Electron spin resonance spectroscopy cations

The first intermediate to be generated from a conjugated system by electron transfer is the radical-cation by oxidation or the radical-anion by reduction. Spectroscopic techniques have been extensively employed to demonstrate the existance of these often short-lived intermediates. The life-times of these intermediates are longer in aprotic solvents and in the absence of nucleophiles and electrophiles. Electron spin resonance spectroscopy is useful for characterization of the free electron distribution in the radical-ion [53]. The electrochemical cell is placed within the resonance cavity of an esr spectrometer. This cell must be thin in order to decrease the loss of power due to absorption by the solvent and electrolyte. A steady state concentration of the radical-ion species is generated by application of a suitable working electrode potential so that this unpaired electron species can be characterised. The properties of radical-ions derived from different classes of conjugated substrates are discussed in appropriate chapters. 

The reaction involves the transfer of an electron from the alkali metal to naphthalene. The radical nature of the anion-radical has been established from electron spin resonance spectroscopy and the carbanion nature by their reaction with carbon dioxide to form the carboxylic acid derivative. The equilibrium in Eq. 5-65 depends on the electron affinity of the hydrocarbon and the donor properties of the solvent. Biphenyl is less useful than naphthalene since its equilibrium is far less toward the anion-radical than for naphthalene. Anthracene is also less useful even though it easily forms the anion-radical. The anthracene anion-radical is too stable to initiate polymerization. Polar solvents are needed to stabilize the anion-radical, primarily via solvation of the cation. Sodium naphthalene is formed quantitatively in tetrahy-drofuran (THF), but dilution with hydrocarbons results in precipitation of sodium and regeneration of naphthalene. For the less electropositive alkaline-earth metals, an even more polar solent than THF [e.g., hexamethylphosphoramide (HMPA)] is needed. 

It is interesting to note that the catalysts that show good selectivities at the higher temperatures generally do not contain easily reducible metal ions, such as V, Mo, or Sb. Many of the catalysts for the lower-temperatures operation, on the other hand, contain these reducible cations. In a study using a Li-Mg oxide, it was established that gas-phase ethyl radicals could be generated by reaction of ethane with the surface at about 600°C (17). These radicals could be trapped by matrix isolation and identified by electron spin resonance spectroscopy. 

F -centers (i.e., two electrons trapped in the same anion vacancy) and M-centers (two electrons occupying two adjacent anion vacancies, i.e., two adjacent F-centers) can also be present. Symons [78] observed by electron spin resonance spectroscopy that in solid alkali halide crystals doped with metal the F-centers are the most stable, while F -centers and M-centers may be formed at higher concentrations of trapped electrons. Durham and Greenwood [79] proposed that the dissolved metal dissociated into metal cations and nearly free electrons scattered in the conduction band. 

Structure and reactivity of neutral pyrazole, its anion and its cation. 2.2 Structure and reactivity of substituted pyrazoles. 2.3 Structure and reactivity of indazoles. 2.4 Dipole moments Structural Methods. 3.1 X-Ray diffraction. 3.2 Microwave spectroscopy. 3.3 H NMR spectroscopy. 3.4 C NMR spectroscopy. 3.5 Nitrogen NMR spectroscopy 3.6 UV spectroscopy. 3.7 JR spectroscopy. 3.8 Mass spectrometry. 3.9 Photoelectron spectroscopy. 3.10 Electron spin resonance spectroscopy Thermodynamic Aspects. 4.1 Intermolecular forces. 4.2 Stability and stabilization. 4.3 Conformation and configuration Tautomerism... 

The work described in the present paper concerns the Influence of water and organic solvents on the ionic interactions in lightly sulfonated polystyrene (SFS) ionomers. The focus will be specifically directed towards the Influence of the solvent environment on the cation-anion and cation-cation interactions. Fourier transform Infrared spectroscopy (FTIR) was used to probe the former while electron spin resonance spectroscopy (ESR) was used to study the latter. Experiments were carried out with dissolved, swollen, and bulk ionomers. 

Electron spin resonance spectroscopy was used to characterize interactions between the Rations in bulk samples and in solution. In the solid state the Mn cations of Mh-SPS are primarily associated. In solution the ratio between associated and isolated ions depends upon the polarity of the solvent. The more polar the solvent, the greater is the extent of isolation of the cations. Preliminary ESR spectra of Cu-SPS are in qualitative agreement with this conclusion. 

The extent to which charge is transferred back from the anion towards the cation in the alkali metal halides themselves is difficult to determine precisely. Calculations indicate that it is probably only a few percent for some salts such as NaCl, whereas for others (e.g. Lil) it may amount to more than 0.33 e per atom. Direct experimental evidence on these matters is available for some other elements from techniques such as Moss-bauer spectroscopy, electron spin resonance spectroscopy, and neutron scattering form factors. ... 

P-07 - Adsorption of DTBN at monovalent cations in zeolite Y as studied by electron spin resonance spectroscopy... 

It would be instructive to apply electron spin resonance spectroscopy in these various metal cation oxidations. Certainly the homogeneous nature of these reactions offers greater possibilities to the kineticist than is afforded by the heterogeneous nature of the metal-catalysed eliminations. Considering the recent discoveries of the synthetic utility of thallium in organic synthesis " , thallous salts might prove to be extremely useful catalysts for dehalogenations. as under the reaction conditions the less stable olefins are rapidly isomerised. 

Among the major analytical tools for detecting and studying radical cations are mass spectrometry, electron spin resonance spectroscopy, nuclear magnetic resonance spectrometry, and particular variations of these methods. 

An important advantage of the anthraquinone system is the stability of its radical anion in the presence of water at neutral pH. When an anthraquinone radical anion podand was added to a two-phase, CH2Cl2-water system containing Li , the cation was transported into the organic phase. This was demonstrated unequivocally using electron spin resonance spectroscopy by detection of a 0.33 G Li-hyperfine splitting. 

In order to introduce vanadium into hydrogen forms of zeolites (H,Na-MOR,H-MOR with nsi/nAi = 5 and H,Na-ZSM-5, H-ZSM-5 with nsi/nAi = 35), mixtures of V2O5 and the zeolites were subjected to heat-treatment at 1073 K in air [92,190, 191 ]. Electron spin resonance spectroscopy (ESR) yielded a spectrum (as shown for the example of H-ZSM-5 in Fig. 52) exhibiting a well-resolved hyperfine (HF) signal ofvanadyl cations with g = 193,gj = 2.02,A = 19.8 mT,and Aj = 8.3 mT. These parameters are typical of isolated cations in an almost square-planar coordination. 

It should be mentioned that the present review does not cover in detail the ground-state electronic and/or molecular structure of diene and polyene radical cations as revealed, for example, by electron spin resonance (ESR) spectroscopy or variants thereof. 

Huttermann J, Ward JF, Myers LS Jr (1971) Electron spin resonance studies of free radicals in irradiated single crystals of 5-methylcytosine. Int J Radiat PhysChem 3 117-129 Huttermann J, Ohlmann J, Schaefer A, Gatzweiler W (1991) The polymorphism of a cytosine anion studied by electron paramagnetic resonance spectroscopy. Int J Radiat Biol 59 1297-1311 Hwang CT, Stumpf CL, Yu Y-Q, Kentamaa HI (1999) Intrinsic acidity and redox properties of the adenine radical cation. Int J Mass Spectrom 182/183 253-259 Ide H, Otsuki N, Nishimoto S, Kagiya T (1985) Photoreduction of thymine glycol sensitized by aromatic amines in aqueous solution. J Chem Soc Perkin Trans 2 1387-1392 Idris Ali KM, Scholes G (1980) Analysis of radiolysis products of aqueous uracil + N20 solutions. J Chem Soc Faraday Trans 176 449-456... 

Electron spin resonance (ESR) spectroscopy is of application to organic species containing unpaired electrons radicals, radical ions and triplet states, and is much more sensitive than NMR it is an extremely powerful tool in the field of radical chemistry (see Chapter 10). Highly unstable radicals can be generated in situ or, if necessary, trapped into solid matrices at very low temperatures. Examples of the application of this techniques include study of the formation of radical cations of methoxylated benzenes by reaction with different strong oxidants in aqueous solution [45], and the study of the photodissociation of N-trityl-anilines [46],... 

The structures of compounds containing a 1,3,2-dithiazole ring (1,3,2-dithiazolidines, 1,3,2-dithiazoles, 1,3,2-dithia-zolium cations and radicals) were extensively investigated by X-ray and electron diffraction between 1980 and 1990 <1996CHEC-II(4)433>. Nuclear magnetic resonance (NMR) spectroscopy was found to be very effective for elucidation of the structure of 1,3,2-dioxazoles and 1,3,2-dithiazoles. Electron spin resonance (ESR) spectra of the 1,3,2-dithiazolyl radicals gave important information on their structures. 

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