Low-temperature neutron scattering spectra of diammonium dodecahydro-closo-dodecaborate [(NH(4))(2)B(12)H(12)] reveal two NH(4)(+) rotational tunneling peaks (e.g., 18.5 μeV and 37 μeV at 4 K), consistent with the tetrahedral symmetry and environment of the cations. The tunneling peaks persist between 4 K and 40 K. An estimate was made for the tunnel splitting of the first NH(4)(+) librational state from a fit of the observed ground-state tunnel splitting as a function of temperature. At temperatures of 50 K-70 K, classical neutron quasi-elastic scattering appears to dominate the spectra and is attributed to NH(4)(+) cation jump reorientation about the four C(3) axes defined by the N-H bonds. A reorientational activation energy of 8.1 ± 0.6 meV (0.79 ± 0.06 kJ/mol) is determined from the behavior of the quasi-elastic linewidths in this temperature regime. This activation energy is in accord with a change in NH(4)(+) dynamical behavior above 70 K. A low-temperature inelastic neutron scattering feature at 7.8 meV is assigned to a NH(4)(+) librational mode. At increased temperatures, this feature drops in intensity, having shifted entirely to higher energies by 200 K, suggesting the onset of quasi-free NH(4)(+) rotation. This is consistent with neutron-diffraction-based model refinements, which derive very large thermal ellipsoids for the ammonium-ion hydrogen atoms at room temperature in the direction of reorientation. 相似文献
We studied the behaviour of Li/Li(4)Ti(5)O(12) cells by electrochemical impedance spectroscopy to gain insight into the changes at the electrode/electrolyte interfaces during extensive cycling. A simple equivalent-circuit model is able to describe the impedance of the complete battery as a function of both state-of-charge and state-of-degradation. The formation of the solid-electrolyte interface and dendrite growth at the Li metal electrode have a strong influence on the impedance measurements although the battery performance is not significantly affected. 相似文献
We numerically investigate the post-pulse alignment of rotationally excited diatomic molecules upon nonresonant interaction with a linearly polarized laser pulse. In addition to the simulations, we develop a simple model which qualitatively describes the shape and amplitude of post-pulse alignment induced by a laser pulse of moderate power density. In our treatment we take into account that molecules in rotationally excited states can interact with a laser pulse not only by absorbing energy but also by stimulated emission. The extent to which these processes are present in the interaction depends, on the one hand, on the directionality of the molecular angular momentum (given by the M quantum number), and on the other hand on the ratio of transition frequencies and pulse duration (determined by the J number). A rotational wave packet created by a strong pulse from an initially pure state contains a broad range of rotational levels, over which the character of the interaction can change from non-adiabatic to adiabatic. Depending on the laser pulse duration and amplitude, the transition from the non-adiabatic to the adiabatic limit proceeds through a region with dominant rotational heating, or alignment, for short pulses and a large region with rotational cooling, and correspondingly preferred anti-alignment, for longer pulses. 相似文献
Contamination with arsenic is a recurring problem in both industrialized and developing countries. Drinking water supplies for large populations can have concentrations much higher than the permissible levels (for most European countries and the United States, 10 μg As per L; elsewhere, 50 μg As per L). Arsenic analysis requires high-end instruments, which are largely unavailable in developing countries. Bioassays based on genetically engineered bacteria have been proposed as suitable alternatives but such tests would profit from better standardization and direct incorporation into sensing devices. The goal of this work was to develop and test microfluidic devices in which bacterial bioreporters could be embedded, exposed and reporter signals detected, as a further step towards a complete miniaturized bacterial biosensor. The signal element in the biosensor is a nonpathogenic laboratory strain of Escherichia coli, which produces a variant of the green fluorescent protein after contact to arsenite and arsenate. E. coli bioreporter cells were encapsulated in agarose beads and incorporated into a microfluidic device where they were captured in 500 × 500 μm(2) cages and exposed to aqueous samples containing arsenic. Cell-beads frozen at -20 °C in the microfluidic chip retained inducibility for up to a month and arsenic samples with 10 or 50 μg L(-1) could be reproducibly discriminated from the blank. In the 0-50 μg L(-1) range and with an exposure time of 200 minutes, the rate of signal increase was linearly proportional to the arsenic concentration. The time needed to reliably and reproducibly detect a concentration of 50 μg L(-1) was 75-120 minutes, and 120-180 minutes for a concentration of 10 μg L(-1). 相似文献
A numerical stochastic model of the high-resolution time series of the wind chill index is considered. The model is constructed under the assumption that time series of the wind chill index are non-stationary non-Gaussian random processes with time-dependent one-dimensional distributions. This assumption makes possible to take into account both daily and seasonal variations of real meteorological processes. Data of the long-term real observations at weather stations were used for estimating the model parameters and for the verification of the model. Based on the simulated trajectories, some statistical properties of rare and adverse weather events, like long periods of time with a low wind chill index, are studied. The model is also used to study the dependence of the statistical properties of the wind chill index time series on a climate change.
Tetrahydrocarbazoles have been prepared in one-flask syntheses from indoles, ketones or aldehydes, and maleimides, with acid catalysis. The reactions involve a condensation of the indole with the ketone or aldehyde, followed by an in situ trapping of the vinylindole in a Diels-Alder addition with a maleimide. Isomerization of the double bond into the indole nucleus gave the tetrahydrocarbazoles which were isolated ( 6, 9 , and 10 ). Variation of the indole, carbonyl compound, and maleimide has been explored. The predominant stereochemistry of the tetrahydro ring in the products is all-cis, although a second stereoisomer has been isolated. Two regioisomers were generated from all unsymmetrical 2-alkanones, except 2-butanone, which gave the single isomer 9a . Aromatization of tetrahydrocarbazoles 6 to carbazoles 7 was accomplished with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. 相似文献
In the paper we present the results of experimental modeling the millimeter wave scattering from elements and fragments of a vegetation canopy. The dependence of amplitude and phase distributions of scattered field in near zone, of back-scattering cross section, and of scattered field distribution in Frenel zone for a fragment of wheat shoots upon the moisture and shape of an individual element is determined. 相似文献
Summary. The role of the enzyme source was studied in the reaction of hydrogen peroxide decomposition by immobilised catalase in acetonitrile. Enzymes isolated from bacterial and mammalian sources were conveniently immobilised on a spectroscopic graphite to obtain an organic-phase enzyme electrode (OPEE). Amperometry at constant potential was employed as basic analytical approach in this study. 相似文献
[reaction: see text] A novel application of photochromic molecules is to mimic physiological oscillatory calcium signals by reversibly binding and releasing calcium ions in response to light. Substituent changes on the largely unexplored photochromic bisbenzospiropyran scaffold led to significant changes in thermal fading rates in several organic solvents. Excellent correlations have been found between fading rates and empirical Hammett constants as well as calculated ground-state energies. These correlations can be used to improve scaffold design. 相似文献