Mee Shelley

The thermal reaction rate of muonium with methane (and ethane) in the gas phase

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 exp...

Hyperfine coupling constants of muonium-substituted cyclohexadienyl radicals in the gas phase: C6H6Mu, C6D6Mu, C6F6Mu

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.

Spin relaxation of muonium‐substituted ethyl radicals (MuCH2ĊH2) in the gas phase

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...

Spin relaxation of muonated radicals in the gas phase

We report on recent results obtained for longitudinal field (T1) spin relaxation of the muonium-substituted (“muonated”) free radicals MuCO, MuC2F4, MuC2H3F, and MuC4H8 (t-butyl), comparing with results reported earlier for MuC2H4 (and MuC2D4). Some comparison with transverse field (T2) data is also given. These data are fit to a phenomenological model based on NMR theory of spin relaxation in gases. The parameters of these fits are presented and discussed.

Reorientation dynamics of cyclohexadienyl radicals in zeolites

The dynamics of the muonium substituted cyclohexadienyl radical adsorbed on silicalite and NaZSM‐5 is investigated by means of avoided level crossing muon spin resonance. The influence of benzene loading on the mobility of the radical is studied. At low loadings the radicals were found to be located on a single adsorption site where they undergo a wobbling type of motion. With increasing loading an additional species adsorbed on a different site is observed.

Melittin Aggregation in Aqueous Solutions: Insight from Molecular Dynamics Simulations

Melittin is a natural peptide that aggregates in aqueous solutions with paradigmatic monomer-to-tetramer and coil-to-helix transitions. Since little is known about the molecular mechanisms of melittin aggregation in solution, we simulated its self-aggregation process under various conditions. After confirming the stability of a melittin tetramer in solution, we observed—for the first time in atomistic detail—that four separated melittin monomers aggregate into a tetramer. Our simulated dependence of melittin aggregation on peptide concentration, temperature, and ionic strength is in good agreement with prior experiments. We propose that melittin mainly self-aggregates via a mechanism involving the sequential addition of monomers, which is supported by both qualitative and quantitative evidence obtained from unbiased and metadynamics simulations. Moreover, by combining computer simulations and a theory of the electrical double layer, we provide evidence to suggest why melittin aggregation in solution likely stops at the tetramer, rather than forming higher-order oligomers. Overall, our study not only explains prior experimental results at the molecular level but also provides quantitative mechanistic information that may guide the engineering of melittin for higher efficacy and safety.

Transverse field dependence of muonium relaxation: Spin exchange and chemical reaction

It has been shown experimentally that the muonium relaxation due to spin exchange is 1.5 times faster in intermediate transverse fields (say, atB=50 G, where the so-called two-frequency muonium signal is observed) than in low fields (say, atB=5 G), in agreement with an earlier theoretical prediction. It has also been confirmed experimentally that this distinct field dependence is totally absent in the case of chemical reaction.

Rate of abstraction of hydrogen atoms from ethane by muonium

Thermal reaction rates for the gas-phase reaction Mu+C2H6→MuH+C2H5 have been measured byμSR over the temperature range 510–730 K. The usual Arrhenius expression,k=Aexp(−Ea/RT), fits the data well, giving parametersA=1.0×10−9 cm3 molecule−1 s−1 andEa=15.35 kcal/mol. The activation energyEa is 5.5 kcal/mol higher than for the H atom variant of this reaction, indicating a marked difference in reaction dynamics. Preliminary analysis indicates a still greater difference between Mu and H for the corresponding CH4 reaction.

Muonium spin relaxation in carbon monoxide

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.

μSR studies of free radicals in the gas phase

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+ → ∶+ + υ∶, 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 (Aμ) of C2H4Mu, C2D4Mu,13C2H4Mu, C2F4Mu, and C2H3FMu 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 onAμ 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.