Emil Roduner

EPR investigation of HO/ radical initiated degradation reactions of sulfonated aromatics as model compounds for fuel cell proton conducting membranes

In order to predict hydroxy radical initiated degradation of new proton conducting polymer membranes based on polystyrene, polyethersulfone, polyetheretherketone, or on polymers obtained by radiation grafting of styrene on different fluoropolymers, eight sulfonated aromatics were chosen as model compounds for EPR experiments, aiming at the identification of products of HO/ radical reactions with these polymers. Photolysis of H 2 O 2 was employed as the source of hydroxyl radicals. A detailed investigation of the pH profile was carried out for p-toluenesulfonic acid. Besides benzyl- and hydroxy-cyclohexadienyl radicals at lower pH values, phenoxyl radicals were identified, predominating in the pH range 10.5-13.0. A large number of new radicals give evidence of multiple hydroxylation of the aromatic rings, confirming reaction mechanisms proposed on the grounds of product analysis, but no evidence of dimerisation was found. The result as regards stability of organic proton exchange membranes for fuel cells is, that all unsaturated bonds and weakly bound atoms are subject to immediate attack by HO/. Ether links open by HO/ ipso addition. Strategies for the reduction of membrane degradation should focus on a minimisation of HO/ formation and of its access to the interior of the membrane.

Selective Catalytic Oxidation of CH Bonds with Molecular Oxygen

Although catalytic reductions, cross‐couplings, metathesis, and oxidation of CC double bonds are well established, the corresponding catalytic hydroxylations of CH bonds in alkanes, arenes, or benzylic (allylic) positions, particularly with O2, the cheapest, “greenest”, and most abundant oxidant, are severely lacking. Certainly, some promising examples in homogenous and heterogenous catalysis exist, as well as enzymes that can perform catalytic aerobic oxidations on various substrates, but these have never achieved an industrial‐scale, owing to a low space‐time‐yield and poor stability. This review illustrates recent advances in aerobic oxidation catalysis by discussing selected examples, and aims to stimulate further exciting work in this area. Theoretical work on catalyst precursors, resting states, and elementary steps, as well as model reactions complemented by spectroscopic studies provide detailed insight into the molecular mechanisms of oxidation catalyses and pave the way for preparative applications. However, O2 also poses a safety hazard, especially when used for large scale reactions, therefore sophisticated methodologies have been developed to minimize these risks and to allow convenient transfer onto industrial scale.

In-situ spin trap electron paramagnetic resonance study of fuel cell processes

A novel method allows the monitoring of radical formation and membrane degradation in-situ in a working fuel cell which is placed in the microwave resonator of an electron paramagnetic resonance (EPR) spectrometer. By introduction of a spin trap molecule at the cathode the formation of immobilized organic radicals on the membrane surface is observed for F-free membranes, revealing the onset of oxidative degradation. For Nafion® there is much less evidence of degradation, and the hydroxyl radical is detected instead. At the anode, free radical intermediates of the fuel oxidation process are observed. No traces of membrane degradation are detected on this side of the fuel cell.

Muonium substituted organic free radicals in liquids. Theory and analysis of μSR spectra

Abstract Muonium substituted free radicals are formed when spin polarized positive muons are stopped in liquid unsaturated organic compounds. They are observed by muon spin rotation (μSR), i.e. via the time evolution of the muon spin polarization caused by Zeeman and hyperfine interactions. A theoretical treatment of μSR spectra of muonic radicals in zero, intermediate and high external magnetic fields is given. Its predictions are verified by observations on radicals derived from tetramethylethylene and benzene. The relation of μSR to magnetic resonance techniques is discussed.

Radiolysis effects in muonium chemistry

Abstract The chemistry of muonium, a light isotope of hydrogen, is investigated for the time period immediately after stopping of its precursor, the positive muon, in the condensed phase. Information is extracted from the initial amplitudes of the muon precession signals arising from muonium and muon-substituted diamagnetic compounds. In liquid water part of the initial spin polarization cannot be accounted for in terms of existing models of muonium formation and reaction. This missing fraction is postulated to arise via a spin exchange interaction of muonium with a hydrated electron. It is also suggested that the chemical fate of the muon is determined by reactions with transient species created in the terminal spur of the muon track.

Muonium-substituted transient radicals observed by muon spin rotation

Abstract We report the first direct observation of radicals formed by formal addition of muonium to liquid organic compounds. They are characterized by muon precession frequencies in transverse magnetic fields of 0.3–5.0 kG. Comparison of the isotropic hyperfine coupling constants with those of hydrogen analogue reveals large isotope effects.

Muonium substituted organic free radicals in liquids. Muonium—electron hyperfine, coupling constants of alkyl and allyl radicals

Abstract Muonium substituted free radicals are observed by muon spin rotation when positive muons are stopped in liquid olefins and dienes. From muon precession the isotropic muon—electron hyperfine coupling constants have been determined for 44 radicals and analyzed to yield the radical structures. Primary, from mono-olefins, and mainly allylic radicals from conjugated dienes. They are formed by addition of the light hydrogen isotope muonium (Mu = μ + the relations between coupling constants and radical structures follow the principles known for the analogous H substituted radicals. Further, the regi to that of H atoms.

Detection of muonated free radicals through the effects of avoided level crossing. Theory and analysis of spectra

Abstract A treatment is developed for the analysis of μSR spectra of muonium-substituted free radicals detected under conditions of avoided level crossing in high longitudinal magnetic fields. Approximate analytical results are compared with those of numerical calculations, and an experimental example is presented.

Muonium-substituted organic free radicals in liquids. Muon-electron hyperfine coupling constants and the selectivity of formation of methyl and fluorine-substituted cyclohexadienyl type radicals

Abstract Muonium-substituted free radicals are observed by muon spin rotation when positive muons are stopped in liquid methyl, and fluorine-substituted benzenes. From muon precession frequencies in high external magnetic fields the isotropic muon-electron hyperfine coupling, constants A gm are determined. They are typical of cyclohexadienyl-type radicals. The individual assignments are based on deuteration and on substituent effects. Comparison of A gm with A p of the hydrogen analogues reveals isotope effects A μ μ p / A p μ μ of 1.15–1.21. Analysis of signal amplitudes yields substituent effects on the relative rates of Mu addition to inequivalent positions in the arenes. Both CH 3 and F are ortho directing. Addition at CH sites is favored over ipso addition by a factor of ≈ 3.

Using polarized muons as ultrasensitive spin labels in free radical chemistry

In a chemical sense, the positive muon is a light proton. It is obtained at the ports of accelerators in beams with a spin polarization of 100%, which makes it a highly sensitive probe of matter. The muonium atom is a light hydrogen isotope, nine times lighter than H, with a muon as its nucleus. It reacts the same way as H, and by addition to double bonds it is implemented in free radicals in which the muon serves as a fully polarized spin label. It is reviewed here how the muon can be used to obtain information about muonium and radical reaction rates, radical structure, dynamics, and local environments. It can even tell us what a fragrance molecule does in a shampoo.