Instant Notes: Analytical Chemistry

Section E – Spectrometric techniques

E9 ULTRAVIOLET AND VISIBLE

MOLECULAR SPECTROMETRY:

APPLICATIONS

Structure effects The structures of organic molecules may be classified in terms of the functional
groups, which they contain. Where these absorb UV or visible radiation in a
particular region they are called chromophores. Some of the chromophores
important for analytical purposes are listed in Table 1.This shows that the
absorption by compounds containing only sbonds such as hexane, or with lone
pairs, such as ethanol, will occur below 200 nm. These compounds are therefore
useful solvents.
The effects of conjugation, that is, the interaction of the molecular orbitals
between alternate single and double bonds in a chain or ring, greatly affect the
absorption characteristics of the chromophore. Since each isolated double bond
has a p-bonding energy level and a higher energy p-antibonding level, when
two double bonds are separated by one single bond, the molecular orbitals
overlap, and there are two plevels and two p levels. The separation in energy

Key Notes

The structure of a molecule determines the nature of its UV or visible

spectrum and facilitates qualitative analysis of a sample.

Measurement of the relation between concentration and absorbance

allows quantitative analysis using the Beer-Lambert Law.

Related topics Ultraviolet and visible molecular Other topics in Section E

spectrometry: principles and

instrumentation (E8)

Structure effects

Quantitative

applications

Table 1. Absorption of simple unconjugated chromophores

Chromophore Transition Approximate wavelength (nm)

s-bonded

æCæCæ/æCæH s–s* ~ 150

Lone pair

O: n-s* ~ 185

æN: n-s* ~ 195

>C=O: n-s* ~ 195

n-p* ~ 300

æN=Næ n-p* ~ 340

p-bonded

>C=C<(isolated) p-p* ~ 180