Resonance Raman enhancement profiles

The resonance Raman enhancement profiles In Figures 7 and 8 show that the maximum Intensity of the Fe-O-Fe symmetric stretch falls to correspond to a distinct absorption maximum In the electronic spectrum. This Implies that the 0x0 Fe CT transitions responsible for resonance enhancement are obscured underneath other, more Intense bands. Although strong absorption bands In the 300-400 nm region (e > 6,000 M" cm"l) are a ubiquitous feature of Fe-O-Fe clusters, the Raman results make It unlikely that they are due to 0x0 -> Fe CT. An alternative possibility Is that they represent simultaneous pair excitations of LF transitions In both of the... 

The different schemes above can also be distinguished by using TRRR techniques. At the moment this technique might take more effort than the optical methods. However, it can be done with more accuracy since vibrational Raman bands are better resolved than optical absorption bands. A detailed study of the observed change of the resonance Raman spectrum with time and with probe laser frequency should, in principle, enable one to distinguish between the different schemes given above. This will be possible if the photoproducts in a certain scheme are produced with different rates or have different optical absorption maxima (and thus different resonance Raman enhancement profiles). 

The reorganization energies for ET between the dinuclear Cua center and cytochrome a in cytochrome c oxidase have been estimated to be between 0.15-0.5 eV from ET experiments. This estimate means that the A for the Cua center is even lower than that for the mononuclear cupredoxin center. An electron transfer study of an engineered Cua azurin showed that the calculated A for Cua in the engineered azurin is 0.4eV, which is about half that of the blue copper center in native azurin. " Furthermore, an excited-state distortion analysis of a resonance Raman enhancement profile for Cua also confirmed that the inner-sphere A for Cua is about 50% that of a blue copper center., i7,ioi lower A for Cua can be attributed to the valence delocalization of the dinuclear center the bond length distortions associated with the redox reaction can be spread over two metal ions and thus reduced to 1/2 that of the mononuclear copper or valence-trapped dinuclear centers. 

A few of these UV resonance Raman studies have reported excitation profiles of oligonucleotides [158, 177], These studies show that the hypochromism in the resonance Raman intensities can be as large as 65% for bands enhanced by the ca. 260 nm absorption band for poly(dG-dC) and that the hypochromism can vary substantially between vibrational modes [177], In the duplex oligonucleotide poly(rA)-poly(rU) [158], similar hypochromism is seen. Although theUV resonance Raman excitation profiles of oligonucleotides have been measured, no excited-state structural dynamics have been extracted from them. 

Figure 2.6 Resonance Raman excitation profiles for Tp MoO(bdt) detailing inplane dithiolene Mo(x -y) and out-of-plane dithiolene Mo(x2jV2) LMCT transition through the selective resonance enhancement of totally symmetric S-Mo-S (diamonds) and Mo O (squares) vibrations, respectively. ...

Nevertheless, a few reports of UV resonance Raman spectra of the purine nucleobases and their derivatives have appeared. Peticolas s group has reported the identification of resonance Raman marker bands of guanine, 9-methylguanine and 9-ethylguanine for DNA conformation [118, 144], In the process of doing that work, very rudimentary excitation profiles were measured, which yielded preliminary structures for two of the ultraviolet excited electronic states. Tsuboi has also performed UV resonance Raman on purine nucleobases in an effort to determine the resonance enhanced vibrational structure [94], Thus far, no excited-state structural dynamics for any of the purine nucleobases have been determined. 

Peticolas was the first to measure the UV resonance Raman spectrum and excitation profile (resonance Raman intensity as a function of excitation wavelength) of adenine monophosphate (AMP) [147, 148], The goal of this work, besides demonstrating the utility of UV resonance Raman spectroscopy, was to elucidate the excited electronic states responsible for enhancement of the various Raman vibrations. In this way, a preliminary determination of the excited-state structures and nature of each excited electronic state can be obtained. Although the excited-state structural dynamics could have been determined from this data, that analysis was not performed directly. 

B2y, and >42 vibrations are expected to be polarized (p), depolarized (dp), and inversely polarized (tp ), respectively. These polarization properties, together with their vibrational frequencies, were used by Spiro and his coworkers to make complete assignments of vibrational spectra of the Fe-porphin skeletons of a series of heme proteins. They showed that the resonance Raman spectrum may be used to predict the oxidation and spin states of the Fe atom in heme proteins. For example, the Fe atom in oxyhemoglobin has been shown to be low-spin Fc(IIl). It should be noted that the A2y mode, which is normally Raman inactive, is observed under the resonance condition. Although the modes are rather weak in Fig. I-19, these vibrations are enhanced markedly and exclusively by the excitation near the B band since the A-term resonance is predominant under such condition. The majority of compounds studied thus far exhibit the A-term rather than the l -term resonance. A more complete study of resonance Raman spectra involves the observation of excitation profiles (Raman intensity plotted as a function of the exciting frequency for each mode), and the simulation of observed excitation proliles based on various theories of resonance Raman spectroscopy. ... 

I. Smova-Sloufova, B. Vlckova, T.L. Snoeck, D.J. Stufkens, P. Matejka, Surface-enhanced Raman scattering and Surface-enhanced resonance Raman scattering excitation profiles of Ag-2,2 -bipyridine surface eomplexes and of [Ru(bpy)3] on Ag colloidal surfaces manifestations of the charge-transfer resonance contributions to the overall surface enhancement of Raman scattering. Inorg. Chem. 39, 3551 (2000)... 

The magnitudes of geometric changes in molecules on electronic excitation can be determined from the excitation profiles of resonance-enhanced Raman bands, most accurately where both the resonant absorption band and the profiles show vibronic structure. 

The Pt + Pt intervalence transitions of such chain complexes occur in the regions 25,000-18,200 cm 1, 23,600-14,300 cm 1 and 20,600-7,500 cm 1 for chloro-, bromo-, and iodo-bridged complexes, respectively, the trend Cl > Br > I being the reverse of that of the conductivity of the complexes. The transition wavenumbers may be determined either by Kramers-Kronig analysis of specular reflectance measurements or from plots of the excitation profiles of Raman bands enhanced at or near resonance with the Pt I-PtIV intervalence band. The maxima have been found to be related to the Pt —PtIV chain distance, the smaller the latter the less being the intervalence transition energy (3). 

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