Showing papers on "Proton published in 2001"

Theoretical Perspectives on Proton-Coupled Electron Transfer Reactions

Abstract: This Account presents a theoretical formulation for proton-coupled electron transfer reactions. The active electrons and transferring protons are treated quantum mechanically, and the free energy surfaces are obtained as functions of collective solvent coordinates corresponding to the proton and electron transfer reactions. Rate expressions have been derived in the relevant limits, and methodology for including the dynamical effects of the solvent and protein has been developed. This theoretical framework allows predictions of rates, mechanisms, and kinetic isotope effects for proton-coupled electron transfer reactions.

Mechanisms of Proton Conductance in Polymer Electrolyte Membranes

Abstract: We provide a phenomenological description of proton conductance in polymer electrolyte membranes, based on contemporary views of proton transfer processes in condensed media and a model for heterogeneous polymer electrolyte membrane structure. The description combines the proton transfer events in a single pore with the total pore-network performance and, thereby, relates structural and kinetic characteristics of the membrane. The theory addresses specific experimentally studied issues such as the effect of the density of proton localization sites (equivalent weight) of the membrane material and the water content of the pores. The effect of the average distance between the sulfonate groups, which changes during membrane swelling, is analyzed in particular, and the factors which determine the temperature dependence of the macroscopic membrane conductance are disclosed. Numerical estimates of the specific membrane conductivity obtained from the theory agree very well with typical experimental data, thereby confirming the appropriateness of the theoretical concepts. Moreover, the versatility of the models offers a useful and transparent frame for combining the analysis of both experimental data and the results of molecular dynamics simulations.

Neutron density distributions deduced from antiprotonic atoms.

Abstract: The differences between neutron and proton density distributions at large nuclear radii in stable nuclei were determined. Two experimental methods were applied: nuclear spectroscopy analysis of the antiproton annihilation residues one mass unit lighter than the target mass and the measurements of strong-interaction effects on antiprotonic x rays. Assuming the validity of two-parameter Fermi neutron and proton distributions at these large radii, the conclusions are that the two experiments are consistent with each other and that for neutron rich nuclei it is mostly the neutron diffuseness which increases and not the half-density radius. The obtained neutron and proton rms radii differences are in agreement with previous results.

Single-voxel proton MRS of the human brain at 1.5T and 3.0T

Abstract: Single-voxel proton spectra of the human brain were recorded in five subjects at both 1.5T and 3.0T using the STEAM pulse sequence. Data acquisition parameters were closely matched between the two field strengths. Spectra were recorded in the white matter of the centrum semiovale and in phantoms. Spectra were compared in terms of resolution and signal-to-noise ratio (SNR), and transverse relaxation times (T(2)) were estimated at both field strengths. Spectra at 3T demonstrated a 20% improvement in sensitivity compared to 1.5T at short echo times (TE = 20 msec), which was lower than the theoretical 100% improvement. Spectra at long echo times (TE = 272 msec) exhibited similar SNR at both field strengths. T(2) relaxation times were almost twofold shorter at the higher field strength. Spectra in phantoms demonstrated significantly improved resolution at 3T compared to 1.5T, but resolution improvements in in vivo spectra were almost completely offset by increased linewidths at higher field.

First O-H-N Hydrogen Bond with a Centered Proton Obtained by Thermally Induced Proton Migration.

Abstract: Within a range of 0.1 A, the H atom in the O-H-N hydrogen bond of the adduct 4-methylpyridine⋅pentachlorophenol could be shifted by a simple adjustment of temperature. At approximately 90 K the H atom is exactly centered between the O and the N atoms, as could be shown by stepwise monitoring by using variable-temperature single-crystal neutron diffraction.

Proton-Controlled Electron Injection from Molecular Excited States to the Empty States in Nanocrystalline TiO2

Abstract: The excited-state and redox properties of Ru(deeb)(bpy)2(PF6)2, Ru(dcb-H2)(bpy)2(PF6)2, Ru(bpy)2(ina)2(PF6)2, and Ru(dpbp)(bpy)2(PF6)2, where bpy is 2,2‘-bipyridine, deeb is 4,4‘-(CO2Et)2-bpy, dcb-H2 is 4,4‘-(CO2H)2-bpy, dpbp is 4,4‘-(PO(OEt)2)2-bpy, and ina is isonicotinic acid, bound to nanocrystalline TiO2 and colloidal ZrO2 films have been studied in acetonitrile at room temperature as a function of the interfacial proton concentration. High surface proton concentrations favor a “carboxylic acid” type linkage(s) where low proton concentrations favor “carboxylate” type binding mode(s) for Ru(II) compounds with ethyl ester or carboxylic acid functional groups. The “carboxylic acid” linkages are unstable when Lewis acids such as Li+ are present in acetonitrile, while desorption is absent for the carboxylate binding under the same conditions. The kinetics for binding are faster when the interfacial proton concentration is high; however, the saturation surface coverage is about 1/3 lower than for base-pret...

Laser generation of proton beams for the production of short-lived positron emitting radioisotopes

Abstract: Protons of energies up to 37 MeV have been generated when ultra-intense lasers (up to 10 20 W cm −2 ) interact with hydrogen containing solid targets. These protons can be used to induce nuclear reactions in secondary targets to produce β+-emitting nuclei of relevance to the nuclear medicine community, namely 11C and 13N via (p, n) and (p,α) reactions. Activities of the order of 200 kBq have been measured from a single laser pulse interacting with a thin solid target. The possibility of using ultra-intense lasers to produce commercial amounts of short-lived positron emitting sources for positron emission tomography (PET) is discussed.

Generalized parton distributions with helicity flip

Abstract: We show that for both quarks and gluons there are eight generalized parton distributions in the proton: four which conserve parton helicity and four which do not. We explain why time reversal invariance does not reduce this number from eight to six, as previously assumed in the literature.

Improved algorithm for corner-cutting tunneling calculations

Abstract: We present an improved version of the large-curvature tunneling method that more accurately treats the anharmonic potentials encountered along low-energy corner-cutting tunneling paths. The method is illustrated by applications to the reaction CH3+H2→CH4+H and to the double proton transfer in formamidine hydrate.

Measurement and QCD analysis of jet cross sections in deep-inelastic positron-proton collisions at $\sqrt{s}$ of 300 GeV

Abstract: Jet production is studied in the Breit frame in deep-inelastic positron-proton scattering over a large range of four-momentum transfers 5 Q^2 15000 GeV^2 and transverse jet energies 7 E_T 60 GeV. The analysis is based on data corresponding to an integrated luminosity of L_int \simeq 33 pb^(-1) taken in the years 1995-1997 with the H1 detector at HERA at a center-of-mass energy sqrt(s)=300 GeV. Dijet and inclusive jet cross sections are measured multi-differentially using k_perp and angular ordered jet algorithms. The results are compared to the predictions of perturbative QCD calculations in next-to-leading order in the strong coupling constant alphas.QCD fits are performed in which alphas and the gluon density in the proton are determined separately. The gluon density is found to be in good agreement with results obtained in other analyses using data from different processes. The strong coupling constant is determined to be alphas(MZ)=0.1186+-0.0059. In addition an analysis of the data in which both alphas and the gluon density are determined simultaneously is presented.

Proton Mobility in Chabazite, Faujasite, and ZSM-5 Zeolite Catalysts. Comparison Based on ab Initio Calculations

Abstract: Ab inito predictions of the proton-transfer reaction rates in chabazite, faujasite, and ZSM-5 zeolites are presented. The reaction studied, a proton jump between neighboring oxygen atoms of the AlO4 tetrahedron, defines proton mobility in an unloaded catalyst. Classical transition state theory is applied, and the potential energy surface is described by the QM-Pot method. The latter combines a quantum mechanical description of the reaction site with an interatomic potential function description of the periodic zeolite lattice. At room temperature, the calculated rates vary over a broad range of 10-6 to 105 s-1, depending on the zeolite type and the particular proton jump path within a given zeolite. Proton tunneling effects appear to be negligible above room temperature. The calculated reaction barriers vary between 52 and 106 kJ/mol. While in all three zeolites both low and high barriers exist, the special structural features of the zeolite frameworks allow the prediction that the proton mobility is gene...

Proton temperature anisotropy constraint in the solar wind: ACE observations

Abstract: The electromagnetic proton cyclotron anisotropy instability may arise in collisionless plasmas in which the proton velocity distribution is approximately bi-Maxwellian with T⊥ p /T ||p > 1, where ⊥ and || denote directions relative to the background magnetic field B o . Theory and simulations predict that enhanced field fluctuations from this instability impose a constraint on proton temperature anisotropies of the form T⊥ p /T ||p - 1 = S p /β ||p αp where β ||p ≡ 8πn p k B T ||p /B o 2 , and the fitting parameters S p ≤ 1 and α p ≃ 0.4. Plasma and magnetic field observations from the ACE spacecraft reported here show for the first time that this constraint is statistically satisfied in the high speed solar wind near 1 AU, and magnetic power spectra provide evidence that this instability is the source of the constraint.

Low-energy Compton scattering and the polarizabilities of the proton

Abstract: Differential cross-sections for Compton scattering from the proton have been measured at the MAMI tagged photon facility using the TAPS setup. The data cover an angular range of θlab γ = 59°-155° and photon energies ranging from 55 MeV to 165 MeV. Our results are in good agreement with those from previous experiments, but yield higher precision. Using dispersion relations the proton polarizabilities have been determined to be = [11.9±0.5stat.±1.3syst.±0.3mod.] . 10-4 fm 3 and = [1.2±0.7stat.±0.3syst.±0.4mod.)] . 10-4 fm 3. These results confirm the Baldin sum rule which was re-evaluated to be + = [13.8±0.4] . 10-4 fm 3. We can also conclude that there is no significant additional asymptotic contribution to the backward spin polarizability γπ beyond the t-channel π0-exchange.

Intruder states in very neutron-deficient Hg, Pb and Po nuclei

Abstract: The gas-filled recoil separator RITU, combined with Ge detector arrays at the Department of Physics, University of Jyvaskyla (JYFL), Finland, has successfully been employed in recoil-decay-tagging (RDT) experiments for probing structures of several very neutron-deficient heavy nuclei for the first time. In this paper new data for Hg, Pb and Po nuclei are summarized and discussed. These data shed new light on the behaviour and role of the proton multiparticle-multihole intruder states in nuclei at the 82

NMR Diffusion, Relaxation, and Spectroscopic Studies of Water Soluble, Monolayer-Protected Gold Nanoclusters†

Abstract: NMR spectroscopy and computer modeling were used to characterize tiopronin monolayer-protected gold clusters (MPCs). These MPCs contain gold cores with a distribution of radii ranging from 0.4 to 2.6 nm. NOESY and HMQC spectra yielded assignments for all NMR sensitive nuclei in the tiopronin ligands. DOSY and T2 experiments provided information about the particle size distribution as a function of proton frequency shift. Further information was obtained from hole-burning and amide-exchange experiments. The spectroscopic data reveal two classes of ligands, a network of hydrogen bonds, and considerable inhomogeneous and homogeneous line broadening. The methyl and methine protons clearly exhibit two components with separations that decrease strongly with the number of bonds separating the proton from the gold core. Spin−echo experiments clearly show that a range of T2 values is associated with each resonance frequency in both the upfield and downfield components for each type of proton but that the most prob...

How Protons Shatter Colored Glass

Abstract: We consider the implications of the Color Glass Condensate for the central region of p+A collisions. We compute the k⊥ distribution of radiated gluons and their rapidity distribution dN/dy analytically, both in the perturbative regime and in the region between the two saturation momenta. We find an analytic expression for the number of produced gluons which is valid when the saturation momentum of the proton is much less than that of the nucleus. We discuss the scaling of the produced multiplicity with A. We show that the slope of the rapidity density dN/dy provides an experimental measure for the renormalization-group evolution of the color-charge density of the Color Glass Condensate (CGC). We also argue that these results are easily generalized to collisions of nuclei of different A at central rapidity, or with the same A but at a rapidity far from the central region.

Internal mixing and surface abundance of [WC]-CSPN

Abstract: Recent advances in constructing stellar evolution models of hydrogen-deficient post-asymptotic giant branch (AGB) stars are presented. Hydrogen-deficiency can originate from mixing and subsequent convective burning of protons in the deeper layers during a thermal pulse on the post-AGB (VLTP). Dredge-up alone may also be responsible for hydrogen-deficiency of post-AGB stars. Models of the last thermal pulse on the AGB with very small envelope masses have shown efficient third dredge-up. The hydrogen content of the envelope is diluted sufficiently to produce H-deficient post-AGB stars (AFTP). Moreover, dredge-up alone may also cause H-deficiency during the Born-again phase (LTP). During the second AGB phase a convective envelope develops. A previously unknown lithium enrichment at the surface of Born-again stellar models may be used to distinguish between objects with different post-AGB evolution. The observed abundance ratios of C, O and He can be reproduced by all scenarios if an AGB starting model with inclusion of overshoot is used for the post-AGB model sequence. An appendix is devoted to the numerical methods for models of proton capture nucleosynthesis in the He-flash convection zone during a thermal pulse.

Baryon rapidity loss in relativistic Au + Au collisions

Abstract: An excitation function of proton rapidity distributions for different centralities is reported from AGS Experiment E917 for Au+Au collisions at 6, 8, and 108GeV/nucleon The rapidity distributions from peripheral collisions have a valley at midrapidity which smoothly change to distributions that display a broad peak at midrapidity for central collisions The mean rapidity loss increases with increasing beam energy, whereas the fraction of protons consistent with isotropic emission from a stationary source at midrapidity decreases with increasing beam energy The data suggest that the stopping is substantially less than complete at these energies