Generalizations Regarding lmax
- If the spectrum of a given compound exhibits an absorption band of very low intensity (e
= 10-100) in the 270-350 nm region, and no other absorptions above 200 nm, the compound contains a simple,
nonconjugated chromophore containing n electrons. The weak band is due to h to
p* transitions.
- If the spectrum of a given compound exhibits many bands, some of which appear even in the visible region, the
compound is likely to contain long-chain conjugated or polycyclic aromatic chromophore. If the compound is colored,
there may be at least 4 to 5 conjugated chromophores and auxochromes.
Exceptions: some nitro-, azo-, diazo-, and nitroso-componds will absorb visible light.
Generalizations Regarding emax
- An e value between 10,000 and 20,000 generally represents a simple
a,b-unsaturated ketone or diene.
- Bands with e values between 1,000 and 10,000 normally show the presence of an aromatic system. Substitution on the aromatic nucleus by a functional group which extends the length of the chromophore
may give bands with e > 10,000 along with some which still have e
< 10,000.
- Bands with e < 100 represent h to
p* transitions.
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