Electronic spectra charge-transfer transitions

The complexity of the low temperature MCD spectra of the oxidized and reduced trinuclear cluster shows the multiplicity of the predominantly S — Fe charge transfer transitions that contribute to the absorption envelope. While MCD spectroscopy provides a method of resolving the electronic transitions, assignment cannot be attempted without detailed knowledge of the electronic structure. However, the complexity of the low temperature MCD spectra is useful in that it furnishes a discriminating method for determining the type and redox state of protein bound iron-sulfur clusters. Each well characterized type of iron-sulfur cluster, i.e. [2Fe-2S], [3Fe-4S], and [4Fe-4S], has been shown to have a characteristic low temperature MCD spectrum in each paramagnetic redox state (1)... 

Charge transfer transitions are best discussed in the language of MO theory (Section 8.2). An L—> M transition involves the transfer of an electron from a nonbonding or weakly-bonding MO localised mainly on the ligand moieties L , to a vacancy in the partly-filled nd subsheli of M. Such a transition is likely to appear in the visible or near UV spectrum if the energy separation between the donor and acceptor orbitals is relatively small. This will be the case if ... 

---