Showing papers by "Emil Roduner published in 1989"

Hyperfine and Structural Isotope Effects in Muonated Cyclohexadienyl and Cyclopentyl Radicals

Abstract: Incorporation of muonium, the ultra-light isotope of hydrogen with a mass of only one-ninth the mass of H, leads to pronounced isotope effects in hyperfine coupling constants of radicals. They reflect the mass dependence of the internal dynamics. A detailed analysis is presented for the cyclohexadienyl and the cyclopentyl radicals. It is concluded that the isotope dependent bond length in an anharmonic potential accounts quantitatively for the observed effects in rigid species.

The separation of chemical reactivity and Heisenberg spin-exchange effects in a radical-radical reaction by avoided level crossing μSR

Abstract: The effect of Heisenberg spin exchange on the avoided level crossing (ALC) μSR spectra of organic free radicals is discussed within the framework of a two-state jump model. Numerical expressions are derived which also apply to chemical exchange processes. A simple expression relating ALC amplitude and linewidth to the exchange and chemical reaction rates is inferred from numerical calculations. The theory is applied to the kinetics of the radical-radical reaction between the muonated cyclohexadienyl radical and the diphenylpicrylhydrazyl radical (DPPH•) which is investigated by means of transverse field (TF) and ALC μSR. A total rate constant of 4.7 (5) × 109 M−1 s−1 is found. ALC experiments allow its separation into a spin exchange contribution of ≈ 75% and a chemical contribution of ≈ 25%. The efficiency of the chemical reaction amounts to ≈ 1 3 per encounter of electron singlet radical pairs. The strong exchange-coupling limit holds true, and 10−10 s is obtained as an upper limit for the radical pair lifetime at room temperature.

Anisotropy of 2-Norbornyl Radical Reorientational Dynamics in the Plastic Phase of Norbornene as Determined by ALC-μSR

Abstract: Avoided level crossing muon spin resonance effects are observed with polycrystalline norbornene in its plastic phase and analysed in terms of axial hyperfine anisotropy. The results agree with those of previous single crystal measurements. It is shown that the technique is sensitive to the detection of small anisotropies, and that it is ideally suited for the study of reorientational dynamics in polycrystalline or amorphous environments.