Paul W. Percival

Muon level-crossing spectroscopy of organic free radicals

Abstract Muon level-crossing spectroscopy has been applied to the study of muonium-substituted radicals formed in liquid benzene, hexadeuterobenzene, furan, 2-methylpropene, 2,3-dimethyl-2-butene, and gaseous ethene. The magnitudes and signs of the proton and deuteron hyperfine constants are reported, and are discussed in terms of isotope effects and intramolecular motion.

Hyperfine constants for the ethyl radical in the gas phase

Abstract Muon spin rotation and level-crossing spectroscopy have been used to measure the muon, proton, deuteron and 13C hyperfine coupling constants for the isotopically substituted ethyl radicals CH2CH2Mu, CD2CD2Mu and 13CH213CH2Mu in the gas phase.

Structure and intramolecular motion of muonium-substituted cyclohexadienyl radicals

Abstract Hyperfine coupling constants of isotopically substituted cyclohexadienyl radicals have been measured as a function of temperature by muon spin rotation and level-crossing spectroscopy. Data are presented for the muon, proton and deuteron hyperfine couplings of the methylene groups in C6H6Mu and C6D6Mu, and also for all the 13C hyperfine couplings of 13C6H6Mu. Comparison of the results with semi-empirical calculations supports a planar ring configuration with complex motion of the methylene substituents.

The structure of C60Mu and other fullerenyl radicals

Abstract The structures of the radicals exo-C 60 Mu, endo-C 60 Mu, and (Mu@C 60 ) are discussed, with particular reference to the spin distribution of the unpaired electron in the most stable isomer (exo). The magnitude of the measured muon hyperfine interaction indicates partial spin delocalization in this radical. This is supported by semi-empirical calculations on C 60 H. Calculations also show that there is a significant energy barrier between the endo radical and (Mu@C 60 ), supporting the assignment of the pseudo-free muonium signal to the latter structure.

Diffusion and CIDEP of H and D atoms in solid H2O, D2O and isotopic mixtures☆

Abstract Hydrogen and deuterium atoms have studied by pulsed EPR spectroscopy in polycrystalline samples of H 2 O ice, D 2 O ice, and various isotopic mixtures. At high temperature (−10°C) the pattern and the time-dependence of the EPR line intensities are similar to previous results for H and D in liquid water. Chemically induced dynamic electron polarization (CIDEP) is generated in second-order atom combination reactions. The CIDEP behavior was found to change over the temperature range studied (−5°C to 130°C), consistent with additional contributions from spur reactions, and at lower temperatures, geminate recombination of (D…OD) radical pairs. Transverse spin relaxation times were measured by the spin-echo technique, and interpreted in terms of translational motion of free atoms diffusing through the ice lattice. One surprising result is that D atoms diffuse faster than H atoms below 200 K. This is explained as a vibrational zero point energy effect, by applying transition state theory to a model in which the diffusing atom must pass through a tight “bottleneck” in the electronic potential surface, as it passes from one minimum energy site in the lattice to the next. The H and D spin relaxation rates were successfully simulated by means of a semiclassical potential which was constructed by pairwise addition of atom-atom contributions represented by modified Buckingham potential functions. Extension of the model to include tunneling resulted in little change to the fit of the H and D data. Although predictions of muonium diffusion rates using the same potential do not give quantitative agreement with published results from spin relaxation measurements, they do serve to illustrate the dominant effect of tunneling over a wide temperature range for that light atom.

Intramolecular motion in the tert-butyl radical as studied by muon spin rotation and level-crossing spectroscopy

Abstract Muon spin rotation and muon level-crossing spectroscopy have been used to determine muon ( A μ ) and proton ( A p ) hyperfine coupling constants for the muon-substituted tert-butyl radical (CH 3 ) 2 CCH 2 Mu over a wide range of temperature in isobutene. A p (CH 3 ) is almost constant, but A μ (CH 2 Mu) falls and A p (CH 2 Mu) rises with increasing temperature, consistent with a preferred conformation of the methyl group in which the CMu bond is coplanar with the symmetry axis of the radical orbital. The A μ data cover the temperature range from 297 K down to 43 K, where the solution is frozen. There is a discontinuity in A μ at the melting point, as well as a change in temperature dependence. It is suggested that the potential barrier for methyl group rotation is lower in the liquid due to simultaneous inversion at the radical centre, and that the inversion mode is somewhat inhibited in the solid. The best fit of the liquid-phase data indicates a V 2 barrier of 1.8 kJ mol −1 , and is consistent with a long CMu bond and a tilt of the CH 2 Mu group in the direction that brings the Mu atom closer to the radical centre.

Isotope and temperature effects on the hyperfine interaction of atomic hydrogen in liquid water and in ice

The Fermi contact hyperfine interaction of hydrogen isotopes in liquid and solid water is below the vacuum value, shows a mass dependence, and has a negative temperature coefficient in the liquid. Furthermore, it shows a pronounced solvent isotope effect in H2O/D2O mixtures. This behavior is analyzed in terms of Dalgarno–Lewis perturbation theory for the atom in a spherical parabolic solvent potential and with a phenomenological model for spin delocalization onto solvent molecules. The results support previous suggestions that hydrogen atoms induce clathrate‐like cages in liquid water. These may resemble the static structures of noble gas clathrates except that they are of transient dynamic nature in liquid water.

Free Radical Reactivity of Mono‐ and Dichlorosilylene with Muonium

Silylenes are generally considered highly reactive species, and it was only in 1994 that the first synthesis of an isolable N-heterocyclic silylene (NHSi) 1 (Scheme 1) was accomplished. Other classes of isolable silylenes have since been discovered, spurred by the utility of silylenes as building blocks in organosilane synthesis and as ligands in transition metal complexes. Understanding the reactivity of silylenes is key to both their isolation and their application in synthesis. The chemistry of analogous carbon compounds is a guide, but there are significant differences between carbon and silicon, most obviously evident in the prevalence of long-chain carbon compounds which are the basis of life on this planet. The stability of unsaturated NHSi compounds is a consequence of p-donor stabilisation of the silicon(II) atom by the nitrogen atoms, as well as pseudoaromaticity in the ring. The latter factor is absent from the corresponding saturated NHSi 2 compounds and they are indeed less stable. An additional (kinetic) factor in the reactivity of silylenes is steric protection of the reaction centre by bulky substituents on neighbouring atoms. Thus, silylene 3, synthesised by Kira, is isolable despite the absence of ring nitrogen atoms. We have explored the reactivity of silylenes by studying the free radicals formed by muonium addition. Muonium (Mu) is a single-electron atom, the nucleus of which is the positive muon; it is chemically equivalent to H, but has only one-ninth the mass. At first we could not reconcile the muon hyperfine constants (hfcs) of the radicals formed from silylenes with the predictions of quantum calculations. The explanation lies in a coupling of the primary muonium adduct with a second silylene, so that the disilanyl product radical 4 is observed. This was confirmed in a subsequent experiment on a series of NHSi compounds with different substituents on the nitrogen atoms. The largest substituent (2,6-diisopropylphenyl) served to slow the silyl coupling reaction so that we were able to detect the primary silyl radical 5, as evident from the record high hfc (931 MHz). In a related study we explored the relative reactivity of carbon and silicon atoms in a silene 6, this being the product of trimethylsilyl migration in silylene 3. We now report our investigation of the reactivity of two novel chlorosilylenes, 7 and 8 (Scheme 2). Both offer multiple sites for potential attack by free radicals. The H atom (and by extension, Mu)

Measurement of the 13C hyperfine constants of the cyclohexadienyl radical using muon level-crossing resonance

Abstract The 13 C hyperfine constants of the cyclohexadienyl radical, formed by muonium addition to 13 C-enriched benzene, have been measured using a novel muon level-crossing resonance technique. The constants are compared with a recent ab initio calculation, Karplus-Fraenkel theory, and with ESR results on the 1,2,3,4,5-pentacarboxyl cyclohexadienyl radical.

Spin depolarization in muonium by hydrated electrons

Abstract The missing fraction of muon polarization in water is shown to originate from encounters between muonium and hydrated electrons. This takes place at ≈1 ns after the primary events in which muonium and hydrated electrons are born.