An efficient transition path sampling algorithm for nanoparticles under pressure

We apply transition path sampling to the simulation of nanoparticles under pressure. As a barostat we use a bath of ideal gas particles that form a stochastically updated atmosphere around the nanoparticle. We justify this algorithm by showing that it preserves the distribution of an ideal gas at constant temperature and pressure by satisfying detailed balance. Based on this result, we present a simple and efficient transition path sampling scheme for the study of activated processes in nanoparticles under pressure. As a first application, we investigate the h-MgO to rocksalt transformation in faceted CdSe nanocrystals. Starting from an artificial mechanism involving a uniform motion of all atoms, trajectories quickly converge towards the dominant mechanism of nucleation and growth along parallel (100) planes.     

A quantitative analysis of Oka’s lemma

In this paper, we will examine a strong form of Oka’s lemma which provides sufficient conditions for compact and subelliptic estimates for the -Neumann operator on Lipschitz domains. On smooth domains, the condition for subellipticity is equivalent to D’Angelo finite type and the condition for compactness is equivalent to Catlin’s condition (P). As an application, we will prove regularity for the -Neumann operator in the Sobolev space W ^s , , on C ² domains.     

Stability of caffeic acid phenethyl ester and its fluorinated derivative in rat plasma

The stability of caffeic acid phenethyl ester (CAPE) and its fluorinated derivative (FCAPE) in rat plasma and conditions preventing their degradation are reported. Reverse-phase high-pressure liquid chromatography (HPLC) using taxifolin as an internal standard was applied for the quantitative determination of CAPE and FCAPE in rat plasma extracted with ethyl acetate. The assay was validated over a linear range of 0.25-10 microg/mL plasma (r(2) > 0.9990, n = 3). No endogenous interferences were observed in the chromatographic region of interest. The limits of quantification and detection were set at 0.25 and 0.1 microg/mL, respectively. The precision ranged from 0.7 to 13.7% for CAPE, and from 0.4 to 10.4% for FCAPE. Accuracy ranged from -2.8 to 12.4% for CAPE and from -0.6 to 6.8% for FCAPE. The stability was conducted at 4, 25 and 37 degrees C. First-order kinetics was observed for the degradation of CAPE and FCAPE. The energies of activation of CAPE and FCAPE were found to be 17.9 and 20.1 kcal/mol, respectively. Addition of 0.4% of sodium chloride and pH adjustment to 6 prevented their degradation in rat plasma for 24 h and at least one month at -20 degrees C. This study provides useful information for the future pharmacokinetic study of CAPE and FCAPE in rat.     

Evidence for in situ catalyst modification during the Pd-catalyzed conversion of aryl triflates to aryl fluorides

A mechanistic investigation of the Pd-catalyzed conversion of aryl triflates to fluorides is presented. Studies reveal that C-F reductive elimination from a LPd(II)(aryl)F complex (L = t-BuBrettPhos or RockPhos) does not occur when the aryl group is electron rich. Evidence is presented that a modified phosphine, generated in situ, serves as the actual supporting ligand during catalysis with such substrates. A preliminary study of the reactivity of a LPd(II)(aryl)F complex based on this modified ligand is reported.     

Assessing chiral self-recognition using chiral stationary phases

Chiral stationary phases were synthesized and their ability to separate racemic precursors from which they were derived was assessed. Taken in conjunction with homochiral recognition previously observed in the solid state, the results of this study reveal that a geometrically controlling π-π interaction has a profound influence on molecular recognition.     

Formation of silica nanofibers with hierarchical structure via electrospinning

Self-standing thiol-functionalized fibrous membrane with bimodal pore structure was successfully fabricated through the combination of electrospinning and surfactant-directing pore formation technique. We also found that the interference of electric field as well as the rapid solvent evaporation during the electrospinning process has a negative effect on the formation of ordered mesoporous structures. As high-performance adsorption-based filtration system, such hierarchically structured membrane is found to efficiently remove Cu (II) ions under high flux.     

Synthesis, characterization, and multielectron reduction chemistry of uranium supported by redox-active α-diimine ligands

Uranium compounds supported by redox-active α-diimine ligands, which have methyl groups on the ligand backbone and bulky mesityl substituents on the nitrogen atoms {(Mes)DAB(Me) = [ArN═C(Me)C(Me)═NAr], where Ar = 2,4,6-trimethylphenyl (Mes)}, are reported. The addition of 2 equiv of (Mes)DAB(Me), 3 equiv of KC(8), and 1 equiv of UI(3)(THF)(4) produced the bis(ligand) species ((Mes)DAB(Me))(2)U(THF) (1). The metallocene derivative, Cp(2)U((Mes)DAB(Me)) (2), was generated by the addition of an equimolar ratio of (Mes)DAB(Me) and KC(8) to Cp(3)U. The bond lengths in the molecular structure of both species confirm that the α-diimine ligands have been doubly reduced to form ene-diamide ligands. Characterization by electronic absorption spectroscopy shows weak, sharp transitions in the near-IR region of the spectrum and, in combination with the crystallographic data, is consistent with the formulation that tetravalent uranium ions are present and supported by ene-diamide ligands. This interpretation was verified by U L(III)-edge X-ray absorption near-edge structure (XANES) spectroscopy and by variable-temperature magnetic measurements. The magnetic data are consistent with singlet ground states at low temperature and variable-temperature dependencies that would be expected for uranium(IV) species. However, both complexes exhibit low magnetic moments at room temperature, with values of 1.91 and 1.79 μ(B) for 1 and 2, respectively. Iodomethane was used to test the reactivity of 1 and 2 for multielectron transfer. While 2 showed no reactivity with CH(3)I, the addition of 2 equiv of iodomethane to 1 resulted in the formation of a uranium(IV) monoiodide species, ((Mes)DAB(Me))((Mes)DAB(Me2))UI {3; (Mes)DAB(Me2) = [ArN═C(Me)C(Me(2))NAr]}, which was characterized by single-crystal X-ray diffraction and U M(4)- and M(5)-edge XANES. Confirmation of the structure was also attained by deuterium labeling studies, which showed that a methyl group was added to the ene-diamide ligand carbon backbone.     

Retention mechanism of a cholesterol-coated C18 stationary phase: van't Hoff and Linear Solvation Energy Relationships (LSER) approaches

This study examines the effect of temperature on the dynamic cholesterol coating of a C18 stationary phase and the effect of this coating on the retention mechanism. In general, an increase in temperature results in a decrease in the mass of cholesterol coated on the stationary phase. Typically, an increase in temperature from 25°C to 55°C results in a nearly 60% reduction in the mass of cholesterol loaded. The inclusion of temperature, along with loading solvent composition and cholesterol concentration in the loading solvent, allows for loading a targeted amount of cholesterol on the stationary phase over an order-of-magnitude range. In addition to loading studies, the retention mechanism of small non-ionizable solutes was examined on cholesterol-coated stationary phases. A van't Hoff analysis was performed to assess retention thermodynamics, while a LSER approach was used to examine retention mechanism. With 50/50 water/organic mobile phases, the addition of cholesterol results in an increase in the entropic contribution to retention, with a decrease in the enthalpic contribution. The opposite trend is seen with 40/60 water/organic mobile phases. LSER system constants are also affected by a cholesterol coating on the stationary phase, with some changing to favor elution and others changing to favor retention.     

Radical O→C transposition: a metal-free process for conversion of phenols into benzoates and benzamides

We report a metal-free procedure for transformation of phenols into esters and amides of benzoic acids via a new radical cascade. Diaryl thiocarbonates and thiocarbamates, available in a single high-yielding step from phenols, selectively add silyl radicals at the sulfur atom of the C═S moiety. This addition step, analogous to the first step of the Barton-McCombie reaction, produces a carbon radical which undergoes 1,2 O→C transposition through an O-neophyl rearrangement. The usually unfavorable equilibrium in the reversible rearrangement step is shifted forward via a highly exothermic C-S bond scission in the O-centered radical, which furnishes the final benzoic ester or benzamide product. The metal-free preparation of benzoic acid derivatives from phenols provides a potentially useful alternative to metal-catalyzed carbonylation of aryl triflates.     

pH responsive self-healing hydrogels formed by boronate-catechol complexation

Here we report the synthesis and characterization of pH-responsive, self-healing hydrogels based on boronate-catechol complexation.