Assisted desolvation as a key kinetic step for crystal growth

The crystallization of materials from a supersaturated solution is a fundamental chemical process. Although several very successful models that provide a qualitative understanding of the crystal growth process exist, in most cases the atomistic detail of crystal growth is not fully understood. In this work, molecular dynamics simulations of the morphologically most important surfaces of barite in contact with a supersaturated solution have been performed. The simulations show that an ordered and tightly bound layer of water molecules is present on the crystal surface. The approach of an ion to the surface requires desolvation of both the surface and the ion itself leading to an activated process that is rate limiting for two-dimensional nucleation to occur. However, desolvation on specific surfaces can be assisted by anions adsorbed on the crystal surface. This hypothesis, corroborated by crystallization and scanning electron microscopy studies, allows the rationalization of the morphology of barite crystals grown at different supersaturations.     

New Components Including Cyclopeptides from Barks of Christiana africana DC. (Tiliaceae)

Phytochemical investigation of barks of Christiana africana led to the identification of cyclopeptide alkaloids, flavonoids, coumarinolignans, iridoids, sesquiterpenoids, and triterpenes. This plant was classified so far in the Tiliaceae family. This study was started while the genomic study of numerous specimens was described in order to establish new criteria for Malvales botanical classification. In the present work, twenty components were identified, belonging to the three major classes of secondary metabolites: alkaloids, phenols, and terpenes. In the first class, cyclopeptides are well‐known compounds in Rhamnaceae and Sterculiaceae. Their presence in Malvaceae (in APG2 sensus) suggests a possible chemical link between the ex‐Tiliaceae and the Malvaceae.     

Variable strategy toward carbasugars and relatives. 5.(1) Focus on preparation of chiral nonracemic medium-sized carbocycles

The feasibility of sequential vinylogous aldol (intermolecular)/silylative aldol (intramolecular) addition reactions involving furan- and pyrrole-based dienoxysilanes, 6 and 12, in the synthesis of carbasugar frameworks is illustrated by the preparation of the scantily investigated carbaseptanose and carbaoctanose representatives of this class of compounds. The target compounds, 1, 2, 3, ent-2, ent-3, and 4, were obtained from readily available carbohydrate precursors (5 and 19) in yields of 21-30% over 8-12 steps. The irreversible silylative ring-closing aldolisation of gamma-substituted dihydro-5H-furan-2-one and pyrrolidin-2-one aldehydes (9, 16, ent-16, and 22) driven by the TBSOTf/Pr(i)(2)EtN Lewis acid-Lewis base couple was shown to be a practical, diastereoselective maneuver to forge the densely functionalized, medium-sized core carbocycles.     

Evaluation of a microHPLC system dedictaed to packed capillary column liquid chromatography

Miniaturization and optimization of the solvent delivery system, mixing device, and detection system for gradient elution at few μl/min is the most important objective of instrumental development in microHPLC using packed capillary columns. Instrumental solutions and evaluation of the performence of a dedicated system for automatic gradient elution with packed capillary columns are reported. Retention time precision shown buy the system results in an RSD of 0.20–0.52% for a PAH model mixture eluted under gradient conditions at few μl/min. Compositional accuracy of gradient profiles is also demonstrated.     

Ethoxycarbonylnitrene addition to vinyl chlorides. Synthesis and thermal rearrangement of α-chloroaziridines

The addition of ethoxycarbonylnitrene to vinyl chlorides gives N-ethoxycarbonyl-α-chloroaziridines which undergo an easy rearrangement mainly or exclusively to N-ethoxycarbonyl-2-chloro-2-alkenylamines.     

Modeling the cavitation free energy

A new expression to compute the cavitation free energy has been derived by integrating a new model to fit its derivative with respect to the cavity radius. The derivatives were obtained from Monte Carlo simulations data of the contact values of distribution functions for hard-sphere solutes in TIP4P water at 298 K and 1 atm. The new expression, formulated in the framework of the thermodynamics of surfaces and unlike the classical simple models, gives good results also for very small cavities with a radius of approximately 1 A. The contribution to the free energy of a term, which depends on the excess number of molecules at the dividing surface, has been taken into account and discussed for the assumed dependence on r of the surface tension. The asymptotic behavior of the derivative has thus been considered, and a function t(r), which is 0 at r = 0 and 1 at infinity, has been introduced to describe the transition from small to large length regimes. The value of the surface tension obtained by fitting is in very good agreement with that obtained from a simulation of the liquid/vapor interface by using the TIP4P model.     

High performance liquid chromatography on the chiral stationary phase (R,R)-DACH-DNB using carbon dioxide-based eluents

Fast and efficient separations of chiral stereolabile compounds were obtained at very low temperature on a π-acid chiral stationary phase (R,R-DACH-DNB) using carbon dioxide-based mobile phases containing alcoholic polar modifiers. Furthermore, efficient separations of the newly discovered spherical carbon cluster buckminsterfullerene (C₆₀) and the related higher fullerenes (C₇₀, etc.) have been performed on the same stationary phase using eluents based on either n-hexane or carbon dioxide.     

X-ray crystal and molecular structure of 2,3-dideoxy-4-thio-d-arabino-heptonic acid 1,4-lactone: a key intermediate for syntheses of 2′, 3′-dideoxy-4′-thio-l-nucleosides

The structure of 2,3-dideoxy-4-thio-d-arabino-heptonic acid 1,4-lactone has been determined by X-ray diffraction. Crystals of the compound are orthorhombic, space groupP2₁2₁2₁, with cell dimensionsa=11.555(5),b=10.451(5),c=9.604(5) Å, andZ=4. The diffraction experiment has allowed to assign the absolute configuration (4R, 5R, 6R) to the chiral centers of the molecule.     

Experimental investigation of an oscillator with discontinuous support considering different system aspects

Non-smooth characteristics are, in general, the source of difficulties for the modeling and simulation of natural systems. These characteristics are usually related to either the friction phenomenon or the discontinuous behavior as intermittent contacts. This article develops an experimental investigation concerning non-smooth systems with discontinuous support. An experimental apparatus is developed in order to analyze the nonlinear dynamics of a single-degree of freedom system with discontinuous support. The apparatus is composed by an oscillator constructed by a car, free to move over a rail, connected to an excitation system. The discontinuous support is constructed considering mass–spring systems separated by a gap to the car position. This apparatus is instrumented to obtain all the system state variables. System dynamical behavior shows a rich response, presenting dynamical jumps, bifurcations and chaos. Different configurations of the experimental set up are treated in order to evaluate the influence of the internal impact within the car and also support characteristics in the system dynamics.     

Bayesian analysis of in vivo dynamic 13C-edited 1H images

We propose an application of a Bayesian methodology to dynamic MR images of protons J-coupled to 13C nuclei for monitoring the in vivo 13C-glucose uptake of mouse brain. The very low population of these protons and the random noise make the analysis of these images extremely difficult. The proposed method restores the images and provides an "activation" map of the mouse brain by means of a hypothesis testing procedure. The restoration step is performed in the Bayesian framework so that among the other advantages of a stochastic approach, it is possible to model spatial and temporal information about neighboring pixels. This leads to a restoration procedure able to reduce the noise level while preserving the information about the edges of signal areas. Based on the restored images, the testing procedure provides us with a reliable map of pixels characterized by the 13C-glucose uptake.