Photoredox-catalyzed [4+2] annulation of cyclobutylanilines with alkenes,alkynes, and diynes in continuous flow

Continuous flow has recently emerged as a powerful enabling technology that greatly improves many reactions’ efficiency. Here, we apply the technology to intermolecular [4+2] annulation of cyclobutylanilines with alkenes, alkynes, and diynes by photoredox catalysis. An across-the-board improvement in the annulation’s efficiency is noticed. Moreover, a gram-scale annulation is successfully demonstrated in continuous flow using a much lower catalyst loading.     

Mixed Type Duality for Set-valued Optimization Problems via Higher-order Radial Epiderivatives

In this article, we introduce a notion of higher-order radial epiderivative for set-valued maps and study its properties. A generalized concept of higher-order strict minimizers in set-valued optimization is proposed as well. By virtue of the radial epiderivative, we establish a mixed dual problem, and then weak, strong, and converse duality theorems are obtained in dealing with generalized strict minimizers.     

Palladium-catalyzed stereoselective formation of alpha-O-glycosides

A novel method for palladium-catalyzed stereoselective formation of alpha-O-glycosides has been developed. This strategy relies on the palladium-biaryl phosphine catalyst-glycal donor complexation to control the anomeric selectivity. It does not depend on the nature of the protecting groups on the substrates, thus eliminating the need for cumbersome protecting group manipulations.     

Local probe and conduction distribution of proton exchange membranes

Proton exchange membranes (Nafion) have been studied using current sensing atomic force microscopy to examine the correlation between the surface morphology and the ionic domains, and to probe the local ionic conduction distribution in the membranes. It is found that the local ionic conduction generated from the current sensing images follows a Gaussian-like distribution, with the peak value and the width of the distribution increasing with the relative humidity in the sample chamber and, thus, the water content in the membranes. Two types of Nafion membranes, Nafion 112 and Nafion 117, were studied using the method. The implications of the distribution in relation to the ionic conducting channels in the membranes are discussed.     

SU-8 as a structural material for labs-on-chips and microelectromechanical systems

Since its introduction in the nineties, the negative resist SU-8 has been increasingly used in micro- and nanotechnologies. SU-8 has made the fabrication of high-aspect ratio structures accessible to labs with no high-end facilities such as X-ray lithography systems or deep reactive ion etching systems. These low-cost techniques have been applied not only in the fabrication of metallic parts or molds, but also in numerous other micromachining processes. Its ease of use has made SU-8 to be used in many applications, even when high-aspect ratios are not required. Beyond these pattern transfer applications, SU-8 has been used directly as a structural material for microelectromechanical systems and microfluidics due to its properties such as its excellent chemical resistance or the low Young modulus. In contrast to conventional resists, which are used temporally, SU-8 has been used as a permanent building material to fabricate microcomponents such as cantilevers, membranes, and microchannels. SU-8-based techniques have led to new low-temperature processes suitable for the fabrication of a wide range of objects, from the single component to the complete lab-on-chip. First, this article aims to review the different techniques and provides guidelines to the use of SU-8 as a structural material. Second, practical examples from our respective labs are presented.     

Structure and dynamics of the homologous series of alanine peptides: a joint molecular dynamics/NMR study

The phi,psi backbone angle distribution of small homopolymeric model peptides is investigated by a joint molecular dynamics (MD) simulation and heteronuclear NMR study. Combining the accuracy of the measured scalar coupling constants and the atomistic detail of the all-atom MD simulations with explicit solvent, the thermal populations of the peptide conformational states are determined with an uncertainty of <5 %. Trialanine samples mainly ( approximately 90%) a poly-l-proline II helix-like structure, some ( approximately 10%) beta extended structure, but no alphaR helical conformations. No significant change in the distribution of conformers is observed with increasing chain length (Ala(3) to Ala(7)). Trivaline samples all three major conformations significantly. Triglycine samples the four corner regions of the Ramachandran space and exists in a slow conformational equilibrium between the cis and trans conformation of peptide bonds. The backbone angle distribution was also studied for the segment Ala3 surrounded by either three or eight amino acids on both N- and C-termini from a sequence derived from the protein hen egg white lysozyme. While the conformational distribution of the central three alanine residues in the 9mer is similar to that for the small peptides Ala(3)-Ala(7), major differences are found for the 19mer, which significantly (30-40%) samples alphaR helical stuctures.     

"Simulating synthesis": ceria nanosphere self-assembly into nanorods and framework architectures

We predict, from computer modeling and simulation in partnership with experiment, a general strategy for synthesizing spherical oxide nanocrystals via crystallization from melt. In particular we "simulate synthesis" to generate full atomistic models of undoped and Ti-doped CeO2 nanoparticles, nanorods, and nanoporous framework architectures. Our simulations demonstrate, in quantitative agreement with experiment [Science 2006, 312, 1504], that Ti (dopant) ions change the shape of CeO2 nanocrystals from polyhedral to spherical. We rationalize this morphological change by elucidating, at the atomistic level, the mechanism underpinning its synthesis. In particular, CeO2 nanocrystals can be synthesized via crystallization from melt: as a molten (undoped) CeO2 nanoparticle is cooled, nucleating seeds spontaneously evolve at the surface and express energetically stable [111] facets to minimize the energy. As crystallization proceeds, the [111] facets grow, thus facilitating a polyhedral shape. Conversely, when doped with Ti, a (predominantly) TiO2 shell encapsulates the inner CeO2 core. This shell inhibits the evolution of nucleating seeds at the surface thus rendering it amorphous during cooling. Accordingly, crystallization is forced to proceed via the evolution of a nucleating seed in the bulk CeO2 region of the nanoparticle, and as this seed grows, it remains surrounded by amorphous ions, which "wrap" around the core so that the energies for high-index facets are drastically reduced; these amorphous ions adopt a spherical shape to minimize the surface energy. Crystallization emanates radially from the nucleating seed, and because it is encapsulated by an amorphous shell, the crystallization front is not compelled to express energetically favorable surfaces. Accordingly, after the nanoparticle has crystallized it retains this spherical shape. A typical animation showing the crystallization (with atomistic detail) is available as Supporting Information. From this data we predict that spherical oxide nanocrystals can be synthesized via crystallization from melt in general by suppressing nucleating seed evolution at the surface thus forcing the nucleating seed to spontaneously evolve in the bulk. Nanospheres can, similar to zeolitic classifications, constitute Secondary Building Units (SBUs) and can aggregate to form nanorods and nanoporous framework architectures. Here we have attempted to simulate this process to generate models for CeO2 and Ti-doped CeO2 nanorods and framework architectures. In particular, we predict that Ti doping will "smooth" the surfaces: hexagonal prism shaped CeO2 nanorods with [111] and [100] surfaces become cylindrical, and framework architectures change from facetted pores and channels with well-defined [111] and [100] surfaces to "smooth" pores and channels (expressing both concave and convex curvatures). Such structures are difficult to characterize using, for example, Miller indices; rather we suggest that these new structural materials may be better described using minimal surfaces.