Solute diffusion in swollen membranes VII. Diffusion in semicrystalline networks

A model is developed to express the solute diffusion coefficient through semicrystalline polymeric networks. The crystallites create impermeable diffusional barriers around the amorphous regions. Solute diffusion is determined by applying a transport model to the amorphous phase and incorporating the crosslinked polymer structure characteristics. This model is tested with theophylline and vitamin B₁₂ permeation experiments through semicrystalline poly(vinyl alcohol) membranes prepared by annealing of amorphous PVA membranes. The degree of crystallinity varies between 23.1 % and 40.5 % on a dry basis. The solute diffusion coefficients correlate well with various parameters of the model.     

Synthesis and reactivity of a series of [14]azaannulenes

Using a metal template reaction, a series of [14]azaannulenes formed from a β-diketone and an aromatic diamine in the presence of nickel(II) have been synthesized. Attempted cyclizations with bulky β-diketones and certain substituted diamines showed that the cyclization is sensitive to steric effects. At least one of the nickel complexes readily reacts with a series of nucleophilic reagents to give methine-substituted products.     

Application and assessment of laser Doppler velocimetry for underwater acoustic measurements

The majority of traditional methods for making underwater acoustic pressure measurements involve placing all or part of a measurement transducer in the acoustic field. A variety of optical metrology techniques have been developed in an attempt to reduce or remove any perturbing effects. An example of this is the use of laser interferometry which has been developed as the primary method of calibrating hydrophones in the frequency range - at the National Physical Laboratory (NPL). This technique involves suspending a thin Mylar pellicle in the acoustic field and recording the displacement of the pellicle surface using a Michelson Interferometer. This study details a comparison of a Laser Doppler Velocimeter (LDV) with the NPL Laser Interferometer, which gives a good correlation where agreement is within approximately 4% and 7% for two different power levels from a plane piston transducer and within 2.5% and 1% for the same power levels from a plane piston transducer. A novel, non-perturbing method of recording temporally resolved acoustic pressure distributions in water using an LDV is also described. The technique is shown to benefit from the consistent frequency response of the LDV detection system, such that the measured output resembles the drive voltage input to the transducer more closely than a similar hydrophone measurement. For the experimental arrangement described, the LDV system is shown to be sensitive to minimum pressure amplitudes of nominally .     

Wasseranalyse

Ohne Zusammenfassung     

Naphthole und Naphtholderivate

Analytical and Bioanalytical Chemistry -     

Path ensembles and path sampling in nonequilibrium stochastic systems

Markovian models based on the stochastic master equation are often encountered in single molecule dynamics, reaction networks, and nonequilibrium problems in chemistry, physics, and biology. An efficient and convenient method to simulate these systems is the kinetic Monte Carlo algorithm which generates continuous-time stochastic trajectories. We discuss an alternative simulation method based on sampling of stochastic paths. Utilizing known probabilities of stochastic paths, it is possible to apply Metropolis Monte Carlo in path space to generate a desired ensemble of stochastic paths. The method is a generalization of the path sampling idea to stochastic dynamics, and is especially suited for the analysis of rare paths which are not often produced in the standard kinetic Monte Carlo procedure. Two generic examples are presented to illustrate the methodology.     

Dynamics of periodic monopoles

BPS monopoles which are periodic in one of the spatial directions correspond, via a generalized Nahm transform, to solutions of the Hitchin equations on a cylinder. A one-parameter family of solutions of these equations, representing a geodesic in the 2-monopole moduli space, is constructed numerically. It corresponds to a slow-motion dynamical evolution, in which two parallel monopole chains collide and scatter at right angles.     

Electron transfer from sodium to oriented nitromethane, CH3NO2: probing the spatial extent of unoccupied orbitals

Beams of sodium atoms with energies of a few eV are crossed with a beam of oriented CH3NO2 molecules to study the effect of collision energy and orientation on electron transfer. The electron transfer produces Na+ ions and free electrons, parent negative ions (CH)NO2-), and fragmentation ions NO2- and O- in proportions that depend on the collision energy. The steric asymmetry is very small or zero and suggests that production of all of the ions is favored by sideways attack with respect to the permanent dipole along the C-N axis. In these experiments, the electron appears to be transferred into the 2B1 state of the anion comprising mainly the pi*NO LUMO, producing a valence-bound state rather than a dipole-bound state.     

Dramatic steric behavior in electron transfer from various donors to CF3Br

Different alkali metal atoms are observed to donate electrons to CF(3)Br molecules that are oriented in space. For collision energies high enough to overcome the Coulomb attraction, a positive ion/negative ion pair is observed and mass-analyzed using coincident time-of-flight mass spectroscopy. The alkali metal cation and various negative ions are observed. The most abundant negative ion is the bromide ion, Br(-), formed preferentially by attack at the Br end of the molecule. The steric asymmetry to produce Br(-) is almost identical for all of the alkali metal donors. Fluoride ions are formed in smaller abundance and reflect completely different behavior among the donors. Sodium and potassium have high thresholds and prefer the CF(3) end of the molecule, whereas cesium prefers the Br end of the molecule. Sodium and potassium apparently interact with the transient CF(3)Br(-) molecular negative ion while it is in the process of decomposing.