Characteristic IR Absorptions | A-Level H3 Chemistry

An infrared spectrum can look like a rather untidy collection of dips, but every important absorption originates from something physical happening inside the molecule: atoms are moving and bonds are vibrating. Once you connect those vibrations to particular bonds, an IR spectrum becomes much easier to interpret.

The central idea is not simply that a molecule contains a particular bond. That bond must take part in a vibration that can interact with infrared radiation. We will therefore begin with molecular vibrations, establish the condition for IR activity, and then use characteristic absorption regions to identify functional groups and interpret spectra.

Diagnostic Quiz

What Do You Already Know About IR Absorptions?

Try these five questions before reading the chapter. Focus on the relationship between molecular vibration, changing dipole moment and the position of absorptions in an IR spectrum.

5 diagnostic questions
IR spectra and molecular examples where useful
Instant explanation for every answer option

Question 1 of 5

What must happen during a molecular vibration for that vibration to absorb infrared radiation?

The molecule must acquire an overall electrical charge.

IR absorption does not require the molecule to become an ion.

The vibration must cause a change in dipole moment.

Correct. A changing dipole moment allows the vibration to interact with the oscillating electric field of infrared radiation.

Every bond in the molecule must stretch simultaneously.

A normal mode involves a particular coordinated motion of the atoms; not every bond must stretch.

A covalent bond must break.

IR spectroscopy excites molecular vibrations. It does not normally supply enough energy to break the bond being observed.

The selection rule to remember is: change in dipole moment → IR-active vibration.

Why is the symmetric stretching vibration of CO₂ IR inactive?

CO₂ contains no covalent bonds.

CO₂ contains two covalent C=O bonds.

C=O bonds cannot vibrate.

C=O bonds certainly vibrate and can give strong IR absorptions in suitable vibrational modes.

The vibration produces no change in the overall dipole moment.

Correct. During the symmetric stretch, the two equivalent bond dipoles continue to cancel.

CO₂ is too small to absorb infrared radiation.

Molecular size is not the reason. Other vibrational modes of CO₂ are IR active.

A molecule need not possess a permanent dipole moment to absorb IR. What matters is whether the particular vibration creates a changing dipole moment.

An IR spectrum contains a broad absorption in the region 3200–3600 cm⁻¹. Which bond is most strongly suggested?

O–H

Correct. An alcohol O–H stretch characteristically produces a broad absorption in this high-wavenumber region.

C=O

A carbonyl absorption occurs much lower, around the 1670–1750 cm⁻¹ region.

C–C

C–C vibrations occur at considerably lower wavenumbers and are not responsible for this broad high-wavenumber feature.

C=C

A C=C absorption occurs at a substantially lower wavenumber.

Both the position and the shape of an absorption can provide structural information.

Which feature most strongly suggests the presence of a carbonyl, C=O, group?

A broad absorption near 3400 cm⁻¹

A broad feature in this region is more characteristic of O–H.

A strong absorption around 1670–1750 cm⁻¹

Correct. The strong C=O stretching absorption is one of the most useful features in an IR spectrum.

No absorptions below 1500 cm⁻¹

Real molecular spectra normally contain many absorptions in the fingerprint region.

A single absorption at 4000 cm⁻¹

This is not the characteristic carbonyl region.

The C=O stretch is usually a prominent absorption and is therefore a valuable first feature to look for during structural elucidation.

Which general statement about vibrational frequency is correct?

Weaker bonds always absorb at higher wavenumbers.

The opposite general trend applies: stronger bonds tend to vibrate at higher frequencies.

Heavier atoms always produce higher vibrational frequencies.

Greater atomic mass generally lowers the vibrational frequency when other factors are comparable.

Bond strength and atomic mass have no effect on an IR absorption.

Both are important in determining the characteristic vibrational frequency.

Stronger bonds and lighter vibrating atoms generally give higher wavenumbers.

Correct. This gives a useful qualitative way of understanding the positions of characteristic absorptions.

Think of a bond as an atomic-scale spring: a stiffer spring vibrates faster, while heavier masses tend to vibrate more slowly.

1. Why molecules absorb infrared radiation

Atoms in molecules are not stationary. Even when the molecule is not rotating or moving from place to place, its atoms can move relative to one another through molecular vibrations. A useful first model is to picture a chemical bond as a spring joining two masses.

Infrared radiation can transfer energy into one of these vibrational modes when two conditions are satisfied: the radiation has an appropriate frequency, and the vibration is capable of interacting with the electric field of the radiation.

IR selection rule: a molecular vibration is IR active only if the vibration causes a change in dipole moment.

Stretching vibrations

In a stretching vibration, the distance between bonded atoms changes. The atoms move approximately along the direction of the bond, so the bond repeatedly lengthens and shortens.

Bending vibrations

In a bending vibration, bond angles change. The atoms move in directions that alter the geometry of the molecule rather than simply changing one bond length.

Stretching: primarily changes bond lengths.

Bending: primarily changes bond angles.

The important examination question is therefore not merely “Can this bond vibrate?” All bonds can participate in molecular vibrations. The more useful question is “Does this particular vibration change the dipole moment?”

2. IR-active and IR-inactive vibrations

The distinction between an IR-active and an IR-inactive mode can be seen particularly clearly by comparing different vibrations of CO₂.

The symmetric stretch of CO₂

CO₂ is linear. During its symmetric stretching mode, the two oxygen atoms move away from the carbon atom together and then move back towards it together. Although the individual C=O bond lengths change, the two bond dipoles remain equal and opposite.

The overall dipole moment therefore remains zero throughout this vibration. There is no change in dipole moment, so the symmetric stretch is IR inactive.

CO₂ symmetric stretch: bond lengths change, but the molecular dipole moment does not. Therefore this mode does not give an IR absorption.

The asymmetric stretch and bending modes of CO₂

The result changes when the two C=O bonds do not stretch equally. During an asymmetric stretch, one side of the molecule stretches while the other changes differently. The bond dipoles no longer cancel at every instant, so the molecular dipole moment changes as the atoms move.

Bending the normally linear CO₂ molecule also destroys the instantaneous symmetry that allowed the bond dipoles to cancel. The bending modes are therefore IR active too.

CO₂ symmetric stretch: no change in dipole moment → IR inactive.

CO₂ asymmetric stretch and bending: changing dipole moment → IR active.

Why SO₂ behaves differently

SO₂ is a bent molecule. Its geometry means that the effects of the two S–O bonds do not cancel in the same way as they do in linear CO₂. Its fundamental stretching and bending vibrations produce changes in dipole moment and are therefore IR active.

A useful lesson from CO₂ and SO₂ is that IR activity is a property of a particular vibrational mode, not simply a label attached to the whole molecule.

3. How to read an IR spectrum

An IR spectrum records how strongly a sample interacts with infrared radiation over a range of wavenumbers. The horizontal axis is normally labelled in cm⁻¹, and by convention the displayed wavenumber usually decreases from left to right.

Do not read the IR x-axis like an ordinary increasing graph. A typical spectrum runs from about 4000 cm⁻¹ on the left towards lower wavenumbers on the right.

How to read this schematic spectrum: the downward features are absorption bands because the vertical axis is shown as transmittance. Their exact positions and shapes depend on the molecule; the diagram is intended to show how the major regions of an IR spectrum are interpreted.

Why absorptions point downwards in many spectra

Many school and university IR spectra plot percentage transmittance on the vertical axis. When the sample absorbs infrared radiation, less radiation is transmitted through it, so the trace moves downward.

If the spectrum is instead plotted using absorbance, the absorptions appear as upward peaks. The chemistry has not changed; only the way the vertical axis is represented has changed.

4. Characteristic absorption regions

Different bonds have different characteristic vibrational frequencies. This is why the position of an absorption can be used to infer which functional groups are present.

In an examination, the exact ranges should be interpreted using the supplied data rather than trying to memorise every possible absorption. Some particularly useful patterns, however, are worth recognising.

Bond or group	Useful region	Typical clue
O–H in an alcohol	About 3200–3600 cm⁻¹	Broad absorption
O–H in a carboxylic acid	Broad high-wavenumber region	Usually exceptionally broad and accompanied by C=O
C–H	Around 3000 cm⁻¹	Useful supporting absorption
C=O	About 1670–1750 cm⁻¹	Usually strong and readily recognised

The carbonyl region

The C=O stretching absorption is particularly useful because it is normally strong and occurs in a relatively recognisable region. A strong absorption around 1670–1750 cm⁻¹ should therefore make you investigate the possibility of a carbonyl-containing functional group.

IR alone does not automatically tell you whether that carbonyl belongs to an aldehyde, ketone, carboxylic acid or another carbonyl compound. Other evidence from the same spectrum and from other spectroscopic information must be considered.

The O–H region

An O–H stretching absorption is distinctive because it is generally broad. An alcohol commonly gives a broad feature around 3200–3600 cm⁻¹.

The O–H absorption associated with a carboxylic acid is usually broader still. When a very broad O–H feature occurs together with a strong C=O absorption, a carboxylic acid becomes an important possibility.

Pattern recognition is stronger than single-peak recognition. A strong C=O absorption plus a very broad O–H absorption is much more informative than treating either feature in isolation.

5. What determines the wavenumber of an absorption?

A bond can be pictured approximately as a spring joining two atoms. Two broad trends help us understand why different bonds absorb at different wavenumbers.

Stronger bond: behaves like a stiffer spring and generally vibrates at a higher frequency and higher wavenumber.

Heavier vibrating atoms: are harder to accelerate and generally produce a lower vibrational frequency and lower wavenumber.

Bond strength

When the masses of the bonded atoms are broadly comparable, increasing bond strength generally increases the vibrational frequency. This is why the bond order and bond environment can influence where an absorption occurs.

Atomic mass

The masses of the atoms also matter. A vibration involving a light atom, such as hydrogen, can occur at a relatively high frequency. This helps explain why O–H and many C–H stretching absorptions occur towards the high-wavenumber side of an IR spectrum.

Useful qualitative rule: stronger bonds tend to give higher wavenumbers, while heavier vibrating atoms tend to give lower wavenumbers.

6. Diagnostic and fingerprint regions

It is useful to divide an IR spectrum conceptually into two broad regions. The higher-wavenumber portion contains many of the absorptions used to identify functional groups. The crowded lower-wavenumber portion is often described as the fingerprint region.

Diagnostic region: particularly useful for locating characteristic functional-group absorptions such as O–H and C=O.

Fingerprint region, below about 1500 cm⁻¹: contains many overlapping absorptions arising from the particular collection of vibrations in the molecule.

Why the fingerprint region is useful

The fingerprint region may initially look too complicated to interpret peak by peak. That complexity is precisely what makes the overall pattern useful for comparison: different molecules can produce different detailed patterns even when they contain some of the same functional groups.

For structural elucidation, however, you should normally begin with the more obvious diagnostic absorptions rather than trying to assign every small feature in the fingerprint region.

7. Using IR spectroscopy for structural elucidation

IR spectroscopy is most powerful when treated as one piece of a larger structural puzzle. It can tell you which functional groups are likely to be present or absent, while other evidence helps determine the complete molecular structure.

Step 1: Look for a carbonyl absorption

Examine approximately the 1600–1850 cm⁻¹ region. A strong absorption around the characteristic C=O range immediately suggests a carbonyl-containing compound.

Step 2: Examine the high-wavenumber region

Look for broad absorptions in the region where O–H stretching occurs. Shape matters here: an O–H absorption is not merely a single narrow line.

Step 3: Combine absorptions rather than reading them separately

Suppose a spectrum contains both a strong carbonyl absorption and a very broad O–H absorption. The two observations together strongly point towards a carboxylic acid, whereas either observation on its own would be less specific.

Step 4: Cross-check with the rest of the evidence

Your proposed functional groups must be consistent with the molecular formula and any other spectroscopic or chemical information supplied. IR spectroscopy should support a structural proposal rather than being forced to provide more information than the spectrum actually contains.

Example: if a molecular formula contains oxygen and the IR spectrum has a very strong absorption near 1700 cm⁻¹ but no broad O–H absorption, the spectrum supports the presence of a carbonyl group but does not by itself identify one unique carbonyl compound. More evidence is needed.

8. IR spectroscopy and greenhouse gases

The same molecular physics used in analytical IR spectroscopy also helps explain the greenhouse effect. Earth absorbs incoming solar energy and subsequently emits energy towards space largely as lower-energy infrared radiation.

Atmospheric molecules that possess IR-active vibrational modes can absorb some of this outgoing infrared radiation. The absorbed energy can subsequently be redistributed through molecular motion and re-emission, affecting the transfer of thermal radiation through the atmosphere.

Greenhouse connection: a molecule can interact strongly with terrestrial IR radiation if it has vibrational modes that produce a changing dipole moment at relevant infrared frequencies.

Why CO₂ can absorb IR even though it is non-polar

A common misconception is that CO₂ cannot absorb IR because its equilibrium structure has zero overall dipole moment. The symmetric stretch is indeed IR inactive, but its asymmetric stretching and bending modes do create changing dipole moments.

CO₂ can therefore absorb infrared radiation at frequencies corresponding to these IR-active vibrations.

Water, methane and fluorinated gases

Other polyatomic gases such as H₂O and CH₄ also possess IR-active vibrational modes. Fluorinated molecules can likewise contain vibrations that interact with infrared radiation.

The important criterion is not simply whether the equilibrium molecule is labelled “polar” or “non-polar”. The key question remains whether a vibrational motion changes its dipole moment.

Why N₂ and O₂ behave differently

N₂ and O₂ are homonuclear diatomic molecules. Stretching one of these molecules does not produce a dipole moment because both atoms are identical. Their fundamental stretching vibration therefore does not satisfy the IR selection rule in the way that the important greenhouse-gas vibrations do.

CO₂: no permanent molecular dipole, but asymmetric stretching and bending produce changing dipole moments → IR-active modes exist.

N₂ and O₂: stretching does not create a dipole moment → the fundamental vibration is IR inactive.

Common Mistakes

Common mistake	Why it is wrong	Better approach
Saying that only polar molecules can absorb IR.	A molecule with no permanent dipole can still possess vibrational modes that create a changing dipole moment.	Apply the selection rule to the particular vibration: does the dipole moment change?
Saying that the symmetric stretch of CO₂ absorbs IR because C=O bonds are polar.	The two bond dipoles continue to cancel during the symmetric stretch, so the molecular dipole moment does not change.	Distinguish the polarity of individual bonds from the changing dipole moment of the complete vibrational mode.
Reading the IR wavenumber axis from low to high from left to right.	Conventional IR spectra normally display high wavenumber on the left and lower wavenumber on the right.	Check the numerical labels before assigning any absorption.
Identifying a whole molecule from one absorption.	One absorption normally identifies a bond or functional-group possibility, not one unique molecular structure.	Combine several IR features with the molecular formula and other available evidence.
Trying to assign every small feature in the fingerprint region first.	The region contains many overlapping molecular vibrations and can be difficult to interpret peak by peak.	Begin with the clearest diagnostic absorptions and use the fingerprint pattern mainly as supporting evidence.

What you should be able to do

Describe molecular vibrations

Distinguish stretching vibrations from bending vibrations.

Apply the IR selection rule

Decide whether a vibration is IR active by considering whether the dipole moment changes.

Interpret characteristic absorptions

Recognise useful O–H, C–H and C=O absorption regions using supplied spectroscopic data.

Read an IR spectrum correctly

Interpret the wavenumber axis, transmittance troughs and the difference between diagnostic and fingerprint regions.

Use IR in structural elucidation

Combine several absorptions with molecular and other spectroscopic evidence rather than relying on a single peak.

Connect IR absorption to the greenhouse effect

Explain why gases such as CO₂ can absorb terrestrial IR radiation while homonuclear diatomic gases such as N₂ and O₂ lack the corresponding IR-active fundamental stretch.

Mastery Quiz

Can You Apply Characteristic IR Absorptions?

These ten questions test whether you can connect molecular vibrations with IR activity and use characteristic absorptions to interpret unfamiliar spectra.

10 mastery questions
Spectroscopic and molecular reasoning
Instant explanation for every answer option

Question 1 of 10

A vibration changes several bond lengths but leaves the molecular dipole moment unchanged. What would you predict?

It must give a strong IR absorption.

Changing bond lengths alone is not sufficient for IR activity.

It is IR inactive.

Correct. Without a changing dipole moment, the vibration does not satisfy the IR selection rule.

The bonds cannot actually be vibrating.

A genuine vibrational mode can be IR inactive.

The molecule must be monatomic.

Polyatomic molecules can contain both IR-active and IR-inactive normal modes.

IR activity depends on a changing dipole moment, not merely on whether atomic positions change.

Which statement correctly compares the symmetric and asymmetric stretching modes of CO₂?

The symmetric stretch is IR inactive, while the asymmetric stretch is IR active.

Correct. Only the asymmetric stretch produces the required changing molecular dipole moment.

Both are IR inactive because CO₂ is non-polar.

Permanent molecular polarity is not the criterion. The asymmetric vibration changes the dipole moment.

Both are IR active because each C=O bond is polar.

The symmetric mode still has cancelling bond dipoles at every instant.

The symmetric stretch is IR active but the asymmetric stretch is IR inactive.

This reverses the actual IR activities.

Always analyse the symmetry and changing dipole moment of the particular normal mode.

A spectrum shows a strong sharp absorption at about 1715 cm⁻¹ but no broad absorption around 3200–3600 cm⁻¹. Which conclusion is safest?

The compound must be an alcohol.

An alcohol should normally show a broad O–H stretching absorption.

The complete molecular structure has been uniquely identified.

A carbonyl absorption alone does not specify one unique molecular structure.

A carbonyl group is strongly indicated, while an alcohol O–H group is not supported.

Correct. This conclusion uses both the observed absorption and an important missing feature.

The molecule contains no oxygen.

A strong absorption around 1715 cm⁻¹ is consistent with a C=O group, which contains oxygen.

Absences can be useful too. Structural elucidation often involves asking which expected absorptions are missing.

The following simplified spectrum has both a very broad high-wavenumber absorption and a strong absorption near 1700 cm⁻¹. Which functional group is most strongly supported?

Alkane only

An alkane alone would not explain the broad O–H and strong C=O absorptions.

Alcohol only

An alcohol explains the broad O–H feature but not the strong carbonyl absorption.

Ketone only

A ketone explains the carbonyl absorption but not the broad O–H feature.

Carboxylic acid

Correct. The combination of a very broad O–H absorption and a strong C=O absorption is characteristic of a carboxylic acid.

Combining two diagnostic features gives a stronger structural assignment than either feature alone.

Why does a stronger bond generally absorb at a higher wavenumber when the vibrating atomic masses are comparable?

Stronger bonds contain more electrons, so they always absorb more radiation.

Absorption position is not explained simply by electron count.

A stronger bond behaves like a stiffer spring and therefore vibrates at a higher natural frequency.

Correct. This mechanical analogy explains the qualitative trend.

Stronger bonds always involve heavier atoms.

Bond strength and atomic mass are separate factors.

Stronger bonds have lower vibrational frequencies.

This is opposite to the general trend.

The “stiffer spring” model is a useful qualitative explanation for the effect of bond strength on vibrational frequency.

Two otherwise similar bond vibrations involve different atomic masses. Which one would generally occur at the higher wavenumber?

The one involving the heavier atoms

Greater vibrating mass generally lowers the frequency.

They must have exactly the same wavenumber.

Atomic masses affect vibrational frequency.

The one involving the lighter atoms

Correct. Smaller vibrating masses generally lead to higher vibrational frequencies.

Atomic mass affects intensity but never position.

Atomic mass is directly relevant to the vibrational frequency and therefore the position of the absorption.

Bond strength and atomic mass should be considered together when rationalising absorption positions.

What is the best first approach when faced with an unfamiliar IR spectrum in a structural elucidation question?

Identify the clearest diagnostic absorptions and then combine them with the other structural evidence.

Correct. Functional-group recognition should guide the initial interpretation.

Assign every small fingerprint-region absorption before doing anything else.

This is usually inefficient because the fingerprint region is crowded and complex.

Choose a structure from the molecular formula and ignore the IR spectrum.

The IR evidence should be used to test and constrain proposed structures.

Assume that the deepest absorption uniquely identifies the molecule.

Absorption intensity alone does not uniquely specify a complete structure.

IR is primarily a functional-group tool in this context. Start with the most diagnostic evidence.

Why can the fingerprint region help distinguish two compounds that contain the same functional group?

Only one compound can absorb below 1500 cm⁻¹.

Many compounds show numerous absorptions in this region.

The fingerprint region gives the molecular formula directly.

IR spectra do not directly provide a molecular formula.

Every absorption in the fingerprint region corresponds to one different functional group.

Many absorptions arise from the complex set of vibrations of the whole molecule.

The detailed collection of molecular vibrations can produce a characteristic overall pattern.

Correct. The complex pattern can act like a spectroscopic fingerprint when spectra are compared.

The fingerprint region is often more useful as an overall pattern than as a list of individually assigned peaks.

CO₂ has no permanent dipole moment. Why can it nevertheless contribute to infrared absorption in the atmosphere?

It becomes permanently ionic in sunlight.

Atmospheric IR absorption does not require CO₂ to become ionic.

Some of its vibrations, including asymmetric stretching and bending, produce changing dipole moments.

Correct. These modes satisfy the IR selection rule.

Every vibrational mode of CO₂ is IR active.

The symmetric stretching mode is an important counterexample: it is IR inactive.

IR absorption requires zero dipole moment at all times.

The opposite is needed: the dipole moment must change during the active vibration.

This is why “non-polar molecule” does not automatically mean “IR inactive molecule”.

Why are the fundamental vibrations of N₂ and O₂ unlike the important IR-active vibrations of greenhouse gases such as CO₂?

N₂ and O₂ do not contain chemical bonds.

Both contain strong covalent bonds.

N₂ and O₂ cannot vibrate.

They do possess vibrational motion.

Stretching a homonuclear diatomic molecule does not produce a changing dipole moment.

Correct. The two identical atoms prevent the stretch from creating the changing dipole required for fundamental IR absorption.

Their vibrational frequencies are outside every possible electromagnetic frequency range.

The key issue here is the IR selection rule, not the absence of a vibrational frequency.

Molecular vibration alone is insufficient. The vibration must also change the dipole moment so that it can interact with infrared radiation.

Frequently asked questions

Questions students often ask about IR absorptions

Once you understand changing dipole moments, most of the apparently separate rules in IR spectroscopy begin to fit together. These questions address the details that most often cause confusion.

Does a molecule need a permanent dipole moment to absorb IR?

No. What matters is whether a particular vibration causes the dipole moment to change. CO₂ is the classic example: its equilibrium molecule has no permanent dipole moment, yet its asymmetric stretching and bending modes generate changing dipole moments and are IR active.

Why is the symmetric stretch of CO₂ IR inactive if the C=O bonds are polar?

The two C=O bond dipoles point in opposite directions. During the symmetric stretch they change equally, so they continue to cancel. The molecular dipole moment therefore remains zero throughout the vibration, making that mode IR inactive.

Why does the wavenumber decrease from left to right on an IR spectrum?

This is simply the conventional way IR spectra are displayed. High wavenumbers, commonly near 4000 cm⁻¹, are placed on the left and the scale decreases towards the fingerprint region on the right. Always read the numerical axis rather than assuming it increases in the usual direction.

Why do absorptions sometimes appear as downward troughs rather than upward peaks?

It depends on the vertical axis. In a transmittance spectrum, absorption means that less IR radiation passes through the sample, so the trace moves downward. In an absorbance spectrum, stronger absorption is represented by a larger positive absorbance, so the feature points upward.

Why are O–H absorptions broad?

O–H groups commonly experience different intermolecular environments, particularly through hydrogen bonding. Instead of every O–H bond vibrating at one identical frequency, there is a spread of closely related vibrational frequencies, which appears as a broad absorption band.

Can an IR spectrum identify a molecule completely?

Not usually from a few characteristic absorptions alone. IR is particularly good at establishing which functional groups are present or absent. A complete structural assignment normally requires the molecular formula and other spectroscopic or chemical evidence as well.

What is the fingerprint region used for?

The region below about 1500 cm⁻¹ contains many overlapping molecular vibrations. It can be difficult to assign each absorption separately, but the complete pattern can be characteristic of a particular compound and is therefore valuable when comparing spectra.

Why does a stronger bond usually absorb at a higher wavenumber?

A stronger bond behaves approximately like a stiffer spring. A stiffer bond has a higher natural vibrational frequency when the vibrating masses are comparable, so its characteristic absorption occurs at a higher wavenumber.

Why do lighter atoms tend to give higher vibrational frequencies?

Lighter masses can respond more rapidly to the restoring force of a vibrating bond. For otherwise comparable bonds, decreasing the masses of the vibrating atoms therefore increases the vibrational frequency and hence the wavenumber of the absorption.

Why are N₂ and O₂ not important IR absorbers like CO₂?

N₂ and O₂ are homonuclear diatomic molecules. Stretching the bond does not create a dipole moment because the two atoms are identical, so their fundamental stretching vibration is IR inactive. CO₂, although non-polar at equilibrium, possesses asymmetric stretching and bending modes that do create changing dipole moments.

What is the quickest way to approach an IR spectrum in an examination?

Begin with the most recognisable diagnostic features. Check for a strong carbonyl absorption, then examine the high-wavenumber region for broad O–H absorptions and other characteristic features. Combine these observations before using the fingerprint region and the remaining structural evidence to test your proposed structure.

Characteristic IR absorptions become much easier to understand when you stop treating the spectrum as a table of numbers and instead picture the molecule moving. Bonds stretch, bond angles bend, and only vibrations that create a changing dipole moment interact with infrared radiation. From that single principle come both the analytical use of IR spectroscopy and its connection with the absorption of infrared radiation by greenhouse gases.