Relate electronic transitions and chromophores to UV/visible absorption

UV/visible spectroscopy usually promotes valence electrons from occupied molecular orbitals to higher unoccupied orbitals. The structural group that supplies a suitable electronic transition is called a chromophore.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Relate electronic transitions and chromophores to UV/visible absorption

Identify the electrons that can be promoted

Begin by identifying σ bonds, π bonds and nonbonding lone pairs, then decide which vacant antibonding orbital can receive the electron.

The four common transitions

The main labels are σ→σ*, n→σ*, π→π* and n→π*. Here n means a nonbonding lone-pair orbital, and the asterisk marks an antibonding orbital.

Their typical energy order is σ→σ* highest, then n→σ*, then π→π*, with n→π* often lowest. Exact positions depend on molecular structure and environment.

A transition may be symmetry- or spin-forbidden, making its absorption weak, even when the energy gap lies in the measured range. ‘Forbidden’ means low probability, not necessarily zero intensity.

Connect structure to transition type

Saturated molecules containing only σ bonds mainly undergo high-energy σ→σ* transitions, often at wavelengths shorter than routinely measured. A heteroatom lone pair can introduce a lower-energy n→σ* transition.

An alkene or aromatic ring contains occupied π and vacant π* orbitals, so π→π* absorption is expected. A carbonyl group contains both a π system and oxygen lone pairs, allowing π→π* and n→π* transitions.

π→π* transitions are commonly allowed and relatively intense. Carbonyl n→π* transitions are often weak because of selection rules, despite occurring at longer wavelength than the corresponding π→π* transition.

Worked example: a carbonyl chromophore

In ethanal, the C=O group supplies an occupied π orbital, a vacant π* orbital and nonbonding electron pairs on oxygen.

It can therefore show a stronger π→π* band and a lower-energy, often weaker n→π* band. The alkyl C–C and C–H σ bonds also have σ→σ* transitions at much higher energy.

Try this

Name two electronic transitions associated with a carbonyl group and compare their typical intensity.

Check your answer

π→π* is usually stronger and allowed; n→π* is usually weaker because it is formally forbidden or less allowed.

Classify transitions from orbital labels

Read the starting orbital first and the destination second. A lone-pair electron promoted into a σ antibonding orbital is n→σ*, not σ→n.

Do not infer the starting orbital from the presence of a bond alone; use the stated or drawn occupied level.

Try this

Classify a transition in which an oxygen lone-pair electron is promoted into a C–O σ* orbital.

Check your answer

It is an n→σ* transition.

Compare ethene and ethane

Ethene is CH₂=CH₂; ethane is CH₃CH₃. Neither has lone pairs.

List the occupied and vacant valence orbital types each molecule has before choosing a transition.

Try this

Which transition distinguishes the simple UV absorption of ethene from ethane, and which molecule absorbs at the longer wavelength?

Check your answer

Ethene has an occupied π and a vacant π* orbital, so it can undergo π→π* absorption; ethane has only σ and σ* valence orbitals, so its main transition is σ→σ*. The π→π* gap is smaller, so ethene absorbs at the longer wavelength.

Common mistake: treating every band as equally allowed

Energy matching is necessary but not sufficient for a strong absorption. Transition probability also matters, so weak bands can arise from formally forbidden transitions.

A carbonyl lone pair does not make every carbonyl transition n→σ*. The destination orbital can be π*, giving n→π*.

Try this

Correct: ‘A forbidden transition never appears in a spectrum.’

Check your answer

Forbidden means low transition probability under the ideal selection rule. Vibronic coupling and other effects can make a weak band observable.

Check your understanding

For a structure, identify occupied σ, π or n orbitals, choose the destination σ* or π*, and state whether the transition is expected to be strong or weak when relevant.

Next, use these ideas to predict whether unfamiliar functional groups should absorb in the UV/visible region without claiming an unjustified exact wavelength.

Try this

List the four common transition types in decreasing typical energy.

Check your answer

σ→σ* > n→σ* > π→π* > n→π*, as a useful general order.