Analytical Chemistry

increases in the energies of π and π orbitals, the π being raised by more than the π, but leaves the
energy of the non-bonding orbital unchanged. Empirical rules have been devised by Woodward, Fieser

and Scott 1 to enable the additive effects of auxochromic substitution on the absorption of aromatic and
other conjugated systems to be predicted. The rules for diene absorption are reproduced in Table 9.4.

Table 9.4 Rules for diene absorption

Wavelength, nm

base value for heteroannular diene 214

base value for homoannular diene 253

increments added for:

double bond extending conjugation 30

alkyl substituent or ring residue 5

exocyclic double bond 5

polar groups OAc 0

OAlk 6

Salk 30

Cl, Br 5

N(Alk) 2 60

solvent correction 0

calculated λmax Total

Solvent Effects

Absorption bands arising from n → π* transitions suffer hypsochromic shifts on increasing the solvent

polarity, whilst those of π → π transitions are shifted bathochromically. Explanations lie in the fact
that the energy of the non-bonding orbital is lowered by hydrogen bonding in the more polar solvent
thus increasing the energy of the n → π transition, but the energy of the π* orbital is decreased relative

to the π orbital. The positions and intensities of π → π* bands in such compounds as phenols and
amines exhibit a marked sensitivity to pH changes because of changes in the interaction of non-bonding
electrons with the π system.

Metal Complexes

Complexes of metals with organic and inorganic ligands which absorb in the visible region of the
spectrum are of importance in quantitative analysis. Transitions giving rise to coloured complexes are
of three types:

(1) d–d transitions within a transition metal ion. These are usually of low intensity and of little use for
determination at trace levels.

(2) Excitations within an organic ligand. These are typical n → π and π → π transitions which are