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Ultraviolet Spectroscopy-2
By
Dr. Mehboob Peeran

Types of electronic transitions

electronic transistions

Conjugated systems

Specific chromophores

A chromophore is a light absorbing group in the UV Visible range which usually has a Pi (Π)bond.

An auxochrome by itself does not absorb light in that range but when present in conjugation with a chomophore will enhance the intensity and the position of the λmax. It usually has an atom with one or more lone pairs of electrons. Tthe λmax is indication of some groups.

C=C                171 nm (15,500)
(C=C)n            n = 2    217 nm (21,000),  n = 6 364 nm (138,000)
Benzene          254 nm (169) and 204 nm (7,500)
C=O                279 nm (15) due to n--->Π * and 189 nm (900) due to Π--->Π*
C=C-C=O        314 nm (38) due to n---> Π* and 224 nm (9750) due to Π--->Π*

Auxochromes:  -OH, -NH2, -OR,
C=C 
171 nm (15,500)
(C=C)n  
n = 2217 nm (21,000),n = 6 364 nm (138,000)
Benzene
254 nm (169) and 204 nm (7,500)
C=O
279 nm (15) due to n--->Π* and 189 nm (900) due to Π--->Π*
C=C-C=O 
314 nm (38) due to n--->Π* and 224 nm (9750) due to Π --->Π*
single chromophores
uvmaxinorganic ions

1. Compare the λmax of NO3 and NO2

a) Rationalise why nitrite has two absorption maxima?

b) Which of them is due to n--->Π* transition?

c) Account for the lower λmax 280 nm of nitrite in comparison with 330 nm of nitrate.

Next page: UV Spectroscopy-3/Influence of solvevnts on λmax
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