Ivan D. Reid

Reorientational dynamics of C60 in the solid state. An avoided level-crossing muon spin resonance study

The dynamics of the muonium (Mu) adduct to C60 in the crystalline state have been investigated using avoided level crossing muon spin resonance (ALC-μSR). The motion of C60 in the orientationally ordered phase (T < 260 K) is well described by a spherical diffusion or isotropic jump-reorientation motion with an Arrhenius activation energy of 176(1) meV and a frequency factor of 2.95(13) × 10−12 s. This is in striking contrast to the behaviour of MuC70 adducts whose reorientational motion fits well in a pseudo-static model with a temperature dependent order parameter.

The μSR facilities at PSI

TheμSR Facility Instruments presently available at PSI and the envisaged medium- and long-term developments are presented. The plans focus on further upgrades of the existing instruments and the development of new techniques using the very high fluxes becoming available at PSI, in particular the setup of a beamline with a fast kicker for ‘muons on request’ (MORE) and the development of very low energy muon beams.

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.

Probing the behaviour of surface-adsorbed free radicals using ALC μSR

Preliminary investigations show that the avoided level crossing muon spin rotation technique is well suited to the study of surface-adsorbed free radicals. The samples used were monolayer films of benzene and anisole on small (7 nm and 14 nm) amorphous silica grains. The observed cyclohexadienyl radical resonances indicate a reduction in mobility down to a temperature of about 125 K, beyond which measurements are unreliable due to weak signals. However, the failure to observe any anisotropy in the resonances shows that the residual mobility is mainly translational. For a benzene multilayer, on the other hand, we found clear evidence of anisotropic behaviour, co-existing with signals from a mobile phase. It seems likely that this arises from a thin fluid surface layer in equilibrium with frozen bulk.

Muonium adduct of benzaldehyde: a novel probe of cation–molecule interactions in zeolite catalysts and of solvation and electronic substituent effects

The adduct radicals (ArCHOMu˙) have been formed by muonium addition to the carbonyl group of benzaldehyde and its derivatives. The muon coupling is found to be highly sensitive to the nature of substituents in the benzene ring, being increased by electron releasing groups. This bears relevance to their properties in Group 1 and Group 2 exchange zeolite-X, in which the coupling is decreased by interactions between the aromatic π-electrons and the cation: the effect increasing in the order of decreasing ionic radius, as the electron withdrawing power of the ring is increased by polarization of the π-electrons towards the cation. For the radical derived from benzaldehyde itself, a strong dependence is also found on the solvent in which it is dissolved. It is suggested that this may be used in the study of solvent effects, particularly H-bonding, pertinent to organic and bio-organic media. Electronic substituent effects are also studied in substituted PhCHOMu˙ radicals, and in cyclohexadienyl radicals formed by muonium addition to the aromatic ring, the latter showing evidence of captodative stabilization, when an electron donor substituent is present with the electron accepting CHO group.

Muonium radical formation in elemental sulphur

Abstract The formation of a molecular radical following muon implantation in sulphur S 8 is established experimentally by level crossing resonance. The position of the resonance determines the hyperfine constant as 233 ± 5 MHz. Of various possible assignments evaluated by ab initio molecular orbital calculations, the most promising appears to be the muonic sulfanyl radical, ṠMu.

Surface diffusion of the cyclohexadienyl radical adsorbed on silica and on a silica supported Pd catalyst studied by means of ALC-μSR

Abstract The dynamic behaviour of the cyclohexadienyl radical adsorbed on plain silica and on a silica supported Pd catalyst is investigated by means of the avoided-level-crossing muon spin resonance technique. The influence of benzene coverage and of surface texture on the hyperfine coupling constants, on the signal amplitudes and on the mobility of the radical is studied. Information about surface diffusion is obtained using a theoretical model based on a stochastic Liouville formalism. The radical mobility increases with coverage of the surface with benzene. Arrhenius parameters are discussed within the framework of a hopping model based on transition-state theory for surface diffusion. It is suggested that the dynamics of the surface hydroxyl groups play a crucial role in the diffusion process.

Structure and charge distribution of multiply charged C70

Abstract Spin-restricted Hartree—Fock molecular orbitals calculations have been performed for C n 70 (−6 ⩽ n ⩽ + 6) at a MNDO type semi-empirical level based on the PM3 parameterization. The optimized geometries reveal interesting variations in bond lengths. The neutral molecule and the triplet states of the two ions with n = ± 4 have D 5h symmetry, the other ions have lower symmetry. Significant charge polarization results in molecular electric quadrupole moments. The consequences of structure and of charge distribution with respect to selectivity towards chemical reaction and to packing in the solid state are discussed.

Hydrogen radioisotopic labelling studies using muonium: properties of thiyl radicals potentially relevant to cellular membrane damage

Thiyl radicals (RS·) are formed labelled with muonium atoms (a radioisotope of hydrogen with a positive muon as the nucleus) and detected using the muon spin rotation (MuSR) method. This approach is shown to be extremely effective in the study of thiyl radicals, in non‐aqueous (membrane‐like) environments, from which details of the structures of these reactive free radicals and their reaction kinetics were determined. In this regard, the method is superior to conventional ESR spectroscopy, to which thiyl radicals are undetectable in liquid solution, and to pulse‐radiolysis experiments, which are limited to aqueous media; the rate constants for the reactions of thiyl radicals with lipids but also with antioxidants such as β‐carotene and (SH)glutathione are found to be enhanced in non‐aqueous media. Copyright

Mobility of dichloroethyl radicals sorbed in kaolin and silica: a potential model of heterogeneous atmospheric processes

An activation energy of 10.6 ± 1.4 kJ mol−1 was determined for the reorientational diffusion of 1,1‐dichloroethyl radicals sorbed in silica gel by transverse‐field muon spin rotation (TF‐MuSR) spectroscopy, and was confirmed by longitudinal‐field muon spin relaxation (LF‐MuSRx) measurements, which gave a value of 10.9 ± 0.7 kJ mol−1. Also according to TF‐MuSR measurements, the activation energy for this radical sorbed in kaolin powder is 11.0 ± 1.3 kJ mol−1, which was within experimental error of that obtained from an LF‐MuSRx study (12.3 ± 0.8 kJ mol−1). The LF‐MuSRx method also revealed the presence of an additional, more mobile, fraction of 1,1‐dichloroethyl radicals in both silica and kaolin with activation energies of 2.6 ± 0.4 and 2.4 ± 0.2 kJ mol−1, respectively. Copyright