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CHEM 121 Studioby Learn4Less · UBC CHEM 121

CHEM 121 glossary

λmax (wavelength of maximum absorption)

The wavelength at which a substance absorbs light most strongly, the peak of its absorption spectrum. For a conjugated molecule it corresponds to the HOMO → LUMO transition, λmax⁡=hc/ΔE\lambda_{\max} = hc/\Delta E; if it falls between about 400 and 750 nm the compound is coloured.

Bar chart of values in nmethene: 171 nm; butadiene: 217 nm; hexatriene: 258 nm; octatetraene: 290 nm; β-carotene: 450 nm0200400600nm171ethene217butadiene258hexatriene290octatetraene450β-carotene

Measured λmax⁡\lambda_{\max} rises with conjugation length; only β-carotene reaches the visible.

Example

Conjugation shifts λmax⁡\lambda_{\max} to longer wavelengths: 1,3-pentadiene (conjugated) absorbs at 224 nm, but 1,4-pentadiene, whose two C=C are separated by a CHX2\ce{CH2}, absorbs at 178 nm, close to ethene's 171 nm.

See alsoHOMO–LUMO gapAbsorption spectrumComplementary colourConjugation