Surface growth models may give rise to instabilities with mound formation whose typical linear size L increases with time (coarsening process). In one dimensional systems coarsening is generally driven by an attractive interaction
between domain walls or kinks. This picture applies to growth models for which the largest surface slope remains constant
in time (corresponding to model B of dynamics): coarsening is known to be logarithmic in the absence of noise ( L(t) ∼ ln t) and to follow a power law ( L(t) ∼t1/3) when noise is present. If the surface slope increases indefinitely, the deterministic equation looks like a modified Cahn-Hilliard
equation: here we study the late stages of coarsening through a linear stability analysis of the stationary periodic configurations
and through a direct numerical integration. Analytical and numerical results agree with regard to the conclusion that steepening
of mounds makes deterministic coarsening faster : if α is the exponent describing the steepening of the maximal slope M of mounds ( Mα∼L) we find that L(t) ∼tn: n is equal to for 1≤α≤2 and it decreases from to for α≥2, according to n = α/(5α - 2). On the other side, the numerical solution of the corresponding stochastic equation clearly shows that in the
presence of shot noise steepening of mounds makes coarsening slower than in model B: L(t) ∼t1/4, irrespectively of α. Finally, the presence of a symmetry breaking term is shown not to modify the coarsening law of model
α = 1, both in the absence and in the presence of noise.
Received 28 September 2001 and Received in final form 21 November 2001 相似文献
The introduction of a formyl group at the anomeric center of 2,3,5-tri-O-benzyl furanoses and substitution of the ring oxygen with a basic nitrogen atom (aminohomologation) was carried out via stereoselective addition of 2-lithiothiazole to N-benzyl, N-furanosylhydroxylamines (masked N-benzyl sugar nitrones), followed by reductive dehydroxylation of the resulting open-chain adducts, and then ring closure via intramolecular displacement of the free hydroxy group by the amino group and unmasking of the formyl group from the thiazole ring. The resulting formyl aza-C-glycosides were transformed into 2,5-dideoxy-2,5-imino-hexitols (pyrrolidine homoazasugars) by reduction of the formyl to the hydroxymethyl group and removal of the O- and N-benzyl groups by hydrogenolysis. This reaction sequence was applied to four furanoses (D-arabino, D-ribo, D-lyxo, L-xylo) to give the hydroxy- and amino-free homoazasugars, including the natural product 2,5-dideoxy-2,5-imino-D-mannitol, in 17% overall yields (six steps). The formyl aza-C-glycosides proved to be valuable intermediates for the synthesis of more complex derivatives. In fact, these sugar aldehydes were employed in Wittig-type coupling reactions with galactose and ribose phosphoranes to give bis-glycosylated alkenes, which upon reduction of the double bond were transformed into methylene isosteres of (1-->6)- and (1-->5)-linked disaccharides in which one of the two sugar moieties was an azasugar (aza-(1-->x)-C-disaccharides). 相似文献
We report a new theoretical study of the mechanism of the parent Staudinger reaction between ketene and formaldimine. This mechanism, in contrast with previously reported computational studies, involves the formation of two different intermediates, gauche and trans, which can be interconnected on the potential energy surface depending upon reaction conditions. This novel CASPT2/CASSCF mechanistic scheme may be considered a new starting point to rationalize stereochemical outcome and solvent effects in these reactions. 相似文献
The rate of axial water exchange in well-defined series of lanthanide complexes depends on the extent of second sphere hydration which is determined by complex hydrophobicity and the nature of the lanthanide ion and its counter-ion. 相似文献
Measurements of the refractive index n for the binary mixtures 2-chloroethanol + 2-methoxyethanol in the 0 < or = t/degree C < or = 70 temperature range have been carried out with the purpose of checking the capability of empirical models to express physical quantity as a function of temperature and volume fraction, both separately and together, i.e., in a two independent variables expression. Furthermore, the experimental data have been used to calculate excess properties such as the excess refractive index, the excess molar refraction, and the excess Kirkwood parameter delta g over the whole composition range. The quantities obtained have been discussed and interpreted in terms of the type and nature of the specific intermolecular interactions between the components. 相似文献
Muscarone analogues are compounds that have been proposed for the prevention or treatment of senile dementias. ARL-16607 (I), ARL-15467 (II), ARL-14995 (III) and YM-796 (IV) are spiromuscar-3-one derivatives and behave as muscarinic M1 agonists, with different binding selectivity and efficacy at the M1 receptors. In this work, we have elucidated the solid-state structures of I-III and compared the results obtained with the data available in the literature for IV.The solid-state arrangements of I-IV have then been used to input a series of theoretical calculations. For each molecule, eight conformations have been modeled and the obtained structures (1-32) have been submitted to a series of molecular dynamics/simulated annealing and molecular mechanics calculations aimed to explore the conformational freedom of the spiromuscar-3-one moiety. Some hints about the reactivity of I-IV have been obtained by performing Hartree-Fock, density functional theory and semiempirical quantum mechanics calculations. These studies analyzed the properties of the frontier orbitals and of the molecular electrostatic potential of I-IV.The information gained has been used to explain the better efficacy and poorer selectivity shown by III. Our results suggest that the behavior of III is due to its smaller size, the features of its molecular surface, the shape of its electrostatic potential and the orientation of its reactive domains. 相似文献
We have recently reported 16 new far-infrared laser emissions of CD2Cl2. While the overwhelming majority of the previously known FIR laser emissions of this molecule were polarized parallel to the polarization of the pump CO2 line, 13 of our 16 new lines are polarized perpendicularly to the exciting CO2 radiation. In principle, the presence of perpendicular/parallel emission pairs is extremely important for the assignment of laser transitions. In the present work we discuss the possible assignments of 16 far-infrared (FIR) laser emission pairs of CD2Cl2. 相似文献
Metal-Insulator-Metal (MIM) and Schottky-barrier diodes have been used extensively in the past years as harmonic generators and mixers for frequency measurements in the spectral range from the far-infrared to the visible. MIM diodes present a very low fabrication cost and are easy to handle, while Schottky diodes are mechanically more stable and long-lived. In the present work we discuss the performance of a metal-semiconductor point-contact diode for the radiation around 1 m. This device, which may be viewed as a hybrid between a MIM and a Schottky diode, combines the simplicity and easiness of fabrication of the MIM diode with the stability and the long contact life typical of the Schottky diode. It proved to be very efficient even for visible light. 相似文献
Summary: The specificity of interactions between pairs of molecules cannot be explicitly given by experimental transport coefficients such as intra‐ or mutual diffusion coefficients. But a microscopic interpretation of the transport properties exists, where distinct diffusion coefficients (DDCs) are related to preferential, correlated motion among distinct molecules. Since in general the DDCs do not play the role of an indicator for molecular self‐association phenomena if not compared with some appropriate standard, here we propose DDCs of hard spheres at the second order of volume fraction as new standard coefficients. The analysis based on these novel DDCs is designed to study intermolecular interaction between macromolecule and solvent. Comparisons of the novel non‐ideal with previous ideal reference states were done, and their combined use is shown to reinforce information conveyed by the usual velocity correlation analysis. The comparison of novel hard sphere standards with real DDCs, corresponding to an homologous chemical series of poly(ethylene glycol)‐water mixtures, provides a look at this polymer‐solvent mixture in a dilute and semi‐dilute regime.
Comparison between real (calculated by using Equation (5)–(7) and experimental data) and hard‐sphere based distinct diffusion coefficients for PEG 200 (1: Dequation/tex2gif-stack-1.gif; 2: Dequation/tex2gif-stack-2.gif and 3: Dequation/tex2gif-stack-3.gif). 相似文献