Resonance Raman spectroscopy dimer

Q. Yan, "Applications of Resonance Raman Spectroscopy to Complexes of Biological and Clinical Significance I. High-Valent Oxo- and Nitridometalloporphyrins II. pu-Oxo Vanadium(III) Dimers III. Antitumor Drug-DNA Intercalators, PhD. Dissertation, University of Houston, 1996. 

ABSTRACT Resonance Raman spectroscopy has been demonstrated to give important structural information on the reactions of aromatic molecules in the interlayer of transition-metal ion-exchanged montmorillonites. Para-substituted benzenes or 4,4 -substituted biphenyls are oxidized to form their cation radicals, which are stabilized in the interlayer of the clay mineral. The oxidative dimerization or polymerization results in the formation of biphenyl type cations and poly-p-phenylene cations from mono-substituted benzenes and benzene, respectively. 

An early success of Raman spectroscopy was in 1934 when Woodward reported the spectrum of mercury(I) nitrate. After the assigimient of lines to the [NOs]" ion, a line at 169 cm remained which he assigned to the stretching mode of the Hg-Hg bond in [Hg2]. This was one of the first pieces of evidence for the dimeric nature of the mercury(I) ion . Resonance Raman spectroscopy is now used extensively for the investigation of coloured if-block metal complexes and for probing the active metal sites in metalloproteins. Application of Raman spectroscopy in pigment analysis is described in Box 4.1. 

The mvestigation of molecular vibrations is a powerful too which can increase our knowledge on structures and on electron molecular vibrations, which are due to charge oscillation between dimerized molecules, coupled with totally symmetric intramolecular modes Raman scattering studies account for totally symmetric vibrations. In addition, Raman spectroscopy can take advantage of resonant effects. In fact, when resonant conditions are fulfilled, selected molecular vibrations are obtained as well as infor> mation on the electronic manifold involved in the resonance process. 

Vibrational spectroscopy is a powerful and unique technique to study the structure and microenvironment of pigments in the RC and their structural changes during the photochemistry. In the case of Raman spectroscopy, the resonance Raman effect selectively enhances the vibrational spectra of pigments even in the protein complexes. For example, the excitation with the 363 8 nm laser line gives a vibrational spectrum of bacterial RCs, to which dimeric BChl, accessory BChl and BPhe equally contribute but the protein moiety does not. Furthermore, by taking a difference of the resonance Raman spectra measured under different conditions, the vibrational bands arising only from the BChl dimer in the RC have been successfully obtained [1]. 

Costas et al. have reported spectroscopic evidence for an Fe Fe complex that can be considered a structural model for the putative Fe Fe (/x-0)2 core of methane monooxygenase intermediate The synthetic complex was prepared at —80 °C in CH2CI2 by decay of a mononuclear low-spin Fe peroxo precursor. The Mossbauer spectra showed that all iron in the sample is intermediate spin (5 = 1) Fe, but the data were compatible with either a mononuclear site or a weakly coupled ( J <5cm ) symmetric dimer. Combination of the Mossbauer technique with resonance Raman and EXAFS spectroscopies provided evidence for a bis-/x-oxo bridged diiron(IV) complex. The complex of Costas et al however, is not an electronic model for intermediate Q, as the latter contains high-spin Fe sites. 

The resonance Raman spectrum of the class III ascorbate peroxidase isoenzyme II from tea leaves was consistent with an unusual five-coordinate quantum mechanically mixed-spin haem. Porphyrin-ring oxidation state marker bands were assigned for a range of Ru (Por)L2 complexes, where Por = TPP and substituted derivatives, L = PhNO. IR spectroscopy was used to probe the coupling of ground state molecular vibrations with low-energy electronic transitions of Ru(III, II) porphyrin dimer species. ... 

Infrared, Raman, microwave, and double resonance techniques turn out to offer nicely complementary tools, which usually can and have to be complemented by quantum chemical calculations. In both experiment and theory, progress over the last 10 years has been enormous. The relationship between theory and experiment is symbiotic, as the elementary systems represent benchmarks for rigorous quantum treatments of clear-cut observables. Even the simplest cases such as methanol dimer still present challenges, which can only be met by high-level electron correlation and nuclear motion approaches in many dimensions. On the experimental side, infrared spectroscopy is most powerful for the O—H stretching dynamics, whereas double resonance techniques offer selectivity and Raman scattering profits from other selection rules. A few challenges for accurate theoretical treatments in this field are listed in Table I. 

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. 

BF Henson, GV Hartland, VA Venturo, PM Felker. Raman-vibronic double-resonance spectroscopy of benzene dimer isotopomers. J Chem Phys 97 2189-2208, 1992. 

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