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This paper reports measurements of the temperature dependence of the rate constants for H-atom abstraction reactions from propane and n-butane by the light isotopic H-atom muonium (Mu), kMu(T), over temperatures in the range 300 K to 435 K. Simple Arrhenius fits to these data yield activation energies, EMua, that are some 2–4 times lower than EHa found from corresponding fits for the H + propane and H + n-butane reactions studied elsewhere, both experimentally and theoretically, and fit over a similar temperature range. These activation energies EMua are also much lower than estimated from zero-point-energy corrected vibrationally adiabatic potential barriers, both results suggesting that quantum tunneling plays an important role in determining kMu(T) and for the Mu + propane reaction in particular. The results are expected to pose a considerable challenge to reaction rate theory for isotopic H-atom reactions in alkane systems.

Calculations of kinetic isotope effects (KIEs) provide challenging tests of quantal mass effects on reaction rates, and muonium KIEs are the most challenging. Here, we show that it can be very important to include reaction-coordinate-dependent vibrational anharmonicity along the whole reaction path to calculate tunneling probabilities and KIEs. For the reaction of propane with Mu, this decreases both the height and width of the vibrationally adiabatic potential barrier, with both effects increasing the rate constants. Our results agree well with the experimental observations.

Precise measurements of the muonium (Mu) hyperfine interaction versus temperature are reported in a silica aerogel and mesoporous silica SBA-15 using a fast-timing spectrometer to detect the precession frequencies of Mu in a magnetic field of 1.14 T. The observed signals are a sensitive monitor of dynamics associated with the binding and unbinding of Mu from the silica surface. Above 100 K the Mu is effectively off the surface. Significant differences are observed in the way the lines show motional narrowing, and are attributed to differences in sample morphology. At room temperature the effective mean free path appears longer in SBA-15, suggesting it may offer advantages over aerogel as a source of Mu in vacuum.

We report the reaction of muonium (Mu = [μ+e-]), a light isotopic analog of hydrogen, with uncapped gold nanoparticles embedded in mesoporous silica. Using the radio-frequency muon spin rotation (RF-μSR) technique, we directly observe and characterize the resulting final state on the nanoparticle surface, showing conclusively its diamagnetic nature. The magnetic environment experienced by the reacted muons is only weakly perturbed compared to that of muons in a silica reference, consistent with the surface of the gold nanoparticles being metallic and non-magnetic. We demonstrate the potential of RF-μSR for the investigation of the surface properties of nanoparticles and show the feasibility of Knight shift measurements of muons on metal surfaces.

Gold nanoparticles (AuNPs) have been a subject of considerable interest in recent years due to both their magnetic and catalytic properties. This paper reports a two-fold study of the reactivity at...

The rate constant for the H atom abstraction reaction from methane by the muonic helium atom, Heμ + CH4 → HeμH + CH3, is reported at 500 K and compared with theory, providing an important test of both the potential energy surface (PES) and reaction rate theory for the prototypical polyatomic CH5 reaction system. The theory used to characterize this reaction includes both variational transition-state (CVT/μOMT) theory (VTST) and ring polymer molecular dynamics (RPMD) calculations on a recently developed PES, which are compared as well with earlier calculations on different PESs for the H, D, and Mu + CH4 reactions, the latter, in particular, providing for a variation in atomic mass by a factor of 36. Though rigorous quantum calculations have been carried out for the H + CH4 reaction, these have not yet been extended to the isotopologues of this reaction (in contrast to H3), so it is important to provide tests of less rigorous theories in comparison with kinetic isotope effects measured by experiment. In this regard, the agreement between the VTST and RPMD calculations and experiment for the rate constant of the Heμ + CH4 reaction at 500 K is excellent, within 10% in both cases, which overlaps with experimental error.

Isotopeneffekte konnen die Bildung und den Bruch chemischer Bindungen im Verlauf chemischer Reaktionen masgeblich beeinflussen. Hier berichten wir uber die Entdeckung einer fundamentalen Anderung der Art der chemischen Bindung durch Isotopensubstitution. Sie beruht auf systematischen quantenchemischen Rechnungen fur die Isotopomere BrLBr, wobei “L” ein Wasserstoffisotop bezeichnet. Demnach werden alle vergleichsweise schwereren Isotopomere BrHBr, BrDBr, BrTBr und Br4HBr durch Van-der-Waals-Bindung stabilisiert, wobei 4H das myonische Helium-Atom bezeichnet, also das schwerste Wasserstoffisotop. Demgegenuber ist das leichteste Isotopomer, BrMuBr, ausschlieslich schwingungsgebunden, wobei “Mu” das Myoniumatom bezeichnet; dies ist in Ubereinstimmung mit seiner moglichen Beobachtung im jungsten Experiment zur Mu+Br2-Reaktion. Demzufolge wird BrMuBr am Sattelpunkt der Potentialenergieflache stabilisiert, wobei der Anstieg der Potentialenergie durch Absenkung der Schwingungsnullpunktsenergie uberkompensiert wird.

Isotopic substitution of muonium for hydrogen provides an unparalleled opportunity to deepen our understanding of quantum mass effects on chemical reactions. A recent topical review in this journal of the thermal and vibrationally state-selected reaction of Mu with H2 raises a number of issues that are addressed here. We show that some earlier quantum mechanical calculations of the Mu + H2 reaction, which are highlighted in this review, and which have been used to benchmark approximate methods, are in error by as much as 19% in the low-temperature limit. We demonstrate that an approximate treatment of the Born–Oppenheimer diagonal correction that was used in some recent studies is not valid for treating the vibrationally state-selected reaction. We also discuss why vibrationally adiabatic potentials that neglect bend zero-point energy are not a useful analytical tool for understanding reaction rates, and why vibrationally non-adiabatic transitions cannot be understood by considering tunnelling through vib...

A first complete μSR study of the T dependences of the (reduced) muon, Aμ′(T), and proton, Ap(T), β-hyperfine coupling constants (hfcc) of the muoniated t-butyl radical is reported in the faujasitic zeolites NaY and USY, and the results are compared with similar data and with early EPR results in condensed bulk phases. The results are also compared with single-molecule UMP2 and DFT/B3LYP calculations in the bulk and in an NaY zeolite fragment of Si and O atoms with both OH- and H-capping. Muon hfcc are reported for the first time for the Mu-isobutyl radical in the bulk phase and are also compared with theory and with EPR data. The present results for the muon and proton hfcc of Mu-t-butyl in the bulk complement earlier work published elsewhere at higher temperatures but are extended here down to 5 K to facilitate comparisons with in vacuo theory at 0 K. Good fits to the data for both Aμ′(T) and Ap(T) for Mu-t-butyl are found from the calculated hfcc in both the bulk and in NaY, assuming a Boltzmann-weight...

We report a new advance in the study of muonium (Mu) reactivity; specifically, we report the rate constant for the Mu + H2(vibrational quantum number n = 1) reaction determined by measurements at 300 K and by converged quantum mechanical calculations. Comparisons are made with earlier results for D + H2 (n = 1) and with the corresponding thermal reaction rates. The measurements are a sensitive probe of the high-curvature region in the entrance valley of the potential energy surface (PES) and thus provide a qualitatively different probe of the PES than that provided by any previous experiment.

Temperature-dependent reaction rates have been measured for the reaction of muonic helium with ammonia 4.11 H + NH 3 ↔ 4.11 HH + NH 2 where 4.11 H representsthe muonicHeatom–Hewithone electron replacedbyanegative muon–whichbehaves chemicallyasaveryheavyisotopeofhydrogen,as emphasizedbythe (nonstandard)symbol. The Arrhenius activation energy was measured as 47 ± 17 kJ/mol between 400 and 600 K. The large uncertainty easily overlaps the value of 57 kJ/mol measured for the normalH + NH3 reaction [Ko et al., J. Phys. Chem. 94 (1990) 1401], and also existing calculations. The use of NH 3 as the electron donor in the production of neutral muonic He is discussed in light of the title reaction, and in comparison with alternatives Xe and CH 4 .

The interaction of muoniated cyclohexadienyl radicals with zeolite environments in NaY, HY, and USY has been studied using mainly avoided level crossing muon spin rotation (ALC-μSR) spectroscopy, w...

The hyperfine coupling constants (HFCCs) of all the butyl radicals that can be produced by muonium (Mu) addition to butene isomers (1- and 2-butene and isobutene) have been calculated, to compare with the experimental results for the muon and proton HFFCs for these radicals reported in paper II (Fleming, D. G.; et al. J. Phys. Chem. A 2011, 10.1021/jp109676b) that follows. The equilibrium geometries and HFCCs of these muoniated butyl radicals as well as their unsubstituted isotopomers were treated at both the spin-unrestricted MP2/EPR-III and B3LYP/EPR-III levels of theory. Comparisons with calculations carried out for the EPR-II basis set have also been made. All calculations were carried out in vacuo at 0 K only. A C-Mu bond elongation scheme that lengthens the equilibrium C-H bond by a factor of 1.076, on the basis of recent quantum calculations of the muon HFCCs of the ethyl radical, has been exploited to determine the vibrationally corrected muon HFCCs. The sensitivity of the results to small variations around this scale factor was also investigated. The computational methodology employed was "benchmarked" in comparisons with the ethyl radical, both with higher level calculations and with experiment. For the β-HFCCs of interest, compared to B3LYP, the MP2 calculations agree better with higher level theories and with experiment in the case of the eclipsed C-Mu bond and are generally deemed to be more reliable in predicting the equilibrium conformations and muon HFCCs near 0 K, in the absence of environmental effects. In some cases though, the experimental results in paper II demonstrate that environmental effects enhance the muon HFCC in the solid phase, where much better agreement with the experimental muon HFCCs near 0 K is found from B3LYP than from MP2. This seemingly better level of agreement is probably fortuitous, due to error cancellations in the DFT calculations, which appear to mimic these environmental effects. For the staggered proton HFCCs of the butyl radicals exhibiting no environmental effect in paper II, the best agreement with experiment is consistently found from the B3LYP calculations, in agreement also with benchmark calculations carried out for the ethyl radical.

Stimulated Raman pumping (SRP) is used to produce H 2 in its first vibrational state, in order to measure, for the first time, the Mu + H 2 * ( v = 1 ) → MuH + H reaction rate at room temperature, as a prototypical example of new directions in gas-phase muonium chemistry, utilizing the pulsed muon beam and a new dedicated laser system at the RIKEN/RAL Laboratory. Reported here is a preliminary result but the final results are expected to provide definitive new tests of reaction rate theory on the highly accurate H 3 potential energy surface. The major difficulty in this experiment, compared to the standard SRP process, is to ensure a homogeneous excitation over a volume of several cm 3 and of sufficient intensity to ensure a measurable Mu relaxation rate. The techniques used to accomplish this are described. The experiment utilizes the 2nd harmonic output of a Nd:YAG laser (532 nm) with pulse energies up to 500 mJ at a repetition rate of 25 Hz. Different optical setups have been constructed and tested in order to optimize the number of laser-pumped H 2 molecules and their overlap with the stopping profile of the muon beam in the reaction cell (total volume ∼ 100 × 40 × 4 mm 3 ). The first result of this experiment gives a measured relaxation rate due to laser excitation of λ * = 0.085 ± 0.051 μ s - 1 , consistent with theory but limited by both low statistics and particularly a high background relaxation rate.

The muonic atom He 4 μ has the composition α ++ μ - e - , and is formed by stopping negative muons in He doped with a small amount of NH 3 (or Xe). It may be regarded as a unique heavy H-atom isotope with a mass of 4.1 amu. As such, the study of its chemical reaction rates and comparison with those of the well-known light Mu atom (0.113 amu) allows unprecedented tests of kinetic isotope effects over a range of 36 in mass. As a first example, and one which is of most fundamental interest, we have begun kinetics studies of the He μ + H 2 → He μ H + H reaction in the gas phase. The first measurements, at 295 K, give a rate constant of k He μ = 4.1 ± 0.7 × 10 - 16 cm 3 molec - 1 s - 1 . In comparison, variational transition state calculations give a value of 2.46 × 10 - 16 cm 3 molec - 1 s - 1 , somewhat below the measurement, despite the large error bar, raising the possibility that the calculations, on an essentially exact potential energy surface, have underestimated the amount of quantum tunneling involved, even for this heavy H-atom isotope.

The adsorption and dynamical behavior of the Mu−ethyl radical (MuC2H4) in NaY, HY, and USY faujasites was investigated by the muon spin resonance (μSR) technique, at loadings of one to five ethene molecules per supercage and over a temperature range of ca. 5−500 K (for NaY). The temperature dependences of both the muon and proton hyperfine coupling constants (Hfc's) are reported and compared with similar studies of MuC2H4 in different environments. Both transverse field (TF) μSR and avoided level crossing resonance (ALC) μSR spectra were recorded, with information on molecular motion mainly provided by the ALC line shapes. The muon Hfc's show only a small sensitivity to different frameworks and loadings but exhibit significant (∼10%) shifts at low temperatures, in comparison with bulk values, due to binding of the ethyl radical to cations at SII sites in NaY and to framework hydroxyls in the case of HY(USY). The Δ1 resonances are symmetric and quite broad at the lower temperatures studied, but dramaticall...

The study of kinetic isotope effects for H-atom abstraction rates by incident H-atoms from the homologous series of lower mass alkanes (CH4, C2H6 and, here, C3H8) provides important tests of reaction rate theory on polyatomic systems. With a mass of only 0.114 amu, the most sensitive test is provided by the rates of the Mu atom. Abstraction of H by Mu can be highly endoergic, due to the large zero-point energy shift in the MuH bond formed, which also gives rise to high activation energies from similar zero-point energy corrections at the transition state. Rates are then far too slow near 300 K to be measured by conventional TF-μSR techniques that follow the disappearance of the spin-polarised Mu atom with time. Reported here is the first measurement of a slow Mu reaction rate in the gas phase by the technique of diamagnetic radio frequency (RF) resonance, where the amplitude of the MuH product formed in the Mu + C3H8 reaction is followed with time. The measured rate constant, kMu = (6.8 ± 0.5) × 10(-16) cm(3) s(-1) at 300 K, is surprisingly only about a factor of three slower than that expected for H + C3H8, indicating a dominant contribution from quantum tunneling in the Mu reaction, consistent with elementary transition state theory calculations of the kMu/kH kinetic isotope effect.

The kinetics of the addition reaction of muonium (Mu) to acetylene have been studied in the gas phase at N2 moderator pressures mainly from ∼800 to 1000 Torr and over the temperature range from 168 to 446 K, but also down to 200 Torr at 168 K and over a much higher range of pressures, from 10 to 44 bar at 295 K, demonstrating pressure-independent rate constants, kMu(T). Even at 200 Torr moderator pressure, the kinetics for Mu + C2H2 addition behave as if effectively in the high-pressure limit, giving k∞ = kMu due to depolarization of the muon spin in the MuC2H2 radical formed in the addition step. The rate constants kMu(T) exhibit modest Arrhenius curvature over the range of measured temperatures. Comparisons with data and with calculations for the corresponding H(D) + C2H2 addition reactions reveal a much faster rate for the Mu reaction at the lowest temperatures, by 2 orders of magnitude, in accord with the propensity of Mu to undergo quantum tunneling. Moreover, isotopic atom exchange, which contributes in a major way to the analogous D atom reaction, forming C2HD + H, is expected to be unimportant in the case of Mu addition, a consequence of the much higher zero-point energy and hence weaker C-Mu bond that would form, meaning that the present report of the Mu + C2H2 reaction is effectively the only experimental study of kinetic isotope effects in the high-pressure limit for H-atom addition to acetylene.

In this paper, we report preliminary data for the first direct evidence of a free radical formed from Mu reactivity with Br 2 in the gas phase, in N 2 moderator at a total pressure of 3 bar. A new experimental setup and target vessel for μSR studies of reactive compounds, such as the halogens and hydrogen halides, suitable as well for RF measurements, is described. The experimental data, obtained from a longitudinal field repolarization curve, yields a hfc of 1770 MHz. We tentatively identify this as the [BrMuBr] radical, a non-conventional bond system, arising from the combination of a van der Waals interaction and dynamics on a repulsive surface. Studies of the dynamics and hfcs of possible radicals, which in principal could form, are also outlined here.

Diamagnetic and muonium (Mu) fractions formed in low-pressure inert gases, by energetic muon implantation, have been studied using the technique of time-delayed radio frequency muon spin resonance (RF-μSR). Results obtained establish the validity of the long-held view that formation of these species is due only to prompt processes, and in turn confirms that the diamagnetic environment is due to a muon molecular ion, MMu+, and not a bare μ+. In addition, polarization fractions for the diamagnetic and Mu environments have been determined at different pressures, thereby complementing earlier data, and demonstrating that the RF-μSR technique provides polarization fractions in good accord with those obtained using conventional transverse-field muon spin resonance measurements.

The precession signals of the muoniated ethyl radical have been studied as a function of ethene pressure in pure ethene, ethene/nitrogen mixtures and ethene/helium mixtures. The purpose was to investigate the kinetics and mechanism of the radical formation process. It is possible to fit the signal amplitudes with a model involving a single reaction step—Mu addition to ethene. However, the rate constant deduced from the model fit is significantly higher than the literature value for a thermal reaction, as determined by direct study of Mu decay in low partial pressures of ethene. The conclusion is that at partial pressures above 1 bar the reaction occurs before the incoming (beam) muons are fully thermalized.

Muonium has been directly detected over a range of temperatures and fields by transverse field μSR in different zeolites: 3A, 13X, USY, ZSM-5, and S-115 (a high-silica form of ZSM-5), as well as in silica gel. The polarizations determined from data at 75 and 150 G were independent of both field and temperature. The amounts of Mu seen vary from ∼20% to 40%, with a large missing fraction seen in every case, which may be partly due to slow Mu formation. There is also a fast Mu relaxation rate seen in all samples. This is the first direct observation of Mu in zeolites.

Muon spin rotation/relaxation measurements show clear evidence for magnetism in 2.2 nm gold nanoparticles capped with butanethiol. At low temperatures (1.8 K), there is significant spin relaxation which decreases as a function of both the applied longitudinal magnetic field and increasing temperature. The results indicate that there are spatially inhomogeneous electronic moments that fluctuate with a wide distribution of correlation times. Possible explanations are discussed.

The adsorption and dynamical behavior of the muonated cyclohexadienyl radical (C6H6Mu) in NaY zeolite, formed by muonium (Mu) addition on adsorbed benzene, was investigated by the muon spin resonance (μSR) technique, primarily at loadings of 2−3 C6H6 molecules per supercage of NaY. The dynamics of this radical are expected to be the same as its isotopic analogue, C6H7, for which there are no similar data available. Both TF-μSR and ALC-μSR spectra were recorded, with the most detailed information provided by the positions and line widths of the avoided level crossing resonances. In concert with 2H NMR, neutron diffraction and molecular dynamics studies of the parent benzene molecule, as well as current theoretical calculations, the dominant adsorption site for the C6H6Mu radical is believed to be the SII Na cation, within a supercage, which gives rise to three observed ALC lines, corresponding to two different orientations for the muon (proton) of the CHMu methylene group: pointing toward (endo) and away ...

Isotope effects are important in the making and breaking of chemical bonds in chemical reactivity. Here we report on a new discovery, that isotopic substitution can fundamentally alter the nature of chemical bonding. This is established by systematic, rigorous quantum chemistry calculations of the isotopomers BrLBr, where L is an isotope of hydrogen. All the heavier isotopomers of BrHBr, BrDBr, BrTBr, and Br(4)HBr, the latter indicating the muonic He atom, the heaviest isotope of H, can only be stabilized as van der Waals bound states. In contrast, the lightest isotopomer, BrMuBr, with Mu the muonium atom, alone exhibits vibrational bonding, in accord with its possible observation in a recent experiment on the Mu+Br2 reaction. Accordingly, BrMuBr is stabilized at the saddle point of the potential energy surface due to a net decrease in vibrational zero point energy that overcompensates the increase in potential energy.

The hyperfine coupling constant A of a hydrogen isotope confined in a cage differs from its vacuum value, demonstrating that the atom acts as a probe of its environment. Room-temperature values of A for muonium in different cube-shaped Si8O12 units (T8 units or octasilsesquioxanes), in H2O, and in D2O were determined with an accuracy of about 1 MHz. The results are compared with those obtained from high-resolution ESR on H in H2O, and D in D2O, and in different silsesquioxanes in the temperature range 40–300 K. Both the strong isotope effect and the temperature dependence are well described by a single-oscillator model of Roduner et al. (J. Chem. Phys. 102 (1995) 5989). Striking differences seen between the different silsesquioxanes and between the porous Optipur and bulk Suprasil reflect varying spatial constraints and electronic interactions.

The rate of the addition reaction H+O2→HO2 is measured at high O2 concentrations (up to 40 bar of O2 at room temperature) through its competition with rapid spin exchange, to give k ch 0 =8.7±0.8×10 −33 cm 6 s −1 .

The hyperfine interactions of the MuC6H6 radical in NaY zeolites at low to moderate benzene loadings have been measured by both the FT-μSR and ALC-μSR techniques over a wide temperature range. From a preliminary interpretation of the data, the hyperfine coupling constants for the muon and proton of the – CHMu methylene group in two different orientations for the radical bound to the SII cation site have been determined. At 322 K these values are: A μ (1)=606±2 MHz and A p (1)=108±2 MHz , for the muon on the opposite side of the ring from the cation; and A μ (2)=430±2 MHz and A p (2)=70±5 MHz , for the muon on the same side. These results as well as their trends with temperature demonstrate that the MuC6H6 radical adopts a non-planar equilibrium geometry due to the strong interaction of the π electron density with the Na cation.

Pyridinium tetra#uoroborate is a ferroelectric with a paraelectric}ferroelectric phase transition of second order, which is exceptional for multidirectional ferroelectrics. Avoided-level-crossing muon-spin resonance has been used to show that the Mu substituted aza-cyclohexadienyl radical derived from the pyridinium ions undergoes fast uniaxial rotation around the axis perpendicular to the molecular plane, con"rming earlier results by Czarnecki et al. The changes in reorientational dynamics between 239 and 244 K also verify the existence of a phase transition in this range, as found by Czarnecki. Furthermore, the results give strong evidence that superimposed on the rotation there is a wobbling motion of the rotational axis which freezes in at the "rst transition temperature, „ 1 "238.7 K. ( 2000 Elsevier Science B.V. All rights reserved.

In order to provide an assessment of the “global” accuracy of the Liu–Siegbahn–Truhlar–Horowitz (LSTH) potential surface for H3, hot atom reaction yields, which are determined from collision processes over an energy range much wider than that of single-collision experiments, have been calculated for the Mu*+H2 and T*+H2 systems. The isotopic comparison of muonium (Mu=μ+e−), an ultralight isotope of hydrogen (mMu/mH≈1/9), with the heaviest H-atom isotope, tritium, is a novel approach in testing the global accuracy of the H3 surface. These reaction yields have been calculated using a formalism developed for (μ+) charge exchange, with input cross sections for elastic, inelastic (rovibrational excitation) and reactive collisions determined from quasi classical trajectories on the LSTH surface, in the center-of-mass energy range 0.5–11 eV. The rate of energy loss of the hot atom (Mu* or T*) due to elastic and inelastic collisions with the moderator (H2) drastically affects the hot atom reaction yield. In parti...

The recombination reaction Mu+NO+M⇌MuNO+M (M=He, N2, CH4) has been studied by the muon spin relaxation/rotation (μSR) technique up to 520 bar at room temperature. The reaction remains in the low pressure regime throughout. The measured termolecular addition rate constant in N2, 8.8±0.3×10−33 cm6 s−1, is essentially the same as that found in our earlier study at pressures below 60 bar [J. J. Pan et al., J. Phys. Chem., 1995, 99, 17160]. It is somewhat smaller in He, 7.7±1.0×10−33, but larger in CH4, 12.8±2.0×10−33. The Mu+NO reaction is about five times slower than the corresponding H+NO reaction. The strong collision limits of the rate constants for three H-isotopes (Mu, H, D) reacting with NO have been estimated with Troe's formalism for unimolecular dissociation in the low pressure regime, based on the ab initio potential energy surface of Guadagnini et al. [J. Chem. Phys., 1995, 102, 774]. The Troe calculations give less than satisfactory agreement with experiment with the corresponding weak collision factor, βc , higher than expected by a factor of ∽2 for H+NO. The calculated kinetic isotope effect in the strong collision limit for Mu/H is weaker than the measured effect by a factor of two giving an apparent large isotope effect in this factor, βcMu≈½βcH, possibly due to mode specific collisional stabilization.

The rate constant of the gas-phase addition reaction of the light hydrogen isotope muonium to molecular oxygen, Mu + O2 → MuO2, was measured over a range of temperatures from 115 to 463 K at a pressure of 2 bar and from 16 to 301 bar at room temperature, using N2 as the moderator gas. The reaction remains in the termolecular regime over the entire pressure range. At room temperature, the average low-pressure limiting rate constant is kch0(Mu) = (8.0 ± 2.1) × 10-33 cm6 s-1, a factor of almost 7 below the corresponding rate constant for the H + O2 addition reaction, kch0(H). In contrast to kch0(H), which exhibits a clear negative temperature dependence, kch0(Mu) is essentially temperature independent. At room temperature, the kinetic isotope effect (KIE) is strongly pressure (density) dependent and is reversed at pressures near 300 bar. The kinetics are analyzed based on the statistical adiabatic channel model of Troe using a Morse potential, which works well in reproducing the overall KIE. The major factor...

The thermal reaction Mu + N2O has been studied by the muon spin rotation (μSR) technique at temperatures from 303 to 593 K and pressures up to 60 atm. The overall reaction rate coefficient depends ...

Muon spin rotation (μSR) and avoided level crossing resonance (ALCR) have been used to determine the hyperfine coupling constants (hfcs) of the muonium-substituted cyclohexadienyl radicals C6H6Mu, C6D6Mu and C6F6Mu in the gas phase, at pressures ∼1 and 15 atm and temperatures in the range 40–80°C. Equivalent studies of polyatomic free radicals in gases, by electron spin resonance (ESR) spectroscopy, are generally not possible in this pressure range. The present gas phase results support the findings of earlier studies of cyclohexadienyl radicals in the condensed phase, by both μSR and ESR. Minor but not insignificant (∼1%) effects on the hfcs are observed, which can be qualitatively understood for such nonpolar media in terms of their differing polarizabilities. This is the first time that comparisons of this nature have been possible between different phases at the same temperatures. These μSR/ALCR gas-phase results provide a valuable benchmark for computational studies on radicals, free from possible effects of solvent or matrix environments.

The spin relaxation of Mu was measured in mixtures of CO and Ar at pressures up to 270 atm and at various magnetic fields. The relaxation rate increased with magnetic field in the way expected for electron spin‐exchange processes, though the effect declined at high pressures. We describe the results in terms of spin relaxation of Mu‐formyl radicals, MuCO, which break up to give depolarized Mu at low pressures, but are increasingly stabilized at higher pressures.

ALC μSR spectra of the muonated ethyl and cyclohexadienyl radicals in the gas phase are reported. They have surprisingly narrow lines for a magnetic resonance type technique under conditions near ambient temperature and near 1 atmosphere pressure. The main reason for this behaviour is the dramatic reduction of electron spin relaxation in high magnetic fields.

Muon spin relaxation has been measured in longitudinal magnetic fields for Mu+CO and Mu+N2O reactions. The interpretation of the results for these small molecules, which are quite different than those obtained in larger molecule systems (e.g., Mu‐ethyl and Mu‐t‐butyl radicals), are made with the phenomenological model for Mu‐radical spin relaxation previously proposed. Proper fitting procedures are important in these cases and are discussed in the present paper.

The spin relaxation of the muonium‐substituted ethyl radical (MuCH2ĊH2) and its deuterated analog (MuCD2ĊD2) has been studied in the gas phase in both transverse and longitudinal magnetic fields spanning the range ∼0.5–35 kG, over a pressure range from ∼1–16 atm at ambient temperature. The Mu13CH213ĊH2 radical has also been investigated, at 2.7 atm. For comparison, some data is also reported for the MuCH2Ċ(CH3)2 (Mu‐t‐butyl) radical at a pressure of 2.6 atm. This experiment establishes the importance of the μSR technique in studying spin relaxation phenomena of polyatomic radicals in the gas phase, where equivalent ESR data is sparse or nonexistent. Both T1 (longitudinal) and T2 (transverse) μSR relaxation rates are reported and interpreted with a phenomenological model. Relaxation results from fluctuating terms in the spin Hamiltonian, inducing transitions between the eigenstates assumed from an isotropic hyperfine interaction. Low‐field relaxation is primarily due to the electron, via both the nuclear h...

The kinetics of the addition reaction of O2 to the ethyl radical has been investigated as a function of temperature (259−425 K) and pressure (1.5−60 bar) using the muon spin relaxation technique in longitudinal magnetic fields. Within this temperature range at 1.5 bar, the chemical reaction is represented by an Arrhenius rate law with an activation energy of −4.4(4) kJ mol-1 and an apparent frequency factor of 1.3(2) × 10-12 cm3 molecule-1 s-1. The high-pressure limit of the rate constant at 294 K amounts to = 8.7(8) × 10-12 cm3 molecule-1 s-1. Within error, this limit has been reached at 1.5 bar. The rate coefficient for spin exchange, kex = 2.8(2) × 10-10 cm3 molecule-1 s-1, is collision controlled. The results for kch agree well with experimental literature values, but the temperature dependence is more pronounced than that predicted on the basis of a RRKM extrapolation from low-pressure data. The theoretical basis for the analysis of experimental data is given, and the results are discussed.

Chemisorption of muonium onto the surface of gold nanoparticles has been observed. Muonium (μ+e−), a light hydrogen-like atom, reacts chemically with uncapped 7 nm gold nanoparticles embedded in mesoporous silica (SBA-15) with a strong temperature-dependent rate. The addition rate is fast enough to allow coherent spin transfer into a diamagnetic muon state on the nanoparticle surface. The muon is well established as a sensitive probe of static or slowly fluctuating magnetic fields in bulk matter. These results represent the first muon spin rotation signal on a nanoparticle surface or any metallic surface. Only weak magnetic effects are seen on the surface of these Au nanoparticles consistent with Pauli paramagnetism.

Rates for the gas‐phase thermal reaction Mu+CH4→MuH+CH3 (Mu=μ+e−), have been measured using the μSR (muon spin rotation) technique, over the temperature range 625–820 K. A good fit is obtained to the usual Arrhenius expression, k=A exp(−Ea/RT), giving an activation energy Ea=24.6±0.9 kcal/mol, ∼12 kcal/mol higher than that of the H‐atom isotopic variant of this reaction, H+CH4→H2+CH3. This Ea difference is the largest yet seen at high temperatures between H and Mu in the gas phase, and seems much too high to be explained in terms of [zero‐point‐energy (ZPE)] differences in their respective transition states, indicating instead a dramatic difference in reaction dynamics. The possible sources of this difference include differing reactivities from vibrationally excited states and/or a more favorable tunneling path for the H+CH4 reaction due to its suspected much earlier (and thinner) reaction barrier. In contrast, the similar H‐atom abstraction reactions with H2 and C2H6 gave Ea differences which matched expectations based on ZPE shifts, suggesting a qualitative difference in dynamics between these otherwise homologous reactions. It is suggested that reaction from vibrationally excited states may be relatively more important in the case of the Mu+CH4 reaction.

Muonium (Mu=μ + +e - ) is the bound state of a positive muon and an electron. Since the positive muon has a mass about 1/9 of the proton, Mu can be regarded as an ultra light isotope of hydrogen with unusually large mass ratios (Mu:H:D:T=1/9:1:2:3). The muon spin rotation technique (μSR) relies on the facts that (1) the muon produced in pion decay, π + →μ + +ν μ , is 100% spin polarized and (2) the positron from muon decay is emitted preferentially along the instantaneous muon spin direction at the time of the muon decay. In transverse field μSR (TF-μSR), the precession of the muon spin in muonium substituted radicals is directly observed by detecting decay positrons time differentially. From observed radical frequencies, the hyperfine coupling constants (A μ ) of C 2 H 4 Mu, C 2 D 4 Mu, 13 C 2 H 4 Mu, C 2 F 4 Mu, and C 2 H 3 FMu are determined. In the longitudinal field avoided level crossing (LF-ALC) technique, one observes the resonant loss of the muon spin polarization caused by the crossing of hyperfine levels at particular magnetic fields. The LF-ALC method together with the information on A μ obtained from TF-μSR allows one to determine the magnitude and sign of the nuclear hyperfine constants at α- and β-positions. Results are compared with hydrogen substituted ethyl-radicals and isotope effects are discussed

New evidence is presented for the observation of a muoniated radical in the Mu + Br(2) system, from μSR longitudinal field (LF) repolarisation studies in the gas phase, at Br(2) concentrations of 0.1 bar in a Br(2)/N(2) mixture at 300 K and at 10 bar total pressure. The LF repolarisation curve, up to a field of 4.5 kG, reveals two paramagnetic components, one for the Mu atom, formed promptly during the slowing-down process of the positive muon, with a known Mu hyperfine coupling constant (hfcc) of 4463 MHz, and one for a muoniated radical formed by fast Mu addition. From model fits to the Br(2)/N(2) data, the radical component is found to have an unusually high muon hfcc, assessed to be ∼3300 MHz with an overall error due to systematics expected to exceed 10%. This high muon hfcc is taken as evidence for the observation of either the Br-Mu-Br radical, and hence of vibrational bonding in this H[combining low line]-L[combining low line]-H[combining low line] system, or of a MuBr(2) van der Waals complex formed in the entrance channel. Preliminary ab initio electronic structure calculations suggest the latter is more likely but fully rigorous calculations of the effect of dynamics on the hfcc for either system have yet to be carried out.

Near-infrared chemiluminescent emission from NeMu*, the analogue of the Rydberg molecule NeH, has been observed in Ne, Ar, and Ne/Ar gas mixtures. Three temporally distinct features were observed: First, a large sharp emission peak at time zero, observed in all gases (Ne, He, N2, Ar), is assigned to scintillation light during muon thermalization, probably caused by spur electrons. Second, a lowintensity broad region observed in all gases is attributed to e+ from muon decay. Finally, NeMu in 1–6 atm Ne with 0.1–2 torr Ar appeared as a high intensitydelayed emission, whose width and intensity depended linearly on the Ar concentration. Its wavelength spectrum from 680–960 nm was measured. Although questions remain as to how NeMu* is formed, the precursor is likely Neμ +. Possible electron donors include metastable Ar* (3 P 2 or3 P 0) and long-lived free (spur) electrons.

The kinetics of the chemical reaction and Heisenberg spin exchange of the ethyl radical with oxygen in the gas phase is investigated using the time differentialμSR technique in longitudinal magnetic fields. At a total pressure of 1.5 bar and a temperature of 311 K, rate constants ofk1=(9.2±0.6)·10−12 cm3 molecules−1 s−1 for chemical reaction andkinex=(4.2±2.3)·10−10 cm3 molecules−1 s−1 for spin exchange are obtained.