Percival, Paul W. | DOOR - DOuglas Open Repository

Person Preferred Name

Paul W. Percival

Related Works

Level crossing resonance due to chlorine nuclei in a free radical

Digital Document

Muonium adds to allyl chloride, CH2=CHCH2Cl, to form two radicals: MuCH2CHCH2Cl (main product) and CH2CHMuCH2Cl (minor product). Both radicals were fully characterized byμSR andμLCR. In the main product, the LCR lines due to the35Cl and37Cl nuclei were observed. Also, the temperature dependence of various hyperfine coupling constants (hfc) indicates that both Mu and Cl eclipse the unpaired electronp 2-orbital in the minimum energy conformation. For the fragment-CH2Cl, the presence of Mu in theβ′-position is found to affect significantly the hfc of Cl in theβ-position; an internal rotational barrier of 12 kJ mol−1 was estimated using a simpleV 2 torsional potential.

Conformational studies of thiyl and selenenyl radicals

Digital Document

Muonium adds to the unsaturated bond in 1,3-dithiolane-2-thione or -2-selenone to form a chalcogen-centered radical. The muon hyperfine coupling constants (hfc) were measured for both compounds over a range of temperatures. In solution, the temperature dependence of the muon hfc indicates that for the thiyl radical the preferred conformation corresponds to the muon eclipsing the unpaired electronp z-orbital, while there is practically free rotation in the case of the selenenyl radical in the temperature range studied. In pure thione the hfc values show a remarkable discontinuity at the melting point. The two radical signals seen in the solid are interpreted as due to the presence of two dominant crystal orientations in the samples studied. In pure selenone data were obtained only in the solid. For both systems, the solid phase results show that interaction between the muon and the unpaired electron spin is enhanced compared to the liquid phase and/or the solution.

Muon investigations of fullerenyl radicals

Digital Document

Four of the five possible isomeric C70Mu radicals have been detected by transversefieldμSR in a C70 powder sample at 298 K. Their assignment is based on the results of semi-empirical MNDO calculations. There are significant changes in intensity and lineshape of the signals at low temperature. The first μSR spectrum of a fullerenyl radical in solution has been obtained.

Detection of an α-Muonium-substituted methyl radical

Digital Document

Reported here is the first observation of anα-Muonium-substituted methyl radical. The key to the success of this experiment was the use of a more stable precursor, trimethylsilyldiazomethane, in lieu of the prototypical diazomethane. The radical, CHMuSi(CH3)3 was observed by TF-μSR, with Aμ=187.6(2) MHz. This result is compared to literature values for the proton analogue and the isotope effect is discussed.

Digital Document

13C hyperfine coupling constants of the MuC60 radical have been measured by muon level-crossing spectroscopy, using a dilute solution of 99% enriched 13C60 in decalin. The signs as well as the magnitudes of the hyperfine constants were determined. The results range from 52.6 to −25.4 MHz and support those calculations which predict an extended distribution of unpaired electron spin density in radical adducts of fullerenes. The hyperfine constants are consistent with published electron spin resonance results for (CD3)3CC60, but contradict a recent report for HC60, where a considerably smaller value is reported for the largest splitting. [ABSTRACT FROM AUTHOR]

Zero frequency resonance: Another way to measure muon-electron hyperfine constants

Digital Document

At a magnetic field determined by the zero crossing of the ω12 frequency of a paramagnetic species like a muonium-substituted radical some muons experience a total magnetic field of zero, simply because the hyperfine field exactly cancels the externally applied field. Since the muon polarization does not rotate under this condition, an integral positron asymmetry is seen even in transverse field if the resonance condition is met. Essentially, the same data acquisition set-up and high beam currents as used in other integral μSR techniques, such as ALCR, can be used to scan for the resonant field, which scales linearly with the hyperfine constant, Aμ. A theoretical treatment of the resonances in the case of isotropic Aμ is presented along with measured spectra demonstrating the technique.

Hyperfine coupling constants of muonium in sub and supercritical water

Digital Document

Muonium, like the hydrogen atom, is a hydrophobic solute in water under standard conditions. Molecular dynamics simulations suggest that the free atom exists in a transient clathrate-like cage of hydrogen-bonded water molecules. The hyperfine constants of Mu and H are very close to their vacuum values, supporting the picture of an atom “rattling” around in a hole in the liquid. Muonium has now been studied in water over a wide range of temperatures and pressures, from standard conditions to over 400°C and 400 bar (the critical point is at 374°C, 221 bar). Drastic changes occur in the properties of water over this range of conditions, so large changes in the muonium hyperfine constant might well be expected. Surprisingly, the changes are small. The hyperfine coupling constant goes through a minimum in the subcritical region, and then increases toward the vacuum value under supercritical conditions. [ABSTRACT FROM PUBLISHER]

Digital Document

Muoniated free radicals have been detected in muon-irradiated aqueous solutions at high temperatures and pressures. Results are presented for the cyclohexadienyl radical, formed by muonium addition to benzene, and for tert-butyl, formed by reaction of muonium with isobutene, itself formed in situ from the dehydration of the starting material, tert-butanol. This is the first report of the direct identification of organic free radicals in near critical and supercritical water.

Digital Document

Muonium is long-lived in pure water and has been studied over a very wide range of temperatures and pressures, from 5°C to over 400°C and from 1 to 400bar. We have determined rate constants for representative reactions of muonium in aqueous solution; equivalent data on H atom kinetics is sparse and stops well short of the maximum temperature and pressure attained in our experiments. The results show remarkable deviations from the predictions of standard reaction theories. In particular, rate constants pass through a maximum with temperature well below the critical point. This seems to be a general phenomenon, since we have observed it for spin-exchange and chemical reactions that are diffusion limited at low temperatures, as well as for activated reactions. We believe that a key factor in the drop of rate constants at high temperature is the cage effect, in particular the number of collisions between a pair of reactants over the duration of their encounter. Whatever the reason, the implications are profound for both the efficiency of supercritical water oxidation reactors and for the modelling of radiation chemistry in pressurized water nuclear reactors.

Digital Document

Muoniated free radicals have been detected in muon-irradiated aqueous solutions of acetone at high temperatures and pressures. At temperatures below 250 °C, the radical product is consistent with muonium addition to the keto form of acetone. However, at higher temperatures, a different radical was detected, which is attributed to muonium addition to the enol form. Muon hyperfine coupling constants have been determined for both radicals over a wide range of temperatures, significantly extending the range of conditions under which these radicals and the keto−enol equilibrium have been studied.

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