ESR spectroscop

Evidence for the radical anion 3 came from esr spectroscopic experiments, thus supporting this mechanism. The radical anion is protonated by the alcohol to give... 

So far, electrochemical measurements have not provided any direct proof for the formation of a bipolaron state in oligbmers or polymers which is significantly more stable than the polaron state. In general, in terms of energy the redox potentials E° for bipolaron formation should be much lower than the potentials Ej for polaron formation (/E / < /E /). However, more recent electrochemical and ESR spectroscopic studies by Nechtschein et al. indicate that the bipolaron state is not much more stable than the polaron state... 

Thus, a more complete study of the spectral properties and the structure of intermediates frozen in inert matrices is achieved when the IR, Raman, UV and esr spectroscopic methods are mutually complementary. Since IR spectroscopy is the most informative method of identification of matrix-isolated molecules, this review is mainly devoted to studies which have been performed using this technique. 

The results described in this review show that matrix stabilization of reactive organic intermediates at extremely low temperatures and their subsequent spectroscopic detection are convenient ways of structural investigation of these species. IR spectroscopy is the most useful technique for the identification of matrix-isolated molecules. Nevertheless, the complete study of the spectral properties and the structure of intermediates frozen in inert matrices is achieved when the IR spectroscopy is combined with UV and esr spectroscopic methods. At present theoretical calculations render considerable assistance for the explanation of the experimental spectra. Thus, along with the development of the experimental technique, matrix studies are becoming more and more complex. This fact allows one to expect further progress in the matrix spectroscopy of many more organic intermediates. 

Another means of observation of such radical dissociation of hydrocarbons, to a lesser extent, is achieved in modern esr spectroscopic studies. Dissociation of bitropyl [49] into the tropyl radical [3 ] was observed in m-xylene solution (11). The radical is a long-lived species but only at low concentration, 10 -10 m (Vincow et al., 1969). The radical dissociation (12) of 7-(triphenylmethyl)-l,3,5-cycloheptatriene [50] has been found to be more facile than that of bitropyl (Okamoto et al., 1970). 

The oxidation of phenol, ortho/meta cresols and tyrosine with Oj over copper acetate-based catalysts at 298 K is shown in Table 3 [7]. In all the cases, the main product was the ortho hydroxylated diphenol product (and the corresponding orthoquinones). Again, the catalytic efficiency (turnover numbers) of the copper atoms are higher in the encapsulated state compared to that in the "neat" copper acetate. From a linear correlation observed [7] between the concentration of the copper acetate dimers in the molecular sieves (from ESR spectroscopic data) and the conversion of various phenols (Fig. 5), we had postulated [8] that dimeric copper atoms are the active sites in the activation of dioxygen in zeolite catalysts containing encapsulated copper acetate complexes. The high substratespecificity (for mono-... 

Meehanistieal studies on the reaction of stable stannylenes with organic halides have been performed by M. F. Lappert and his group161 163>. On the basis of ESR spectroscopic data they proposed a radicalic pathway for this reaction. Initially, one electron of the stannylene is transferred to the organic halide the... 

Tetrachloro-5,6-diphenyl- and -5,6-di-n-propyl calicene were found to yield intensely colored radical anions when interacted with alkali metals (Na, K)291. As ESR-spectroscopic investigation showed an unpaired electron spin is located on three-ring carbons Cs/C6 and their substituents. 

In the radical anions of the di(2-anthryl) system [9] and of the ort/io-cyclophane [13] a spin-localized situation prevails independently of temperature and ion pairing. On the other hand, for the corresponding di(9-anthryl) species and even for the di(l-anthryl) species a rapid hopping process can be detected if suitable ion pairing is established (Becker et al., 1991). Thus, according to both cyclic voltammetric and esr spectroscopic... 

This hypothesis was supported by results of model studies as well as ESR spectroscopic investigations. The use of alternative Ni-containing dendrimers in which the distance between the Ni sites was increased (Figure 4.30), led to significantly improved catalytic efficiency. 

Only one compound containing an E5 moiety has previously been reported this is the Ss+ radical cation, which is always observed upon oxidation of sulfur with MF5 (M = As, Sb) or oleum (H2S04/S03 mixture). No isolated S5+ compound is known and the existence of S5+ was deduced from an ESR spectroscopic study of a solution that contained 92.4% 33S.42 It is believed that S5+ is formed in equilibrium over S8[AsF6]2 in solution in S02, i.e. Equation (l) 43... 

ESR spectroscopical investigations of the anion radical of fluorene indicated a first-order decay, and it was concluded that the CH bond undergoes homolytic cleavage [261]. Voltammetric studies of fluorene in DME reached the same conclusion [195]. 

Esr-spectroscopic measurements 145 C—H Bond-dissociation energies 151 C—-C Bond-dissociation energies 154 Rotational barriers 159 Isomerization reactions 163 Addition reactions 170 Azoalkane decomposition 171... 

ESR methods unambiguously establishes the presence of species bearing unpaired electrons (ion-radicals and radicals). The ESR spectrum quantitatively characterizes the distribution of electron density within the paramagnetic particle by a hyperfine structure of ESR spectra. This establishes the nature and electronic configuration of the particle. A review by Davies (2001) is highly recommended as a guide to current practice for ESR spectroscopic studies (this quotation is from the title of the review). The ESR method dominates in ion-radical studies. Its modern modifications, namely, ENDOR and electron-nuclear-nuclear triple resonance (TRIPLE) and special methods to observe ion-radicals by swiftness or stealth are described in special literatures (Moebius and Biehl 1979, Kurreck et al. 1988, Werst and Trifunac 1998). 

The ligands 369 react with [RuCl2(dmso)4] to yield [RuCl2(dmso)2(369-A, 0)], characterized W spectroscopic and electrochemical methods. Complexes in the families [Ru"(bpy)(370)2] and [Ru" (aca( (370)2] have been reported. The complexes [Ru(bpy)(370)2] undergo a reversible Ru"/Ru" oxidation followed by an irreversible Ru /Ru process the bpy-centered one-electron reduction is also observed. Chemical oxidation of the complexes [Ru(bpy)(370)2] gives [Ru(bpy)(370)2] (isolated as the iodides), the electronic and ESR spectroscopic properties of which have been described. The crystal structure of [Ru(acac)(371)2] has been established, and the electrochemical and chemical redox reactions of [Ru(acac)(370)2] and [Ru(acac)(371)2] generate Ru" and Ru species that have been characterized by spectroscopic and electrochemical techniques. ... 

Additions of perfluoroalkyl groups generated from perfluoroalkyl iodides (Rpl) and perfluoroacyl peroxides [RpC(0)00(0)CRp] have also been carried out [50]. Another usable source of CEj radicals is the stable [(CF3)2CF]2 CCF2CE3 radical (Scheme 6.8) [51]. As with alkyl radical additions, ESR spectroscopic investigations... 

The covalent 8a-N(3)-histidyl FAD of mitochondrial succinate dehydrogenase functions as a two-electron acceptor in the oxidation of succinate to fumarate and as a one-electron donor in the reduction of the iron-sulfur centers of the enzyme. Recent ESR spectroscopic data have shown the covalent flavin semiquinone... 

Fluorinated radicals play a significant role in synthetic organo-fluorine chemistry, for example, in electrophilic radical addition to alkenes, single-electron transfer reactions (SET), telomerization of fluoroalkenes with perfluoroalkyl iodides, polymerization to fluoropolymers and copolymers, and thermal, photochemical and radiation destruction of fluorocarbons. Furthermore, such free radicals are of interest for studying structures, reaction kinetics and ESR spectroscopic parameters.38... 

A pentopyranosyl radical is much more flexible than a hexopyranosyl radical. Because the alkyl-anchor at C-5 is absent, the radical is now so flexible that several species of similar energy can coexist. According to ESR spectroscopic data, the arabinopyranosyl radical 9 exists as an equilibrium between the 4C19a and the B03 9b conformation, which both realize a coplanar arrangement of the C-O bond and the SOMO [9] (Scheme 6). Reactions with alkenes are unselective. However, the arabinofuranosyl radical 10 reacts with high stereoselectivity [9]. This is due to its 2E conformation in which the si-side of the radical center is sterically hindered by the large benzoyl group. 

Cuprous cyclopropylacetylide, prepared from Cul and cyclopropylacetylene in ammo niacal solution couples with 4-iodonitrobenzene in pyridine, yielding 4-nitrophenyl cyclopropylacetylene (equation 154)234. Reaction of the acetylide with tropylium tetra-fluoroborate in acetonitrile, in the presence of LiBr, affords 7-(cyclopropylethynyl)cyclo-heptatriene. The anion radicals obtained by reduction of these compounds were utilized for ESR spectroscopic analysis of the cyclopropyl P hyperfine splittings. 

Additional information has been obtained from single crystal, polarized optical and ESR spectroscopic studies924 on poplar plastocyanin, which have allowed a correlation of the electronic structure of the blue copper active site with its geometric structure. In summary, the three dominant absorption bands at 13 350, 16 490 and 17 870 cm-1 were assigned to CysS- Cu (d 2-,2 charge-transfer transitions. The methionine makes only a small contribution, due to the long Cu—S(Met) bond (2.9 A) and the poor overlap of the methionine sulfur orbitals with the dx y orbital of copper. Histidine-Cu charge transfer contributes to the weaker absorptions at 21 390 and... 

As shown by means of electrochemical ESR spectroscopic methods and confirmed by ab initio calculations (Alberti et al. 1995), the first electron is accommodated in the more accessible nitrobenzopyran moiety. Next, the second electron fits into the nitroindoline moiety in virtually the same orbital that in the anion radical of mon<9-nitro-spiro(indoline-benzopyran), is occupied by the unpaired electron. The mono-nitro anion radical is also formed by the aforementioned reaction with terf-BuOK in DMSO, Scheme 1-63. 

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