole moment, so all are IR active.

Intensity. The larger the change in dipole moment during a vibration, the stronger the IR absorption. This is why C=O stretches, involving a highly polar bond, give some of the most intense bands in IR spectra, whereas the C=C stretch of a symmetrical alkene can be weak or absent.

Step-by-step reasoning

To decide whether a vibration is IR active:

1. Sketch the molecule and its bond dipoles. 2. Sketch the molecule at the extremes of the vibration. 3. Ask whether the overall dipole moment is different at the two extremes. 4. If it changes, the mode is IR active; if it stays the same, the mode is inactive.

Visual explanation

Draw CO₂ as O=C=O with arrows representing the two bond dipoles pointing in opposite directions. In the symmetric stretch, both arrows lengthen together and still cancel. In the asymmetric stretch, one arrow becomes longer than the other, leaving an uncancelled net arrow. In the bend, the arrows no longer point in opposite directions, and a net dipole appears.

Real-world analogy

Pushing a child on a swing only works if you have something to push against. Infrared radiation "pushes" on oscillating charge. A vibration that does not move any charge relative to the molecule's centre offers nothing to push on.

Real-world example

Greenhouse gases such as CO₂, CH₄, H₂O and N₂O all have IR-active vibrations in the range of wavelengths emitted by the warm Earth. N₂ and O₂ lack IR-active vibrations and therefore contribute almost nothing directly to the greenhouse effect despite their abundance.

Why?

Why is the symmetric stretch of CO₂ IR inactive even though each C=O bond is polar? The two bond dipoles are equal in size and opposite in direction. As both bonds stretch together, they remain equal and opposite, so the overall dipole stays zero throughout the vibration.

Common misconception

"Only polar molecules absorb infrared radiation." Non-polar molecules such as CO₂ and CH₄ absorb strongly, because some of their vibrations create a temporary dipole. The requirement is a change in dipole moment, not a permanent one.

Worked example

Question: Which of these molecules are IR active: H₂, HBr, CH₄?

Reasoning: H₂ is homonuclear; its only vibration causes no dipole change, so it is inactive. HBr has a polar bond whose stretch changes the dipole, so it is active. CH₄ has no permanent dipole, but its asymmetric C–H stretches and some bends distort the tetrahedron and create a temporary dipole.

Answer: HBr and CH₄ are IR active; H₂ is IR inactive.

Quick check

1. Explain why oxygen, O₂, does not absorb infrared radiation. Answer: Its only vibration is a stretch of identical atoms, which causes no change in dipole moment, so it is IR inactive.

Exam focus

Use the precise phrase "the vibration must cause a change in dipole moment". A popular question asks why CO₂ is a greenhouse gas while N₂ and O₂ are not; answer with the asymmetric stretch and bending modes of CO₂ changing its dipole moment.

Advanced insight

Vibrations that are IR inactive are often detected by Raman spectroscopy, which depends on a change in the polarisability of the electron cloud rather than a change in dipole moment. For molecules with a centre of symmetry, such as CO₂, modes that are IR active are Raman inactive and vice versa — the rule of mutual exclusion.

Summary

A vibration absorbs IR radiation only if it causes a change in dipole moment. Homonuclear diatomics such as N₂ and O₂ are IR inactive. Polar diatomics such as HCl are active. In CO₂, the symmetric stretch is inactive but the asymmetric stretch and bend are active. The larger the dipole change, the stronger the absorption.

Practice questions

1. State the condition for a molecular vibration to be IR active. Answer: The vibration must cause a change in the dipole moment of the molecule. 2. Which vibrational mode of CO₂ is IR inactive? Explain. Answer: The symmetric stretch, because the two equal and opposite bond dipoles remain cancelled throughout the vibration. 3. Why does a C=O stretch usually give a stronger IR absorption than a C=C stretch? Answer: The C=O bond is much more polar, so stretching it causes a larger change in dipole moment. 4. Would you expect carbon monoxide, CO, to absorb IR radiation? Explain. Answer: Yes; the bond joins different atoms and is polar, so stretching it changes the dipole moment.