Information charge-transfer transitions

Additional information has been obtained from single crystal, polarized optical and ESR spectroscopic studies924 on poplar plastocyanin, which have allowed a correlation of the electronic structure of the blue copper active site with its geometric structure. In summary, the three dominant absorption bands at 13 350, 16 490 and 17 870 cm-1 were assigned to CysS- Cu (d 2-,2 charge-transfer transitions. The methionine makes only a small contribution, due to the long Cu—S(Met) bond (2.9 A) and the poor overlap of the methionine sulfur orbitals with the dx y orbital of copper. Histidine-Cu charge transfer contributes to the weaker absorptions at 21 390 and... 

The continuous absorption with maximum near 1935 A gives no useful information. Theoretically it has been assigned to a charge-transfer transition leading to a state in which the dipolar structure (7) makes a major contribution (Nagakura, 1960). 

In the previous chapter it was shown how measurements of polarized absorption spectra in the visible to near-infrared region can provide information on such crystal chemical problems as oxidation states of transition metal ions, coordination site symmetries and distortions, cation ordering and the origins of colour and pleochroism of minerals. Much attention was focused in chapter 4 on energies of intervalence charge transfer transitions appearing in electronic absorption spectra of mixed-valence minerals. 

The final physical method to be considered here, which allows further probing of an absorption band, is resonance Raman spectroscopy. The excitation laser wavelength is tuned into an absorption band and the vibrations enhanced in the Raman spectrum are detected. Only those vibrational modes associated with distortion of the excited electronic state relative to the ground-state geometry will be resonance enhanced. This method, therefore, not only allows observation of vibrations directly associated with the active site but also provides valuable information on the nature of the excited state. Usually, charge transfer transitions are probed due to the high intensity (e > 500 M"1 cm-1) required for resonance enhancement. These points are well illustrated by reso-... 

Evidently /iv ax/AG is <4 for a weakly coupled, endergonic charge-transfer reaction and >4 for a weakly coupled, exergonic charge-transfer reaction or for charge transfer in a moderately coupled symmetrical double-well system. The value of /zVmax/AG Can thus provide information about the degree of electronic interaction. However, in practice the latter is more readily obtained from the intensity of the charge transfer transition. 

In Chap. 3 of this volume the luminescence of the octahedral uranate group is discussed. The emission is also due to a charge-transfer transition, but the spectra show a rich vibrational structure which yields additional information not available for the closed-shell ions under discussion (with the possible exception of the CrO complex). These studies can be helpful for a better understanding of the luminescence of the closed-shell transition-metal ions. 

Prospects for MCD spectroscopy in the near-IR are also bright. The weak bands of heme and other metalloproteins are usually either rf — rf transitions of the electrons on the metal ion or charge transfer transitions involving orbitals of both the metal and its ligands. Thus, this spectral region provides direct information on the state of the metal ion, which usually is of critical importance in the function of the protein. The work... 

Patterson and coworkers [5-18] have reported detailed liuninescence studies of selected transition metal complex ions in a varietjr of different environments such as in single crystals, in host crystals and on surfaces. For example, for vanadium oxide on a Si02 substrate the vanadium oxide (d ) is present in low concentrations on the substrate and detailed information can be obtained from the luminescence spectra about the vanadium oxide system [5]. This is a case where charge transfer transitions are present. This d° system is the first example to be discussed in Sect. 3. 

To obtain more information on this point, let us examine the data given in Table 3.6<42-47> for some substituted benzophenones. The data in Table 3.6 indicate that benzophenone derivatives having lowest triplet states of n->TT character undergo very efficient photoreduction in isopropyl alcohol. Those derivatives having a lowest it- -it triplet, on the other hand, are only poorly photoreduced, while those having lowest triplets of the charge-transfer type are the least reactive toward photoreduction. In additon, in some cases photoreduction is more efficient in the nonpolar solvent cyclohexane than in isopropanol. This arises from the solvent effect on the transition energies for -> , ir- , and CT transitions discussed in Chapter 1 (see also Table 3.7). 

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