Electronic and vibrational spectroscopy

Transitions between different electronic states result in absorption of energy in the ultraviolet, visible, and, for many transition metal complexes, the near infrared region of the electro-magnetic spectrum. Spectroscopic methods which probe these electronic transitions can, in favorable conditions, provide detailed information on the electronic and magnetic properties of both the metal ion and its ligands. 

FIGURE 6.4 (a) Visible absorptioa spectrum of cytochrome c in its reduced and oxidised states, (b) The three separate a-bands in the visible 

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Molecular clusters are weakly bound aggregates of stable molecules. Such clusters can be produced easily using supersonic expansion, and have been extensively studied by both electronic and vibrational spectroscopy [146,... 

Methane-to-methanol conversion by gas-phase transition metal oxide cations has been extensively studied by experiment and theory see reviews by Schroder, Schwarz, and co-workers [18, 23, 134, 135] and by Metz [25, 136]. We have used photofragment spectroscopy to study the electronic spectroscopy of FeO" " [47, 137], NiO [25], and PtO [68], as well as the electronic and vibrational spectroscopy of intermediates of the FeO - - CH4 reaction. [45, 136] We have also used photoionization of FeO to characterize low lying, low spin electronic states of FeO [39]. Our results on the iron-containing molecules are presented in this section. 

N. Seurre, K. Le Barbu Debus, F. Lahmani, A. Zehnacker Rentien, N. Borho, and M. A. Suhm, Chiral recognition between lactic acid derivatives and an aromatic alcohol in a supersonic expansion Electronic and vibrational spectroscopy. Phys. Chem. Chem. Phys. 8, 1007 1016 (2006). 

Electronic and vibrational spectroscopy continues to be important in the characterization of iron complexes of all descriptions. Charge-transfer spectra, particularly of solvatochromic ternary diimine-cyanide complexes, can be useful indicators of solvation, while IR and Raman spectra of certain mixed valence complexes have contributed to the investigation of intramolecular electron transfer. Assignments of metal-ligand vibrations in the far IR for the complexes [Fe(8)3] " " were established by means of Fe/ Fe isotopic substitution. " A review of pressure effects on electronic spectra of coordination complexes includes much information about apparatus and methods and about theoretical aspects, though rather little about specific iron complexes. ... 

There are no comprehensive data files for CD spectra for standard reference materials (SRM) that compare with the exhaustive libraries which have been compiled for absorbance data in the electronic and vibrational spectroscopy ranges. Analysts are required to create their own CD spectral files using SRM prepared by the usual purveyors of fine chemicals. A significant problem with an SRM is that although it might meet the industry specifications for chemical purity, its enantiomeric purity is open to question. The few cases in which absolute enantiomeric purity might be assured involve natural products whose syntheses are under total enzymatic control. To prove 100% enantiomeric purity is beyond current capabilities. The problem is compounded even more with the risk that the material might racemize after its extraction from its natural environment. Therefore, it is not possible to assume absolute enantiomeric purity with firm conviction. 

Other spectroscopic techniques that have been used with electrochemistry to probe nanoparticles include electronic and vibrational spectroscopies. The spec-troelectrochemistry of nanosized silver particles based on their interaction with planar electrodes has been studied recently [146] using optically transparent thin layer electrodes (OTTLE). Colloidal silver shows a surface plasmon resonance absorption at 400 nm corresponding to 0.15 V vs. Ag/AgCl. This value blue shifts to 392 nm when an Au mesh electrode in the presence of Ag colloid is polarized to —0.6 V (figure 20.12). The absorption spectrum is reported to be quite reproducible and reversible. This indicates that the electron transfer occurs between the colloidal particles and a macroelectrode and vice versa. The kinetics of electron transfer is followed by monitoring the absorbance as a function of time. The use of an OTTLE cell ensures that the absorbance is due to all the particles in the cell between the cell walls and the electrode. The distance over which the silver particles will diffuse has been calculated to be 80 pm in 150 s, using a diffusion coef-... 

By studying the properties of CU/AI2O3 and Cu-ZSM-5 solids with electronic and vibrational spectroscopies, we have already concluded that Cu and Cu° are not detected by the FTIR spectroscopy of the adsorbed CO probe molecule. The IR bands belong to CO adsorbed on Cu ions, these surface ions being generated by the reduction of Cu ions under vacuo and/or by the CO probe itself (12). Furthermore the zeolite framework acts as a host for isolated Cu" ions (10). In fact for a ZSM-5 zeolite a ZO (CuOH)" species may be formed during the exchange process, where ZO represents the zeolite framework Cu (OH) is thus linked to only one Al atom (18). 

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