Charge transfer band temperature sensitivity

The optical spectra of Cu(III)(//-0)2Cu(III) complexes are unique with intense charge transfer bands at around 300 and 400 run. Resonance Raman studies on [(Cu(III)tacn )2(/i-oxo)2] found isotopicaUy sensitive bands at 602 and 612 cm that shifted to a single peak at 583 cm when the complex was prepared with 2. This isotopic shift is too small to be from a peroxo ligand but is consistent for a Cu-O vibration. The Cu-O vibration in the isotopomer is split into a Fermi doublet, giving rise to two observable peaks. Conclusive structural evidence was obtained from low-temperature XRD measurements that revealed Cu Cu and Cu-O distances of 2.794(2) and 1.81 A (Fig. 6.24) - these distances are significantly shorter than those found in complexes with Cu(ll)(y -OH)2Cu(II) or Cu(II)(,u- / / -00)Cu(II) cores. 

The weak band at 14.4 kK (e = 800) in the essentially low-spin cyto-chrome-c has been the subject of much discussion. Its sensitivity to conformation, temperature and pH (52, 53) were considered by Williams (4, 5) to be consistent with electron transfer from a distal oxygen or sulphur atom to the metal. This assignment is also compatible with single-crystal spectra (90) which show that the band is z-polarized. However, it is equally possible to assign the band to charge transfer from the porphyrin a2 orbital to the dzz orbital (90). 

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