Measurement of the electric form factor of the neutron at Q2 = 0.3-0.8 (GeV/ c) 2 ⋆

The electric form factor of the neutron, G_E,n, has been measured at the Mainz Microtron by recoil polarimetry in the quasielastic D(¯e, e¯n)p reaction. Three data points have been extracted at squared four-momentum transfers Q ² = 0.3, 0.6 and 0.8 (GeV/c)². Corrections for nuclear binding effects have been applied.This revised version was published online in March 2005. In the previous version, the email address of one author was inadvertently assigned to multiple authors.     

A kinetic study of the decomposition of N-bromoserine

This article reports the kinetics of the decomposition of N-bromoserine formed rapidly by bromation of serine by BrO^?. The main decomposition products are glycolaldehyde, ammonia, carbon dioxide, and bromide ions at pH < 11.5, and β-hydroxypyruvic acid, ammonia, and bromide ions at pH > 11.5. The reaction is of order one with respect to N-bromoserine, and is independent of ionic strength and excess serine. The rate constant increases with increasing pH at pH > 11 and with decreasing pH at pH < 8, and over the range pH 8–11 has the constant value 1.67 × 10^?3 s^?1 at 298 K.     

Beam-helicity asymmetry in photon and pion electroproduction in the Δ(1232)-resonance region at Q2 = 0.35(GeV/c)2

The beam-helicity asymmetry has been measured simultaneously for the reactions p→epγ and p→epπ ⁰ in the Δ(1232)-resonance region at Q ² = 0.35(GeV/c)². The experiment was performed at MAMI with a longitudinally polarized beam and an out-of-plane detection of the proton. The results are compared with calculations based on dispersion relations for virtual Compton scattering and with the MAID model for pion electroproduction. There is an overall good agreement between experiment and theoretical calculations. The remaining discrepancies may be ascribed to an imperfect parametrization of some γ ^(*) N→πN multipoles, mainly contributing to the non-resonant background. The beam-helicity asymmetry in both channels (γ and π ⁰) shows a good sensitivity to these multipoles and should allow future improvement in their parametrization.     

Decomposition kinetics of the N-bromoaminoacids,N-bromoalanine,N-bromo-2-aminobutyric acid,and N-bromonorvaline

A kinetic study of the decomposition reactions of N-bromoalanine, N-bromo-2-aminobutyric acid, and N-bromonorvaline shows them to be first-order with respect to N-bromoaminoacid concentration and independent of both excess aminoacid and pH over the interval pH 9–11. In this pH range the mean rate constants at 298 K were 1.20 × 10^?3 s^?1, 1.37 × 10^?3 s^?1 and 1.28 × 10^?3 s^?1, respectively. © 1993 John Wiley & Sons, Inc.     

A kinetic study of the chlorination of tertiary alcoholamines in alkaline media

The kinetics of the chlorination of various tertiary alcoholamines by hypochlorite have been studied in alkaline media. A reaction mechanism consistent with the experimental results is put forward, and a relationship is established between the rate constants and the polar parameter (Σσ*) of the substituents.     

Synthesis, characterization and solid state structures of thiosemicarbazone palladacycles: Influence of hydrogen bonding in the molecular arrangement

Treatment of the thiosemicarbazones 4-FC₆H₄C(Me)NN(H)C(S)NHR, (R = Me, a; Ph, b) and 2-ClC₆H₄C(Me)NN(H)C(S)NHR (R = Ph, c) with lithium tetrachloropalladate(II) in methanol or palladium(II) acetate in acetic acid gave the tetranuclear cyclometallated complex [Pd{4-FC₆H₃C(Me)NNC(S)NHR}]₄ (1a, 1b) and [Pd{2-ClC₆H₃C(Me)NNC(S)NHPh}]₄ (1c). Reaction of these tetramers with the diphosphines dppe, t-dppe, dppp or dppb in a 1:2 molar ratio gave the dinuclear cyclometallated complexes [(Pd{4-FC₆H₃C(Me)NNC(S)NHR})₂(μ-Ph₂P(CH₂)_nPPh₂)], (n = 2, 2a, 2b; 3, 4a, 4b; 4, 5a, 5b), [(Pd{4-FC₆H₃C(Me)NNC(S)NHPh})₂(μ-Ph₂PCHCHPPh₂)], (3a, 3b) and [(Pd{2-ClC₆H₃C(Me)NNC(S)NHR})₂(μ-Ph₂P(CH₂)_nPPh₂)], (n = 2, 2c, 2d; 3, 4c, 4d; 4, 5c, 5d), [(Pd{2-ClC₆H₃C(Me)NNC(S)NHPh})₂(μ-PPh₂CHCHPPh₂)], (3c, 3d). The X-ray crystal structure of the ligand b and the complexes 3c, 4a and 4d were determined. The structures of complexes 4a and 4d show that the different disposition of the chain cyclometallated of the thiosemicarbazones (in the same orientation or in the opposite one) is due to the different H bonds produced.     

Kinetic study of the formation of N-chloramines

We studied the kinetics of the chlorination of amines by sodium hypochlorite in strongly alkaline aqueous solution. A reaction mechanism compatible with experimental results is proposed and discussed. © 1995 John Wiley & Sons, Inc.     

Kinetics of the N-chlorination of 2-aminobutyric, 3-aminobutyric, 3-aminoisobutyric and 4-aminobutyric acids in aqueous solution

We studied the kinetics of the N-chlorination of 2-aminobutyric, 3-aminoisobutyric, and 4-aminobutyric acids by sodium hypoclorite in strongly alkaline aqueous solution. As in the case of other amines, the rate of formation of the four N-chloroamino acids was proportional to the concentrations of hypochlorite and amino acid, and inversely proportional to the concentration of hydroxyl ions. A reaction mechanism compatible with these results is proposed and discussed. © John Wiley & Sons, Inc.     

Oxidation of Bromide by Tert‐Butyl Hypochlorite

The kinetics of the oxidation reaction of bromide by tert‐butyl hypochlorite (^tBuOCl) was studied at 25°C, ionic strength 0.5 M, and under isolation conditions. A stopped‐flow spectrophotometer was employed for monitoring the reactions. Kinetic studies show that the reaction is first order with respect to [Br^?] and [^tBuOCl]. Linear dependences of the proton concentration, in perchloric acid medium, and the buffer solution concentration were found on the rate constant. The activation parameters were calculated using the Arrhenius and Eyring equations from the kinetic studies performed to analyze the influence of temperature on the rate constant. The results are consistent with a reaction mechanism of general acid catalysis. The catalytic constants were obtained for the oxidation of bromide by tert‐butyl hypochlorite. The slope obtained for the Brönsted relationship was 0.36.     

Surface and bulk properties of two anionic amphiphilic penicillins in a selective solvent

The surface physicochemical properties of two anionic penicillins-cloxacillin and dicloxacillin-in mixed ethanol-water solvent were investigated by surface tension and dynamic light scattering (DLS). The data were analyzed according to the treatment of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to study the stability of the systems. The aim of the study is to obtain information about the effects of ethanol on the surface activity, bulk properties, and aggregate stability of these amphiphilic drugs, keeping in mind that both penicillins have the same counterion, and the difference in their structures is only a Cl atom in the phenyl ring that makes dicloxacillin more hydrophobic. The surface tension data show a minimum area per molecule increment with ethanol concentration that is related to the variation of the dielectric constant with the alcohol. Dicloxacillin has lower values of the standard Gibbs energies of adsorption than does cloxacillin, which gives this drug a more marked escaping tendency from the aqueous environment to the air-water monolayer. DLS data was fitted to an exponential function for cloxacillin at any drug or alcohol concentration in the range of concentrations studied that indicates that the system can be modeled as an ergodic system of dilute diffusing monodisperse particles. Dicloxacillin DLS data at an ethanol concentration of 5% (v/v) had to be fitted at a sum of an exponential and a stretched exponential function, which indicates that, besides the drug aggregates, a small population of penicillin clusters with longer relaxation times is present. The stability curves predicted by the DLVO theory, for both penicillins, indicate the predominance of electrostatic repulsion, leading to a stable system over the drug-ethanol concentration range studied, but the height of the reduced pair interaction potential energy barrier decreases with ethanol concentration, thus it is expected to undergo a transition from a stable dispersion to a coagulated one.