Synthesis of SnO2−x nanoparticles tuned between 0 x 1 in a premixed low pressure H2/O2/Ar flame

The transition from tin monoxide to tin dioxide is investigated by doping a low pressure premixed H₂/O₂/Ar flame with dilute concentrations of tetramethyl tin (TMT) Sn(CH₃)₄. The H₂/O₂ ratio was varied between 1.97 and 0.97; the reactor pressure was set to 30 mbar. The inlet gas velocity was kept constant and the precursor concentration was varied between 200 and 700 ppm. Mean particle diameters between 3 nm d_p 7 nm were measured using a particle mass spectrometer (PMS), while X-ray diffraction (XRD) and transmission electron microscope (TEM) analysis revealed information on crystal structure as well as morphology of the synthesized oxides. A gray SnO film was observed in the reactor at low oxygen concentrations, while for higher oxygen concentrations light yellow sub-stoichiometric tin oxides and white SnO₂ were obtained. Further analysis of the sub-stoichiometric oxides, using atomic electron spectroscopy AES revealed that SnO_2−x (0.2 x 0.6) was obtained. Furthermore, electrical characterization of all materials, deposited on interdigital capacitors, showed a decrease in the conductivity of the nanoparticles with increasing oxidation (decreasing x).     

Intermolecular Electron Spin Transfer between Naphthalene π-Systems Separated by a Variable Number of Spirobonded Cyclobutane Rings,An. ESR and ENDOR Study

The radical anions of the compounds N1N, N3N and N5N , in which two naphthalen π-systems are separated by 1, 3 and 5 spirobonded cyclobutane rings, respectively, and tha tof the reference compound N1 , containing one naphthalene π-system and one cyclobutane ring, have been studied by ESR and ENDOR spectroscopy under a variety of experimental conditions. The intramolecular electrons spin transfer between the two π-moieties in N3N and N5N is slow on the hyperfine time-scale, irrespective of the applied conditions. It is also slow in N1N , except for media of high solvating power. In such media, with a slight reduction of N1N to its radical anion, a paramagnetic species is observed, the hyperfine data for which are consistent with N1N to its radical anion, a paramagnetic species is observed, the hyperfine data of which are consistent with N1N , undergoing a fast intramolecular electron spin tansfer. The ESR and ENDOR spectra of this species are superimposed on those characteristic of a slow transfer. It is suggested that the fast and slow transfer involve the syn- and anti-conformations, respectively, since the distance, r, between the two naphthalene π-systems of N1N is considerably shorter in the former than in the latter (r = 740 vs. 880 Pm for the distance between the centres of the π-systems). Glassy solutions of exhaustively reduced N1N display signals of the dianion triplet state, whereas no such signals are found for N3N and N5N . The zero-field splitting parameter, D , is 4.7 mT, corresponding to r ≈ 480 pm.     

Oganoboron compounds : XVIII. Chlorodiualkylaminophenylboranes

The synthesis and properties of a series of chlorodialkylaminophenylboranes, PhB(NR₂)CL, are reported and their ¹H and ¹³C NMR spectra discussed     

Complete relative stereochemistry of multiple stereocenters using only residual dipolar couplings

Residual dipolar couplings (RDCs), in combination with molecular order matrix calculations, were used to unambiguously determine the complete relative stereochemistry of an organic compound with five stereocenters. Three simple one-dimensional experiments were utilized for the measurements of (13)C-(1)H, (13)C-(19)F, (19)F-(1)H, and (1)H-(1)H RDCs. The order matrix calculation was performed on each chiral isomer independently. The fits were evaluated by the comparison of the root-mean-square deviation (rmsd) of calculated and measured RDCs. The order tensor simulations based on two different sets of RDC data collected with phage and bicelles are consistent. The resulting stereochemical assignments of the stereocenters obtained from using only RDCs are in perfect agreement with those obtained from the single-crystal X-ray structure. Six RDCs are found to be necessary to run the simulation, and seven are the minimum to get an acceptable result for the investigated compound. It was also shown that (13)C-(1)H and (1)H-(1)H RDCs, which are the easiest to measure, are also the most important and information-rich data for the order matrix calculation. The effect of each RDC on the calculation depends on the location of the corresponding vector in the structure. The direct RDC of a stereocenter is important to the configuration determination, but the configuration of stereocenters devoid of protons can also be obtained from analysis of nearby RDCs.     

A practical and scaleable synthesis of A-224817.0, a novel nonsteroidal ligand for the glucocorticoid receptor

A practical and scaleable synthesis of a novel nonsteroidal ligand for the glucocorticoid receptor A-224817.0 1A is described. The synthesis proceeds in seven steps starting from 1,3-dimethoxybenzene. The biaryl intermediate 5 was prepared by an optimized high-yielding and high-throughput Negishi protocol. The quinoline core 8 was constructed by using a modified Skraup reaction. The final product was obtained by a direct allylation reaction of lactol 10 with allyltrimethylsilane. The process was accomplished efficiently to produce 1A in 25% overall yield and >99% purity with simple and practical isolation and purification procedures.     

Carbodiimide-Driven Dimerization and Self-Assembly of Artificial,Ribose-Based Amphiphiles

The aqueous self-assembly of amphiphiles into aggregates such as micelles and vesicles has been widely investigated over the past decades with applications ranging from materials science to drug delivery. The combination of characteristic properties of nucleic acids and amphiphiles is of substantial interest to mimic biological self-organization and compartmentalization. Herein, we present ribose- and ribonucleotide-based amphiphiles and investigate their self-assembly as well as their fundamental reactivity. We found that various types of aggregates are formed, ranging in size from nanometers to micrometers and all amphiphiles exhibit aggregation-induced emission (AIE) in solution as well as in the solid state. We also observed that the addition of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) leads to rapid and selective dimerization of the amphiphiles into pyrophosphates, which decreases the critical aggregation concentration (CAC) by a factor of 25 when compared to the monomers. Since the propensity for amphiphile dimerization is correlated with their tendency to self-assemble, our results may be relevant for the formation of rudimentary compartments under prebiotic conditions.     

Synthesis of 13C and 2H Labeled Vinyl Pyruvate and Hyperpolarization of Pyruvate

The hyperpolarization of nuclear spins has enabled unique applications in chemistry, biophysics, and particularly metabolic imaging. Parahydrogen-induced polarization (PHIP) offers a fast and cost-efficient way of hyperpolarization. Nevertheless, PHIP lags behind dynamic nuclear polarization (DNP), which is already being evaluated in clinical studies. This shortcoming is mainly due to problems in the synthesis of the corresponding PHIP precursor molecules. The most widely used DNP tracer in clinical studies, particularly for the detection of prostate cancer, is 1-¹³C-pyruvate. The ideal derivative for PHIP is the deuterated vinyl ester because the spin physics allows for 100 % polarization. Unfortunately, there is no efficient synthesis for vinyl esters of β-ketocarboxylic acids in general and pyruvate in particular. Here, we present an efficient new method for the preparation of vinyl esters, including ¹³C labeled, fully deuterated vinyl pyruvate using a palladium-catalyzed procedure. Using 50 % enriched parahydrogen and mild reaction conditions, a ¹³C polarization of 12 % was readily achieved; 36 % are expected with 100 % pH₂. Higher polarization values can be potentially achieved with optimized reaction conditions.     

Transformations of Strained Three-Membered Rings a Common,Yet Overlooked,Motif in Heavy-Atom Tunneling Reactions

Quantum mechanical tunneling has long been recognized as an important phenomenon when considering transformations dominated by a lightweight hydrogen atom. Tunneling of heavier atoms like carbon, initially dismissed as negligible, has seen a quickly increasing number of computationally predicted and/or experimentally confirmed examples over the last decade, thus highlighting its importance for a wide variety of reactions. However, no common structural motif has been pointed out within these seemingly unconnected examples, strongly limiting the predictability of the impact of heavy-atom tunneling on a given reaction. This Concept article will provide this perspective and showcase how the recognition of the formation and cleavage of three-membered rings as common motif can inform the prediction of and research into heavy-atom tunneling reactions.     

Aggregation and consensus for preference relations based on fuzzy partial orders

We propose a framework for eliciting and aggregating pairwise preference relations based on the assumption of an underlying fuzzy partial order. We also propose some linear programming optimization methods for ensuring consistency either as part of the aggregation phase or as a pre- or post-processing task. We contend that this framework of pairwise-preference relations, based on the Kemeny distance, can be less sensitive to extreme or biased opinions and is also less complex to elicit from experts. We provide some examples and outline their relevant properties and associated concepts.     

Pivotal Role of Holes in Photocatalytic CO2 Reduction on TiO2

Evidence is provided that in a gas-solid photocatalytic reaction the removal of photogenerated holes from a titania (TiO₂) photocatalyst is always detrimental for photocatalytic CO₂ reduction. The coupling of the reaction to a sacrificial oxidation reaction hinders or entirely prohibits the formation of CH₄ as a reduction product. This agrees with earlier work in which the detrimental effect of oxygen-evolving cocatalysts was demonstrated. Photocatalytic alcohol oxidation or even overall water splitting proceeds in these reaction systems, but carbon-containing products from CO₂ reduction are no longer observed. H₂ addition is also detrimental, either because it scavenges holes or because it is not an efficient proton donor on TiO₂. The results are discussed in light of previously suggested reaction mechanisms for photocatalytic CO₂ reduction. The formation of CH₄ from CO₂ is likely not a linear sequence of reduction steps but includes oxidative elementary steps. Furthermore, new hypotheses on the origin of the required protons are suggested.