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91.
Direct numerical simulations are used to examine the locking of quantized superfluid vortices and normal fluid vorticity in evolving turbulent flows. The superfluid is driven by the normal fluid, which undergoes either a decaying Taylor-Green flow or a linearly forced homogeneous isotropic turbulent flow, although the back reaction of the superfluid on the normal fluid flow is omitted. Using correlation functions and wavelet transforms, we present numerical and visual evidence for vortex locking on length scales above the intervortex spacing.  相似文献   
92.
93.
Robust, selective, and stable in the presence of ethylene, ruthenium olefin metathesis pre-catalyst, {[3-benzyl-1-(10-phenyl-9-phenanthryl)]-2-imidazolidinylidene}dichloro(o-isopropoxyphenylmethylene)ruthenium(II), Ru-3 , bearing an unsymetrical N-heterocyclic carbene (uNHC) ligand, has been synthesized. The initiation rate of Ru-3 was examined by ring-closing metathesis and cross-metathesis reactions with a broad spectrum of olefins, showing an unprecendented selectivity. It was also tested in industrially relevant ethenolysis reactions of olefinic substrates from renewable feedstock with very good yields and selectivities.  相似文献   
94.
A series of 4-alkoxybenzoic acids 1[m,n] and their methyl esters 2[m,n] containing a partially fluorinated alkyl chain was prepared and their physical properties were investigated by optical, thermal and powder X-ray diffraction (XRD) methods. The former exhibits an SmC phase, while the latter form an SmA phase. XRD analysis indicated liquid-like character of the linking alkyl chains, and showed a decreasing layer thermal expansion coefficient from positive to negative upon increasing the degree of fluorination. The new esters 2[m,n] expanded the series and permitted analysis of trends in thermal stability of the SmA phase. The results demonstrate that the SmA–I transition temperature increases by about +20 K per CF2CF2 unit, and is destabilised by –5.6 K upon extending the hydrocarbon part by each CH2CH2 group. Data for the expanded series 2[m,n] were used for comparative analysis of mesogenic behaviour in two other series of derivatives of 1[m,n]. Synthetic methods for 1[m,n] and 2[m,n] are reviewed.  相似文献   
95.
Exciton and charge delocalization across π‐stacked assemblies is of importance in biological systems and functional polymeric materials. To examine the requirements for exciton and hole stabilization, cofacial bifluorene ( F 2) torsionomers were designed, synthesized, and characterized: unhindered (model) Me F 2, sterically hindered tBu F 2, and cyclophane‐like C F 2, where fluorenes are locked in a perfect sandwich orientation via two methylene linkers. This set of bichromophores with varied torsional rigidity and orbital overlap shows that exciton stabilization requires a perfect sandwich‐like arrangement, as seen by strong excimeric‐like emission only in C F 2 and Me F 2. In contrast, hole delocalization is less geometrically restrictive and occurs even in sterically hindered tBu F 2, as judged by 160 mV hole stabilization and a near‐IR band in the spectrum of its cation radical. These findings underscore the diverse requirements for charge and energy delocalization across π‐stacked assemblies.  相似文献   
96.
The γγ′ Co-based superalloys are newly developed class of refractory alloys which may replace commercial Ni-based superalloys owing to their favorable properties at high temperature. In case of new Co-based superalloys, the heat treatment aims to obtain microstructure composed of appropriate volume fraction of small cuboidal γ′-Co3(Al,W) precipitates within the γ-Co matrix. However, due to a high tendency to interdendritic segregations of alloying elements, the alloys based on Co–Al–W system should be normally homogenized before further steps of heat treatment (solutionizing and aging). In this study, thermal analysis was applied for determination of temperature range for primary heat treatment of the Co–9Al–9W (at.%). The differential thermal analysis (DTA) measurements were carried out on the thermal analyzer NETZSCH STA 449 F3 Jupiter. On the base of obtained results, respectively, solvus of γ′ phase and solidus temperatures were determined, as well as the thermal range of Co3W (DO19) phase precipitation. As a consequence, the heat treatment without homogenizing (only solution and aging) was proposed as a most suitable way to obtain beneficial microstructure.  相似文献   
97.
Jung ME  Allen DA 《Organic letters》2008,10(10):2039-2041
Silyl triflate-promoted rearrangement of cis-2,3-epoxycycloalkanols A, prepared by epoxidation of the cyclic allylic alcohol and then silylation, afforded good yields ( approximately 70-75%) of the cis-2-alkyl-3-silyloxycycloalkanones B, presumably via the intermediates C and D, even with quite large alpha-substituents, e.g., tert-butyl. Finally, it has been shown that the stereochemistry of the epoxy alcohol is crucial as one would expect from the mechanism.  相似文献   
98.
99.
We present results of molecular dynamics (MD) computer simulations of hexane (C6H14) adlayers physisorbed onto a graphite substrate for coverages in the range 0.5 < or = rho < or = 1 monolayers. The hexane molecules are simulated with explicit hydrogens, and the graphite substrate is modeled as an all-atom structure having six graphene layers. At coverages above about rho congruent with 0.9 the low-temperature herringbone solid loses its orientational order at T(1) = 140 +/- 3 K. At rho = 0.878, the system presents vacancy patches and T(1) decreases to ca. 100 K. As coverage decreases further, the vacancy patches become larger and by rho = 0.614 the solid is a connected network of randomly oriented islands and there is no global herringbone order-disorder transition. In all cases we observe a weak nematic mespohase. The melting temperature for our explicit-hydrogen model is T(2) = 160 +/- 3 K and falls to ca. 145 K by rho = 0.614 (somewhat lower than seen in experiment). The dynamics seen in the fully atomistic model agree well with experiment, as the molecules remain overall flat on the substrate in the solid phase and do not show anomalous tilting behavior at any phase transition observed in earlier simulations in the unified atom (UA) approximation. Energetics and structural parameters also are more reasonable and, collectively, the results from the simulations in this work demonstrate that the explicit-hydrogen model of hexane is substantially more realistic than the UA approximation.  相似文献   
100.
The substitution of a deuterium for a hydrogen is known to perturb the NMR chemical shift of a neighboring hydrogen atom. The magnitude of such a perturbation may depend on the specifics of bonding and stereochemical relationships within a molecule. For deuterium-labeled cyclohexanes held in a chair conformation at -80 degrees C or lower, all four possible perturbations of H by D as H-C-C-H is changed to D-C-C-H have been determined experimentally, and the variations seen, ranging from 6.9 to 10.4 ppb, have been calculated from theory and computational methods. The predominant physical origins of the NMR chemical shift perturbations in deuterium-labeled cyclohexanes have been identified and quantified. The trends defined by the Delta delta perturbation values obtained through spectroscopic experiments and by theory agree satisfactorily. They do not match the variations typically observed in vicinal J(H-H) coupling constants as a function of dihedral angles.  相似文献   
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