Electronic spectroscopy charge transfer transitions

Electronic absorption spectroscopy charge transfer transitions, 19 71 d-d transitions, 19 70, 71 flavocytochrome b, 36 269-271 intraligand transitions, 19 71-80 of organometallics, 19 69-80 Electronic coupling, between donor and acceptor wave functions, 41 278 Electronic nuclear double resonance spectroscopy, molybdenum center probes, 40 13... 

UV-vis(-NIR) Ultraviolet-visible(-near-infrared) spectroscopy Electron and 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)... 

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-... 

Fluorescence and phosphorescence are emission processes which originate directly or indirectly (see 5 section ll.B) from the electronically excited singlet state and triplet state, respectively, produced by charge-transfer processes (Eqs. 1 and 2). Many publications deal with such charge-transfer transitions by diffuse reflectance spectroscopy (DRS) (2-6) showing the link between the latter technique and photoluminescence. It is worthwhile to recall that the emergence of the coordination chemistry of solid-state anions, namely, of surface lattice oxide ions, has almost entirely been based on the results of both photoluminescence and DRS analyses (7, 66). For some catalytic systems, vibrational structures can be detected (see Section IV.B) with an associated vibrational constant, which may be determined directly and independently by IR or Raman spectroscopy, evidencing the relation between these spectroscopies and photoluminescence (33, 34). 

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... 

In addition to the characterization of the intermediates in such reactions, electronic absorption spectroscopy can suggest the nature of donor atoms and often the geometric isomer from the energies and splitting of the d-d transitions in the visible (vis) absorption spectrum (80). However, sometimes these transitions are masked by charge-transfer transitions. 

Chromoproteins are characterized by an electronic absorption band in the near-UV, visible or near-IR spectral range. These bands may arise from Jt Jt" transitions of prosthetic groups or from charge-transfer transitions of specifically bound transition metal ions. Thus, chromoproteins which may serve as electron transferring proteins, enzymes or photoreceptors, are particularly attractive systems to be studied by RR spectroscopy since an appropriate choice of the excitation wavelength readily leads to a selective enhancement of the Raman bands of the chromo-phoric site. Moreover, these chromophores generally constitute the active sites of these biomolecules so that RR spectroscopic studies are of utmost importance for elucidating structure-function relationships. 

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