Resonance Raman spectroscopy charge transfer transitions

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-... 

It usually turns out that there are several such vibrations. They will help electron transfer from A to B. The reason is obvious e.g., the empirical formula for Vjip says that a vibration that makes the AB distance smaller will increase the transfer probability. This could be visible in what is known as resonance Raman spectroscopy close to a charge transfer optical transition. In such spectroscopy, we have the opportunity to observe particular vibronic transitions. The intensity of the vibrational transitions (usually from u = 0 to u = 1) of those normal modes that facilitate electron transfer will be highest. 

The vibronic coupling between the chromophores of ZnTRP has been studied by resonance Raman spectroscopy using six laser lines in the 450-515 nm range, in which there is a superposition of the Soret, Ru (dTr) (p7T )bipy and Ru (dTr) (p7T )pyP charge-transfer transitions. Intense peaks... 

Most charge-transfer transitions show less vibronic resolution than the examples in Figure 2. Resonance Raman spectroscopy has often been used in these cases to analyze the structural changes between the initial and final states of the transition, an approach especially relevant to metal centers in enzymes and to bioinorganic model compounds. The full ensemble of optical spectroscopic techniques has been applied to the study of the lowest-energy metal-to-ligand charge-transfer (MLCT) bands in Ru(bipyridine)3 and related complexes. Other well-studied cases of MLCT transitions with resolved vibronic structure include a number of W(CO)sL complexes. "... 

The electronic absorption spectra of several complexes containing a [Mn (0)2Mn ] core were investigated in detail using MCD spectroscopy in conjuction with resonance Raman and absorption spectroscopies. The MCD spectra of the Mn-oxo complexes were recorded at 300-2,500 nm, deconvoluted, and the component peaks assigned. The majority of peaks corresponded to metal-centered d-d transitions and oxo-to-Mir ligand-to-metal charge transfer (LMCT) bands. Although contributions from the other chromophores in PSII and low protein... 

Calculations of eight frontier molecular orbitals for D2h dimers 9 = 90°), and six frontier orbitals for D2 dimers were carried out from the four frontier orbitals of the monomers. A simplified description of the frontier orbitals evolution is reproduced in Figure 13.22, along with a transition dipole moment representation in the orthogonal and oblique bis-porphyrins. Raman resonance spectroscopy of the excited states shows that some transitions involve charge transfer between the two subunits, and that the contribution of charge transfer increases with the degree of coplanarity of the dimers. This is consistent with previous electric dipole measurements in the excited states. 

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