Charge transfer absorption spectra

Charge-transfer absorption is important because it produces very large absorbances, providing for a much more sensitive analytical method. One important example of a charge-transfer complex is that of o-phenanthroline with Fe +, the UV/Vis spectrum for which is shown in Figure 10.17. Charge-transfer absorption in which the electron moves from the ligand to the metal also is possible. 

As examples. Table 8 records some observations on d—d and charge transfer absorption bands in metal/protein systems. The examination of the spectrum of cobalt carbonic anhydrase (d—d) and of iron conalbumin (charge-transfer) permitted a prediction of the ligands from the protein to the metal. The predictions have now been substantiated by other methods. 

Fig. 5 (A) Typical time-resolved picosecond absorption spectrum following the charge-transfer excitation of tropylium EDA complexes with arenes (anthracene-9-carbaldehyde) showing the bleaching (negative absorbance) of the charge-transfer absorption band and the growth of the aromatic cation radical. (B) Temporal evolution of ArH+- monitored at Amax. The inset shows the first-order plot of the ion...

Fig. 12 Typical time-resolved absorption spectrum following the charge-transfer excitation of nitrosonium-EDA complexes with arene (hexamethylbenzene) showing the bleaching of charge-transfer absorption and growth of the donor cation radical...

The Cu11 atoms are well separated and are not coupled magnetically, but absorb intensely (3800 1 mol-1 cm-1 at 635 nm) in the visible region. This charge-transfer absorption is reversibly lost on electrochemical reduction and regained on reoxidation by 02. These properties, together with A hyperfme coupling apparent in the ESR spectrum, are closely related to those of the distorted tetrahedral type-1 Cu site of blue Cu proteins,310 despite the different Cu coordination stereochemistry in the cluster. 

Fig. 12. Band analysis of the (rans-[IrCl4F2]2 charge transfer absorption spectrum (10 K in KC1 discs) by deconvolution into Gaussians (points, experimental, solid line superposition of components)...

Fig. 5. Charge-transfer absorption spectrum of 3.5 mMCo(CO)3(PBu3)2+ Co(CO)4 (A) Solvent effect in THF, CH2C12, MeCN, and Et20. (B) Salt effect of added TBAP (top to bottom) none, 1, 2, 3, 4, 5, 10, and 20 equivalents, relative to 10 mM Co(CO)3(PBu3)2+ Co(CO)4 in THF (27).

A number of physical studies have been performed on Mn111 porphyrins in an attempt to understand their electronic structure. The visible spectra of such compounds have been of particular interest. For most metal porphyrins the visible spectrum is insensitive to the nature of the coordinated metal and this has been interpreted as indicating little interaction between the metal and the porphyrin 7r-orbitals in such compounds. However this is not the case for Mn111 porphyrins which exhibit metal-dependent charge transfer absorptions. This dependence appears to reflect significant n orbital-Mn"1 interaction. Resonance Raman and linear dichroism spectral studies are also consistent with this conclusion.667... 

Figure 12 Vibrational enhancement selectivity available from resonance Raman spectroscopy. The UV-visible spectrum of a P. aeruginosa azurinis shown together with two different Raman spectra (frozen solution at 77 K) that derive from laser excitation within the S(Cys) — Cu(II) charge-transfer absorption band at 625run (647.1 nm) and away from the absorption (488.Onm). Excitation within resonance leads to dramatically increased Raman scattering from the Cu active site, whereas off-resonance excitation produces a spectrum dominated by bands of nonchromophoric ice...

It is extremely common for coordination compounds also to exhibit strong charge-transfer absorptions, typically in the ultraviolet and/or visible portions of the spectrum. These absorptions may be much more intense than d-d transitions (which for octahedral complexes usually have e values of 20 L moF cm or less) molar absorp-tivities of 50,000 L mole cm or greater are not uncommon for these bands. Such absorption bands involve the transfer of electrons from molecular orbitals that are primarily ligand in character to orbitals that are primarily metal in character (or vice versa). For example, consider an octahedral d complex with cr-donor ligands. The ligand electron pairs are stabilized, as shown in Figure 11-15. 

Fe(CN)6] exhibits two sets of charge transfer absorptions, one of lower intensity in the visible region of the spectrum, and one of higher intensity in the ultraviolet. [Fe(CN)5]" , however, shows only the high-intensity charge-transfer in the ultraviolet. Explain. 

The blue eopper proteins are characterized by intense S(Cys) Cu charge-transfer absorption near 600 nm, an axial EPR spectrum displaying an unusually small hyperfine coupling constant, and a relatively high reduction potential. 

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