Determination of oxygenated and native polycyclic aromatic hydrocarbons in urban dust and diesel particulate matter standard reference materials using pressurized liquid extraction and LC–GC/MS

The objective of this study was to develop a novel analytical chemistry method, comprised of a coupled high-performance liquid chromatography–gas chromatography/mass spectrometry system (LC–GC/MS) with low detection limits and high selectivity, for the identification and determination of oxygenated polycyclic aromatic hydrocarbons (OPAHs) and polycyclic aromatic hydrocarbons (PAHs) in urban air and diesel particulate matter. The linear range of the four OPAHs, which include 9,10-anthraquinone, 4H-cyclopenta[def]phenanthrene-4-one, benzanthrone, and 7,12-benz[a]anthraquinone, was 0.7 pg–43.3 ng with limits of detection (LODs) and limits of quantification (LOQs) on the order of 0.2–0.8 and 0.7–1.3 pg, respectively. The LODs in this study are generally lower than values reported in the literature, which can be explained by using large-volume injection. The recoveries of the OPAHs spiked onto glass fiber filters using two different pressurized liquid extraction (PLE) methods were in the ranges of 84–107 and 67–110 %, respectively. The analytical protocols were validated using the following National Institute of Standards and Technology standard reference materials: SRM 1649a (Urban Dust), SRM 1650b (Diesel Particulate Matter), and SRM 2975 (Diesel Particulate Matter, Industrial Forklift). The measured mass fractions of the OPAHs in the standard reference materials (SRMs) in this present study are higher than the values from the literature, except for benzanthrone in SRM 1649a (Urban Dust). In addition to the OPAHs, 44 PAHs could be detected and quantified from the same particulate extract used in this protocol. Using data from the literature and applying a two-sided t test at the 5 % level using Bonferroni correction, significant differences were found between the tested PLE methods for individual PAHs. However, the measured mass fractions of the PAHs were comparable, similar to, or higher than those previously reported in the literature.     

Diphenylberyllium Reinvestigated: Structure,Properties, and Reactivity of BePh2, [(12-crown-4)BePh]+, and [BePh3]−

The first synthesis of BePh₂ was accomplished almost a century ago. However, its structure has remained unknown so far, while the corresponding aryls of the elements adjacent to beryllium in the periodic table are well investigated. Herein, we present an improved synthesis for diphenylberyllium and show by X-ray diffraction that it forms a trinuclear complex in the solid state. NMR spectroscopy revealed that this structure is also retained in solution but exhibits dynamic behavior. Its stability against heat and coordinating solvents is discussed and the possible obstacles to the synthesis of BePh₂ from BeCl₂ are examined. In the process of this study two ether adducts, BePh₂⋅Et₂O and Be₂Ph₄⋅Et₂O, have been characterized as well as the previously unknown triphenylberyllate anion. From the latter several single-crystal structures were obtained under various conditions, in which [BePh₃]⁻ is either isolated or acts as a ligand for Li⁺. Furthermore, the crown ether induced selfionization of BePh₂ is described and the resulting [(12-crown-4)BePh]⁺ cation was isolated, which shows an unusual 4+1 coordination around the Be atom.     

Growth kinetics of heterostructured GaP-GaAs nanowires

We have studied the vapor-liquid-solid (VLS) growth dynamics of GaP and GaAs in heterostructured GaP-GaAs nanowires. The wires containing multiple GaP-GaAs junctions were grown by the use of metal-organic vapor phase-epitaxy (MOVPE) on SiO(2), and the lengths of the individual sections were obtained from transmission electron microscopy. The growth kinetics has been studied as a function of temperature and the partial pressures of the precursors. We found that the growth of the GaAs sections is limited by the arsine (AsH(3)) as well as the trimethylgallium (Ga(CH(3))(3)) partial pressures, whereas the growth of GaP is a temperature-activated, phosphine(PH(3))-limited process with an activation energy of 115 +/- 6 kJ/mol. The PH(3) kinetics obeys the Hinshelwood-Langmuir mechanism, indicating that the dissociation reaction of adsorbed PH(3) into PH(2) and H on the catalytic gold surface is the rate-limiting step for the growth of GaP. In addition, we have studied the competitive thin layer growth on the sidewalls of the nanowires. Although the rate of this process is 2 orders of magnitude lower than the growth rate of the VLS mechanism, it competes with VLS growth and results in tapered nanowires at elevated temperatures.     

Immobilization and Continuous Recycling of Photoredox Catalysts in Ionic Liquids for Applications in Batch Reactions and Flow Systems: Catalytic Alkene Isomerization by Using Visible Light

A catalytic (E)‐ to (Z)‐isomerization of olefins using a photoredox catalyst under mild reaction conditions is presented. A variety of (Z)‐alkenes can be prepared in the presence of visible light. A new reaction system allows an easy and efficient scale‐up, as well as a continuous flow process in which the photocatalyst is immobilized in an ionic liquid and continuously recycled by simple phase separation.     

Predissociation of chlorobenzene, beyond the pseudo-diatomic model

Dissociation of chlorobenzene via the lowest singlet excited state has been investigated by means of pump–probe femtosecond spectroscopy and spin–orbit corrected ab initio quantum chemistry. We have found that the so far accepted model with a ¹ππ^* → ³π/nσ^* reaction mechanism has to be amended. We suggest that the mechanism goes via a transition from ¹ππ^* to a πσ^* state that is to 90% a singlet. Further, three nuclear degrees of freedom required to describe the dissociation have been defined.     

C–F Bond Cleavage Reactions with Beryllium,Magnesium, Gallium,Hafnium, and Thorium Halides

The work describes unexpected stoichiometric C–F bond cleavage reactions of beryllium, magnesium, gallium, hafnium and thorium halides with α,α,α-trifluorotoluene. The reaction of BeBr₂ / GaBr₃ or MgBr₂ / GaBr₃ mixtures as well as neat GaI₃ with α,α,α-trifluorotoluene in the presence of (OSi₂Me₄)₂ ( I ) yields the carbenium ion containing compounds [Ph-C(O₂Si₂Me₄)][GaX₄] (X = Br: 1 , X = I/F: 2 ). Both compounds were successfully characterized and a defluorination type reaction under incorporation of a siloxy unit was observed. Compound 1 was also characterized by single-crystal X-ray diffraction analysis. The conversion of α,α,α-trifluorotoluene with BeI₂, HfI₄ or ThI₄ turned out to be a halodefluorination-type reaction with formation of α,α,α-triiodotoluene ( 3 ). An adequate NMR spectroscopic and the X-ray crystallographic characterization of 3 were performed for the first time.     

NMR‐Structural Investigations of a β3‐Dodecapeptide with Proteinogenic Side Chains in Methanol and in Aqueous Solutions

The structural properties of an all‐β³‐dodecapeptide with the sequence H‐β‐HLys(N^ε‐CO(CH₂)₃‐S Acm)‐β‐HPhe‐β‐HTyr‐β‐HLeu‐β‐HLys‐β‐HSer‐β‐HLys‐β‐HPhe‐β‐HSer‐β‐HVal‐β‐HLys‐β‐HAla‐OH ( 1 ) have been studied by two‐dimensional homonuclear ¹H‐NMR and by CD spectroscopy. In MeOH solution, high‐resolution NMR spectroscopy showed that the β‐dodecapeptide forms an (M)‐3₁₄‐helix, and the CD spectrum corresponds to the pattern expected for an (M)‐3₁₄‐helical secondary structure. In aqueous solution, however, the peptide adopts a predominantly extended conformation without regular secondary‐structure elements, which is in agreement with the absence of the characteristic trough near 215 nm in the CD spectrum. The NMR and CD measurements with solutions of 1 in MeOH containing 3M urea further indicated that the peptide retains the regular secondary structural elements under these conditions, whereas, after addition of 40% (v/v) H₂O to the MeOH solution, the large ¹H‐chemical‐shift dispersion indicative of a defined spatial peptide fold was lost. The β³‐dodecapeptide is – so far – the longest β‐peptide shown to adopt a regular (M)‐3₁₄‐helix conformation in an organic solvent. The observation that the structure of this long β³‐peptide is not maintained in aqueous solution indicates that the (M)‐3₁₄‐fold is primarily stabilized by short‐range interactions.     

Ortho‐Quinone Methides as Reactive Intermediates in Asymmetric Brønsted Acid Catalyzed Cycloadditions with Unactivated Alkenes by Exclusive Activation of the Electrophile

An efficient method for the highly enantioselective synthesis of chiral chromanes bearing multiple stereogenic centers was developed. A chiral BINOL‐based N‐triflylphosphoramide proved to be an effective catalyst for the in situ generation of ortho‐quinone methides (o‐QMs) and their subsequent cycloaddition reaction with unactivated alkenes provided chromanes with excellent diastereo‐ and enantioselectivity.     

Scalable Synthesis of Piperazines Enabled by Visible‐Light Irradiation and Aluminum Organometallics

The development of more active C? H oxidation catalysts has inspired a rapid, scalable, and stereoselective assembly of multifunctional piperazines through a [3+3] coupling of azomethine ylides. A combination of visible‐light irradiation and aluminum organometallics is essential to promote this transformation, which introduces visible‐light photochemistry of main‐group organometallics and sets the basis for new and promising catalysts.