Describe stretching vibrations

Compare three snapshots of linear CO₂: both C=O bonds 116 pm, then both 119 pm, then both 113 pm.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Describe stretching vibrations

See bond length change

Compare three snapshots of linear CO₂: both C=O bonds 116 pm, then both 119 pm, then both 113 pm.

The nuclei move along the bond axes; the O=C=O angle remains 180°, so this is stretching rather than bending.

Symmetric and asymmetric stretch

A stretching vibration periodically changes bond length. In a symmetric stretch equivalent bonds lengthen or shorten together.

In an asymmetric stretch one C=O bond lengthens while the other shortens; the changing dipole makes the CO₂ asymmetric stretch IR active.

Why an IR photon is absorbed

A vibration has quantised energy levels. Absorption occurs when the IR photon wavenumber matches the gap and the normal mode changes molecular dipole moment.

For ideal CO₂ the symmetric stretch leaves zero net dipole and is IR inactive, whereas the asymmetric stretch near 2350 cm⁻¹ changes dipole and absorbs.

Classify two CO₂ motions

Motion A changes both C=O lengths from 116 pm to 119 pm together. Motion B changes them to 119 pm and 113 pm.

A is symmetric stretching; B is asymmetric stretching and is the IR-active one.

Try this

Name each motion and state which gives the strong CO₂ band near 2350 cm⁻¹.

Check your answer

A strong answer gives symmetric stretch for A, asymmetric stretch for B, and the 2350 cm⁻¹ absorption to B because its dipole changes.

Track nuclei, length and angle

For H₂O, draw arrows along both O–H bonds moving outwards together; label both bond lengths increasing.

Then draw one O–H increasing as the other decreases and keep the H–O–H angle approximately fixed.

Try this

Classify the two arrow diagrams as symmetric or asymmetric stretching.

Check your answer

The together motion is symmetric; the opposite bond-length changes are asymmetric. Neither diagram should be labelled bending because the angle is fixed.

Transfer to SO₂

SO₂ is bent, with two S–O bonds and an O–S–O angle of about 119°.

For each stretch, decide whether the molecular dipole changes during the vibration; a permanent dipole alone does not settle IR activity.

Try this

Draw the two SO₂ stretching modes and predict whether each can be IR active.

Check your answer

Both modes are IR active. In the symmetric stretch both S–O bonds lengthen together, which changes the size of the dipole along the bisector; in the asymmetric stretch one lengthens as the other shortens, which changes its direction. The arrows must show together versus opposite S–O length changes.

A moving atom is not automatically a stretch

A vibration is classified by the internal coordinate that changes, not by whether atoms move.

Bond-axis motion changing length is stretching; transverse motion changing angle is bending.

Try this

Better reasoning: “Any vibration in which oxygen atoms move is a stretching vibration.”

Check your answer

Reject it; name bond-length change as the criterion and contrast it with bond-angle change.

Check stretching before mode counting

After the check questions, try the HCN case without notes. Return later for a different N₂O question.

Try this next: describe bending vibrations.

Try this

Explain CO₂ symmetric and asymmetric stretching using arrows, dipole change and one fixed band.

Check your answer

A complete response identifies bond-length motion, distinguishes phase, and links the asymmetric stretch—not the symmetric stretch—to about 2350 cm⁻¹.