Showing papers by "Emil Roduner published in 1990"
TL;DR: In this paper, a positive-muon avoided-level-crossing spin resonance was used to detect cyclohexadienyl radicals on silica, which can detect radical concentrations many orders of magnitude less than can be studied by conventional magnetic spectroscopy.
Abstract: HETEROGENEOUSLY catalysed reactions usually proceed at outer or inner surfaces of materials that have large and often non-uniform surface area. Many intermediates of such chemical transformations are short-lived and escape direct detection; such transient species may include free radicals. The structure, adsorption and surface dynamics of diamagnetic intermediates have been well studied by thermodynamic or spectroscopie methods, but paramagnetic species (that is, radicals) recombine at catalytically relevant temperatures so that they are never present in sufficiently high concentrations to allow their observation by these techniques. The otherwise powerful laser methods are also hampered by the non-transparent nature of heterogeneous systems. Here we demonstrate for the model case of cyclohexadienyl radicals on silica that a novel technique, positive-muon avoided-level-crossing spin resonance, which uses positive muons as spin probes, is sufficiently sensitive to allow the detection and study of radicals under such constrained conditions. The technique can detect radical concentrations many orders of magnitude less than can be studied by conventional magnetic spectroscopy.
34 citations
01 Nov 1990
TL;DR: In this paper, rate constants have been measured for the reaction of the light hydrogen isotope muonium with benzene and 2,3-dimethylbutadiene-1,3 in the gas phase and in several solvents.
Abstract: Rate constants have been measured for the reaction of the light hydrogen isotope muonium with benzene and 2,3-dimethylbutadiene-1,3 in the gas phase and in several solvents. Comparison with H atom data reveals (1) appreciable kinetic isotope effects, (2) a strong contribution of tunnelling for the activated reaction with benzene, (3) encounter controlled reaction for Mu addition to the diene, (4) a mass dependence of the diffusion constant of hydrogen isotopes in solution, and (5) an enhancement of the rate constant in solution over that in the gas phase by a factor, except for water, of the inverse Ostwald solubility coefficient for hydrogen. The isotope-dependent reduction of the solvent effect in water is tentatively ascribed to the dynamics of the clathrate-like structure when the hydrogen atom reacts.
20 citations
TL;DR: In this paper, positive muons were inserted into samples of PhCCPh and PhCCMe at 298 K to generate cyclohexadienyl radicals, in lower yield, allowing spin-delocalisation parameters for the CCR substituents.
Abstract: Implantation of positive muons into samples of PhCCPh and PhCCMe at 298 K led to the formation of the radicals PhĊC(Mu)Ph and PhĊC(Mu)Me; in the case of the latter compound, the alternative MeĊC(Mu)Ph radical was not detected. Additionally, cyclohexadienyl radicals formed by muonium atom addition to the aromatic rings were observed, in lower yield, allowing spin-delocalisation parameters (Δx) to be determined for the –CCR substituents. During irradiation of Me3SiCCMe, the Me3SiĊC(Mu)Me radical was observed.
13 citations
TL;DR: In this article, the muonium-substituted radicals formed in 3-methyl-2-butenal were observed and the reduced hyperfine coupling constants at 273 K were 59.96 MHz for addition to the C-C bond and 2.89 MHz for attachment to CO.
Abstract: We report the observation of muonium-substituted radicals formed in 3-methyl-2-butenal. The reduced hyperfine coupling constants (A′µ) at 273 K are 59.96 MHz for addition to the C—C bond and 2.89 MHz for muonium attachment to CO.
8 citations
01 Jan 1990
TL;DR: In this article, the muonium-substituted radicals formed in 3-methyl-2-butenal were observed and the reduced hyperfine coupling constants at 273 K were 59.96 MHz for addition to the C-C bond and 2.89 MHz for attachment to CO.
Abstract: We report the observation of muonium-substituted radicals formed in 3-methyl-2-butenal. The reduced hyperfine coupling constants (A′µ) at 273 K are 59.96 MHz for addition to the C—C bond and 2.89 MHz for muonium attachment to CO.
7 citations
TL;DR: In this article, the addition of muonium atoms to hexamethyl (Dewar) benzene has been studied by ESR spectroscopy for comparison with the H-atom analogue of the endo isomer.
5 citations
TL;DR: In this article, it was shown that the ascorbate anion reacts faster than the neutral molecule by a factor of 20. This is ascribed to differences in the free energies of reaction.
Abstract: (CH3)2COH was generated photolytically in aqueous solution in the pH range 1.0 to 7.3 in presence of ascorbic acid. It reacts with ascorbate to yield the ascorbate radical anion. The kinetics of this oxidation and of the radical termination reactions were measured using time resolved ESR. It is shown that the ascorbate anion reacts faster than the neutral molecule by a factor of 20. This is ascribed to differences in the free energies of reaction.
1 citations