Page 11 - Instrumental Analysis - Pharm D Clinical- 07-PA403

UV-visible Spectrophotometry

Structural considerations

• For a compound to absorb light in the UV-visible region,
it should contain a chromophore.

• Chromophores are unsaturated organic functional
groups that absorb in the UV or visible regions. These
include multiple (double or triple) bonds: C=C, C=O,
C=N, N=N, N=O, C≡C and C≡N. Such groups are
responsible for  → * and n → * transitions.

• It may be conjugated with other groups to form complex
chromophores.

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Structural considerations

The greater the number of conjugated multiple bonds in a compound,
the longer the wavelength of the light that the compound will absorb.
This can be explained by less energy requirement for excitation of the
compound.

Compound Formula Wavelength
absorbed

Ethylene CH2=CH2 171 nm
1,3-butadiene CH2=CH−CH=CH2 217 nm
1,3,5-hexatriene CH2=CH−CH=CH−CH=CH2 274 nm
β-carotene (11 conjugated double bonds) 450 nm

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