Validity limits of the superposition principle when applied to reactions the kinetics of which is diffusion controlled

The superposition principle which is commonly used to solve the Smoluchowsky equation can be proved to be unreliable by elementary calculations applied to a particular example. But the practical kinetic consequences are small and usually hidden whithin experimental errors.

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Ion adsorption at the rutile-water interface: linking molecular and macroscopic properties

A comprehensive picture of the interface between aqueous solutions and the (110) surface of rutile (alpha-TiO2) is being developed by combining molecular-scale and macroscopic approaches, including experimental measurements, quantum calculations, molecular simulations, and Gouy-Chapman-Stern models. In situ X-ray reflectivity and X-ray standing-wave measurements are used to define the atomic arrangement of adsorbed ions, the coordination of interfacial water molecules, and substrate surface termination and structure. Ab initio calculations and molecular dynamics simulations, validated through direct comparison with the X-ray results, are used to predict ion distributions not measured experimentally. Potentiometric titration and ion adsorption results for rutile powders having predominant (110) surface expression provide macroscopic constraints of electrical double layer (EDL) properties (e.g., proton release) which are evaluated by comparison with a three-layer EDL model including surface oxygen proton affinities calculated using ab initio bond lengths and partial charges. These results allow a direct correlation of the three-dimensional, crystallographically controlled arrangements of various species (H2O, Na+, Rb+, Ca2+, Sr2+, Zn2+, Y3+, Nd3+) with macroscopic observables (H+ release, metal uptake, zeta potential) and thermodynamic/electrostatic constraints. All cations are found to be adsorbed as "inner sphere" species bonded directly to surface oxygen atoms, while the specific binding geometries and reaction stoichiometries are dependent on ionic radius. Ternary surface complexes of sorbed cations with electrolyte anions are not observed. Finally, surface oxygen proton affinities computed using the MUSIC model are improved by incorporation of ab initio bond lengths and hydrogen bonding information derived from MD simulations. This multitechnique and multiscale approach demonstrates the compatibility of bond-valence models of surface oxygen proton affinities and Stern-based models of the EDL structure, with the actual molecular interfacial distributions observed experimentally, revealing new insight into EDL properties including specific binding sites and hydration states of sorbed ions, interfacial solvent properties (structure, diffusivity, dielectric constant), surface protonation and hydrolysis, and the effect of solution ionic strength.     

Search for massive photon pair production at the CERN intersecting storage rings

A search for massive photon pair production at \(\sqrt s = 63 GeV\) has been carried out on the data sample previously employed for the electron pair production study. Positive evidence is reported form _γγ>6GeV, with a production cross-section similar to Drell-Yan electron pairs. The ratio γγ/π⁰π⁰ was measured to be ～10^?3 for ap _T of each γ or γ or π⁰ above 3 GeV/c.     

The dissociation quotients of formic acid in sodium chloride solutions to 200°C

The dissociation quotients of formic acid were measured potentiometrically from 25 to 200°C in NaCl solutions at ionic strengths of 0.1, 0.3 1.0, 3.0, and 5.0 mol-kg^–1. The experiments were carried out in a concentration cell with hydrogen electrodes. The resulting molal acid dissociation quotients for formic acid, as well as a set of infinite dilution literature values and a calorimetrically-determined enthalpy of reaction, were fitted by an empirical equation involving an extended Debye Hückel term and seven adjustable parameters involving functions of temperature and ionic strength. This regressional analysis yielded the following thermodynamic quantities for 25°C: logK=–3.755±0.002, H^o=–0.09±0.15 kJ-mol^–1, S^o=–72.2±0.5 J-K^–1-mol^–1, and C _p ^o =–147±4 J-K^–1-mol^–1. The isocoulombic form of the equilibrium constant is recommended for extrapolation to higher temperatures.     

Alkyl modified anionic siloxanes as pseudostationary phases for electrokinetic chromatography. I. Synthesis and characterization

Anionic water soluble siloxane polymers have been synthesized and characterized for electrokinetic chromatography. Siloxane polymers are of interest in electrokinetic chromatography because of the wide variety of chemistries that can be developed based on these backbones, including much of the stationary phase chemistry developed in the last 30 years. The siloxanes in this study have a sulfonate functional group. The siloxanes have different length alkyl chains (C8, C12, C18) attached to the backbone in differing densities. The methylene selectivity generally increases with increasing alkyl chain length and with increasing alkyl chain density. The electrophoretic mobility appears to pass through a maximum as more alkyl chain is added to the siloxane backbone. The efficiency also would seem to pass through a maximum as more alkyl chain is added. The chemical selectivities of the siloxane polymers are very different from sodium dodecyl sulfate but are similar to each other.     

Synthetic entries to 6-fluoro-7-substituted indole derivatives

Three practical synthetic entries of functionalized 6-fluoro-7-substituted indole derivatives were developed in connection with the preparation of 7-fluoro-8-substituted-1,3,4,9-tetrahydropyrano[3,4-b]indole-1-acetic acid derivatives 11 . The first route, which permits group modification about position 8 of the pyranoindole skeleton, employs 2-bromo-3-fluoroaniline ( 18 ) as a key intermediate, the preparation of which was achieved by either a novel ortho metalation of 15 or via the intermediacy of 22 . The second route utilizes 32 to append a terminally functionalized three carbon side chain onto the indole template and in addition leads to 43 from 40 . The third route to the 7-fluoro-8-substituted-pyranoindole skeleton complements route two in that the synthetic pathway exploits 32 in a nucleophilic fashion to construct a terminally functionalized two carbon appendage onto the indole nucleus.     

Subnanosecond high-pressure iodine photodissociation laser oscillator

A simultaneously Q-switched and acousto-optic mode-locked high-pressure iodine photodissociation (=1.315m) oscillator system, capable of generating pulse-widths as short as 160 ps is described. Good agreement is found between theory and experiment for the pressure-dependence of the modelocked pulse-width. The results indicate that pulse-widths approaching 100ps should be obtainable with this type of system.     

Combined Chemical Modification and Collision Induced Unfolding Using Native Ion Mobility-Mass Spectrometry Provides Insights into Protein Gas-Phase Structure

Native mass spectrometry is now an important tool in structural biology. Thus, the nature of higher protein structure in the vacuum of the mass spectrometer is an area of significant interest. One of the major goals in the study of gas-phase protein structure is to elucidate the stabilising role of interactions at the level of individual amino acid residues. A strategy combining protein chemical modification together with collision induced unfolding (CIU) was developed and employed to probe the structure of compact protein ions produced by native electrospray ionisation. Tractable chemical modification was used to alter the properties of amino acid residues, and ion mobility-mass spectrometry (IM-MS) utilised to monitor the extent of unfolding as a function of modification. From these data the importance of specific intramolecular interactions for the stability of compact gas-phase protein structure can be inferred. Using this approach, and aided by molecular dynamics simulations, an important stabilising interaction between K6 and H68 in the protein ubiquitin was identified, as was a contact between the N-terminus and E22 in a ubiquitin binding protein UBA2.     

Some limit theorems for negatively associated random variables

Let {X _n , n ≥ 1} be a sequence of negatively associated random variables. The aim of this paper is to establish some limit theorems of negatively associated sequence, which include the L ^p -convergence theorem and Marcinkiewicz–Zygmund strong law of large numbers. Furthermore, we consider the strong law of sums of order statistics, which are sampled from negatively associated random variables.