Frustrated Lewis Pairs Comprising Nitrogen Lewis Acids for Si–H Bond Activation

N-heterocyclic nitrogen Lewis acids are a recent addition to the field of organic chemistry. Based on nitrenium cations, these acids where previously shown to generate Lewis adducts when combined with the appropriate Lewis bases. Herein, a triazinium-based Lewis acid was combined with tBu₃P to generate a frustrated Lewis pair (FLP) capable of cleaving, for the first time, Si−H bonds in silanes. Whereas low yields were initially encountered owing to insufficient Lewis acidity, a new nitrenium-based Lewis acid was synthesized, and its superior Lewis acidity was experimentally and computationally confirmed. A FLP based on this acid cleaved the Si−H bond in PhSiH₃, generating the triazane product in a quantitative yield. This unprecedented N−H triazane was fully characterized by multinuclear NMR techniques and single-crystal X-ray crystallography. A new class of compounds, N-H triazanes display the potential capacity to participate in hydride transfer reactions.     

Truthful mechanism design via correlated tree rounding

A powerful algorithmic technique for truthful mechanism design is the maximal-in-distributional-range (MIDR) paradigm. Unfortunately, many such algorithms use heavy algorithmic machinery, e.g., the ellipsoid method and (approximate) solution of convex programs. In this paper, we present a correlated rounding technique for designing mechanisms that are truthful in expectation. It is elementary and can be implemented quickly. The main property we rely on is that the domain offers fractional optimum solutions with a tree structure. In auctions based on the generalized assignment problem, each bidder has a publicly known knapsack constraint that captures the subsets of items that are of value to him. He has a private valuation for each item and strives to maximize the value of assigned items minus payment. For this domain we design a truthful 2-approximate MIDR mechanism for social welfare maximization. It avoids using the ellipsoid method or convex programming. In contrast to some previous work, our mechanism achieves exact truthfulness. In restricted-related scheduling with selfish machines, each job comes with a public weight, and it must be assigned to a machine from a public job-specific subset. Each machine has a private speed and strives to maximize payments minus workload of jobs assigned to it. Here we design a mechanism for makespan minimization. This is a single-parameter domain, but the approximation status of the optimization problem is similar to unrelated machine scheduling: The best known algorithm obtains a (non-truthful) 2-approximation for unrelated machines, and there is 1.5-hardness. Our mechanism matches this bound with a truthful 2-approximation.     

Direct measurement of the flow field around swimming microorganisms

Swimming microorganisms create flows that influence their mutual interactions and modify the rheology of their suspensions. While extensively studied theoretically, these flows have not been measured in detail around any freely-swimming microorganism. We report such measurements for the microphytes Volvox carteri and Chlamydomonas reinhardtii. The minute (～0.3%) density excess of V. carteri over water leads to a strongly dominant Stokeslet contribution, with the widely-assumed stresslet flow only a correction to the subleading source dipole term. This implies that suspensions of V. carteri have features similar to suspensions of sedimenting particles. The flow in the region around C. reinhardtii where significant hydrodynamic interaction is likely to occur differs qualitatively from a puller stresslet, and can be described by a simple three-Stokeslet model.     

High-resolution terahertz spectroscopy of crystalline trialanine: extreme sensitivity to beta-sheet structure and cocrystallized water

High-resolution terahertz absorption spectra (0.06-3 THz) have been obtained at 4.2 K for three crystalline forms of trialanine [H2+-(Ala)3-O-]. The crystal structures differ in their beta-sheet forms (parallel vs antiparallel) and in their water composition (hydrated vs dehydrated antiparallel beta-sheet). The spectra are nearly vibrationally resolved, with little absorption below 1 THz. In sharp contrast to observations made in the mid-IR region, the spectral patterns of all three forms are qualitatively different, illustrating the extreme sensitivity to changes in the intermolecular hydrogen-bonding networks that stabilize peptide crystals. Predictions obtained from a classical force field model (CHARMM) and density functional theory (DFT/PW91) for periodic solids are compared with the X-ray structural data and the terahertz absorption spectra. In general, the results for the parallel beta-sheet are in better agreement with experiment than those for the antiparallel beta-sheet. For all three structures, however, most hydrogen bond distances are underestimated at both levels of theory, and the predicted absorption features are significantly red-shifted for the two antiparallel beta-sheet structures. Moreover, the nuclear motions predicted at the two levels of theory are qualitatively different. These results indicate that the PW91 functional is not sufficient to treat the weak intersheet hydrogen bonding present in the different beta-sheet forms and strongly suggest the need for improved force field models that include three-atom hydrogen-bonding terms for periodic solids.     

Transparent gold as a platform for adsorbed protein spectroelectrochemistry: investigation of cytochrome c and azurin

The majority of protein spectroelectrochemical methods utilize a diffusing, chemical mediator to exchange electrons between the electrode and the protein. In such methods, electrochemical potential control is limited by mediator choice and its ability to interact with the protein of interest. We report an approach for unmediated, protein spectroelectrochemistry that overcomes this limitation by adsorbing protein directly to thiol self-assembled monolayer (SAM) modified, thin (10 nm), semitransparent gold. The viability of the method is demonstrated with two diverse and important redox proteins: cytochrome c and azurin. Fast, reversible electrochemical signals comparable to those previously reported for these proteins on ordinary disk gold electrodes were observed. Although the quantity of protein in a submonolayer adsorbed at an electrode is expected to be insufficient for detection of UV-vis absorption bands based on bulk extinction coefficients, excellent spectra were detected for each of the proteins in the adsorbed state. Furthermore, AFM imaging confirmed that only a single layer of protein was adsorbed to the electrode. We hypothesize that interaction of the relatively broad gold surface plasmon with the proteins' electronic transitions results in surface signal enhancement of the molecular transitions of between 8 and 112 times, allowing detection of the proteins at much lower than expected concentrations. Since many other proteins are known to interact with gold SAMs and the technical requirements for implementation of these experiments are simple, this approach is expected to be very generally applicable to exploring mechanisms of redox proteins and enzymes as well as development of sensors and other redox protein based applications.     

Nature's microfluidic transporter: rotational cytoplasmic streaming at high Péclet numbers

Cytoplasmic streaming circulates the contents of large eukaryotic cells, often with complex flow geometries. A largely unanswered question is the significance of these flows for molecular transport and mixing. Motivated by "rotational streaming" in Characean algae, we solve the advection-diffusion dynamics of flow in a cylinder with bidirectional helical forcing at the wall. A circulatory flow transverse to the cylinder's long axis, akin to Dean vortices at finite Reynolds numbers, arises from the chiral geometry. Strongly enhanced lateral transport and longitudinal homogenization occur if the transverse Péclet number is sufficiently large, with scaling laws arising from boundary layers.     

Thermal and x-ray studies of binary rare earth sesquioxides — Alkali metal persulfate systems

Three binary systems, each consisting of mixtures of different molar ratios of a rare earth (4f metal) sesquioxide and sodium or potassium persulfate (peroxodisulfate) have been investigated under static (air) atmospheres using a derivatograph. For each system (R₂O₃ - M₂S₂O₈, whereR is Sm, Gd, or Dy, andM is K or Na), the molar ratios that represent stoichiometric reactions were fixed. Calculations were based on data obtained from T, TG, DTG and DTA curves. Intermediate and final products were mainly identified by X-ray diffractometry. Double salts of Sm, Gd, and Dy were thermally prepared by solid-state reactions. It was found that one such double salt KDy(SO₄)₂, forms a eutectic mixture with K₂SO₄ and also that both Gd and Dy sesquioxides behave asp-type semiconductors and therefore exhibit catalytic activities towards the thermal decomposition of persulfates and pyrosulfates. Optimal catalysis was obtained with molar ratios of 13. The activities increased as the ionic radii of these metals decrease: Sm, Gd, Dy.

---

Zusammenfassung Drei binäre Systeme, jeweils aus Gemischen verschiedener Molverhältnisse der Sesquioxide der seltenen Erden mit Natrium- oder Kaliumpersulfat (Peroxodisulfat) bestehend, werden in statischer Luft-Atmosphäre unter Einsatz des Derivatographen untersucht. Für jedes System (R₂O₃M₂S₂O₈, mitR = Sm, Gd oder Dy undM = K oder Na) sind die Molverhältnisse, welche stöchiometrischen Reaktionen entsprechen, festgelegt. Die Berechnungen beruhen auf aus T-, TG-, DTG- und DTA-Kurven erhaltenen Angaben. Die Zwischen- und Endprodukte werden hauptsächlich durch Röntgendiffraktometrie identifiziert. Doppelsalze von Sm, Gd und Dy werden thermisch durch Festphasenreaktionen gebildet. Es wurde gefunden, dass eines dieser Doppelsalze, KDy(SO₄)₂, mit K₂SO₄ ein eutektisches Gemisch bildet. Weiterhin zeigte sich, dass sowohl Gd- als auch Dy-Sesquioxide sich wie Halbleiter vom p-Typs verhalten und dadurch gegenüber der thermischen Zersetzung von Persulfaten und Pyrosulfaten katalytische Aktivitäten aufweisen. Die optimale Katalyse wird mit Molverhältnissen von (13) erzielt. Die Aktivitäten steigen mit abnehmenden Ionenradien der Metalle: Sm, Gd und Dy.

---

Résumé Trois systèmes binaires consistant chacun en des mélanges en proportions molaires différentes de sesquioxydes de métaux de terres rares 4f et de persulfate (peroxodisulfate) de sodium et de potassium ont été étudiés en atmosphère statique (air) avec un Derivatograph. Les proportions molaires correspondant aux réactions stoechiométriques ont été fixées pour chaque système (R₂O₃M₂S₂O₈ oùR=Sm, Gd ou Dy etM=K ou Na). Les calculs reposent sur des données obtenues à partir des courbes T, TG, TGD et ATD. L'identification des produits intermédiaires et finaux a été effectuée principalement par diffractométrie des rayons X. Les sels doubles de Sm, Gd et Dy sont préparés par voie thermique, par réactions en phase solide. On a trouvé que l'un de ces sels doubles, (KDy/SO₄)₂ forme un mélange eutectique avec K₂SO₄. On a trouvé de même que les deux sesquioxides de Gd et de Dy ont un comportement de semiconducteurs du typep et qu'ils montrent une activité catalytique sur la décomposition thermique des persulfates et pyrosulfates. La catalyse optimale est obtenue avec les proportions molaires de (1/3). L'activité augmente quand les rayons ioniques des métaux diminuent: Sm, Gd, Dy.

---

4f . R₂O₃ M₂S₂O₈, R-Sm, Gd, Dy, aM- Na, , . , -, -, - -. . Sm, Gd Dy . , - KDy(SO₄)₂ K₂SO₄. , Gd Dy p- . 13 . : Sm, Gd, Dy.

     

Electrochemically Driven Cation Exchange Enables the Rational Design of Active CO2 Reduction Electrocatalysts

Metal oxides or sulfides are considered to be one of the most promising CO₂ reduction reaction (CO₂RR) precatalysts, owing to their electrochemical conversion in situ into highly active electrocatalytic species. However, further improvement of the performance requires new tools to gain fine control over the composition of the active species and its structural features [e.g., grain boundaries (GBs) and undercoordinated sites (USs)], directly from a predesigned template material. Herein, we describe a novel electrochemically driven cation exchange (ED‐CE) method that enables the conversion of a predesigned CoS₂ template into a CO₂RR catalyst, Cu₂S. By means of ED‐CE, the final Cu₂S catalyst inherits the original 3 D morphology of CoS₂, and preserves its high density of GBs. Additionally, the catalyst's phase structure, composition, and density of USs were precisely tuned, thus enabling rational design of active CO₂RR sites. The obtained Cu₂S catalyst achieved a CO₂‐to‐formate Faradaic efficiency of over 87 % and a record high activity (among reported Cu‐based catalysts). Hence, this study opens the way for utilization of ED‐CE reactions to design advanced electrocatalysts.     

Chiral symmetry breaking during crystallization: an advection-mediated nonlinear autocatalytic process

When a chiral chemical compound crystallizes from solution or from its melt, stirring often results in the formation of crystals of just one of the two possible enantiomers, while without fluid advection both enantiomers are formed. We demonstrate with simulations of the dynamics of the system that secondary nucleation is a nonlinear autocatalytic phenomenon that can explain these observations. Furthermore, we present theoretical arguments and experimental results that suggest that at the microscale the mechanism of secondary nucleation is whisker crystal growth and dispersion in the fluid flow.