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131.
The synthesis of functionalized β-lactam-substituted, tricyclic chromenoisoxazolidine and tetracyclic naphthopyranoisoxazolidine derivatives by intramolecular nitrone cycloaddition reaction is described. The O-allyl hydroxyaldehyde derivatives, obtained from Baylis–Hillman adducts (BHA) derived from β-lactam aldehyde underwent intramolecular cycloaddition reaction to give β-lactam-substituted, polycyclic isoxazolidine derivatives in good yield. All the products were confirmed by spectral analysis. The products with a β-lactam substituent may find good applications in biological chemistry.  相似文献   
132.
Structures have been built at micro scales with unique failure mechanisms that are not yet understood, in particular, under high-rate loading conditions. Consequently, microelectromechanical systems (MEMS) devices can suffer from inconsistent performance and insufficient reliability. This research aims to understand the failure mechanisms in micro-scaled specimens deforming at high rates. Single-crystal silicon (SCS) micro specimens that are 4 μm thick are subjected to tensile loading at an average strain rate of 92 s?1 using a miniature Hopkinson tension bar. A capacitance displacement system and piezoelectric load cell are incorporated to directly measure the strain and stress of the silicon micro specimens. The average dynamic elastic modulus of the silicon micro specimens is measured to be 226.8?±?18.50 GPa and the average dynamic tensile strength of the silicon is measured to be 1.26?±?0.310 GPa. High-speed images show that extensive fragmentation of the specimens occurs during tensile failure.  相似文献   
133.
The proton N.M.R. lineshape of polycrystalline Langbeinite, (NH4)2Cd2(SO4)3, has been studied in the temperature range 300 K to 1·8 K. The resonance line is motionally narrowed over the entire temperature range, and the low temperature proton line shows clear evidence for tunnelling motion of the ammonium ion between spin-symmetry states. From a computer simulation of the lineshape, we obtain an estimate for the tunnelling splitting parameter, J, of the torsional ground state of the ammonium ion, as 375 ± 125 gauss. For an undistorted tetrahedral crystal field this corresponds to a tunnelling splitting Δ = 4J = 6·3 ± 2·1 MHz.

Pulsed proton N.M.R. studies have also been carried out on the above compound at 30·8 MHz and 48·2 MHz and the spin-lattice relaxation time (T 1) has been measured by the π - t - π/2 pulse sequence as a function of temperature down to 77 K. At 30·8 MHz, a T 1 minimum of 13 ms occurs at 105·8 K, and is ascribed to random reorientations of the NH4 + ion. An activational energy barrier of 0·74 ± 0·1 kcal/mole and an associated pre-exponential factor of 8·0 × 10-13 s are calculated for the above motional process, and the value of the activation energy is correlated with the tunnelling splitting of the torsional ground state.

An anomaly in T 1 has been observed at the ferroelectric Curie point (95 K), indicating the order-disorder nature of the transition. This is the first experimental evidence relating to the nature of the transition in Langbeinite.  相似文献   
134.
135.
We investigate single-file osmosis of water through a semipermeable membrane with an uncharged, a positively and a negatively charged nanopore. Molecular dynamics simulations indicate that the osmotic flux through a negatively charged pore (J_) is higher compared to the osmotic flux in a positively charged pore (J+) followed by the osmotic flux in the uncharged pore (J(0)), i.e., J_ > J+ > J(0). The molecular mechanisms governing osmosis, steady state osmosis, and the observed osmotic flux dependence on the nanopore charge are explained by computing all the molecular interactions involved and identifying the molecular interactions that play an important role during and after osmosis. This study helps in a fundamental understanding of osmosis and in the design of advanced nanoporous membranes for various applications of osmosis.  相似文献   
136.
In trying to manipulate the intensity distribution of a focused field, one typically uses amplitude or phase masks. Here we explore an approach, namely, varying the state of spatial coherence of the incident field. We experimentally demonstrate that the focusing of a Bessel-correlated beam produces an intensity minimum at the geometric focus rather than a maximum. By varying the spatial coherence width of the field, which can be achieved by merely changing the size of an iris, it is possible to change this minimum into a maximum in a continuous manner. This method can be used, for example, in novel optical trapping schemes, to selectively manipulate particles with either a low or high index of refraction.  相似文献   
137.

Abstract  

Click chemistry was used to synthesize a series of biaryl-based bis(1,2,3-triazoles). Their antifungal activity was evaluated against three soil-borne plant pathogenic fungi, viz. Rhizoctonia bataticola, Sclerotium rolfsii, and Fusarium oxysporum, using the food poison technique at concentrations of 62.5–500 μg/cm3.  相似文献   
138.
Explicitly time-dependent density functional theory (TDDFT) is a formally exact theory, which can treat very large systems. However, in practice it is used almost exclusively in the adiabatic approximation and with standard ground state functionals. Therefore, if combined with coherent control theory, it is not clear which control tasks can be achieved reliably, and how this depends on the functionals. In this paper, we continue earlier work in order to establish rules that answer these questions. Specifically, we look at the creation of wave packets by ultrashort laser pulses that contain several excited states. We find that (i) adiabatic TDDFT only works if the system is not driven too far from the ground state, (ii) the permanent dipole moments involved should not differ too much, and (iii) these results are independent of the functional used. Additionally, we find an artifact that produces fluence-dependent excitation energies.  相似文献   
139.
The Jiles-Atherton (JA) theory of hysteresis has been extended in the present paper to model hysteresis in two-phase magnetic materials. Two-phase materials are those that exhibit two magnetic phases in one hysteresis cycle: one at lower fields and the other at higher fields. In magnetic hysteresis, the transition from one phase to the other i.e. low field phase to high field phase depends mainly on the exchange field. Hence, the material-dependent microstructural parameters of JA theory: spontaneous magnetization, MS, pinning factor, k, domain density, a, domain coupling, α, and reversibility factor, c, are represented as functions of the exchange field. Several cases based on this model have been discussed and compared with the measured data from existing literature. The shapes of the calculated and measured hysteresis loops are in excellent agreement.  相似文献   
140.
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