Electron Attachment to 5-Fluorouracil: The Role of Hydrogen Fluoride in Dissociation Chemistry

We investigate dissociative electron attachment to 5-fluorouracil (5-FU) employing a crossed electron-molecular beam experiment and quantum chemical calculations. Upon the formation of the 5-FU⁻ anion, 12 different fragmentation products are observed, the most probable dissociation channel being H loss. The parent anion, 5-FU⁻, is not stable on the experimental timescale (~140 µs), most probably due to the low electron affinity of FU; simple HF loss and F⁻ formation are seen only with a rather weak abundance. The initial dynamics upon electron attachment seems to be governed by hydrogen atom pre-dissociation followed by either its full dissociation or roaming in the vicinity of the molecule, recombining eventually into the HF molecule. When the HF molecule is formed, the released energy might be used for various ring cleavage reactions. Our results show that higher yields of the fluorine anion are most probably prevented through both faster dissociation of an H atom and recombination of F⁻ with a proton to form HF. Resonance calculations indicate that F⁻ is formed upon shape as well as core-excited resonances.     

Experimental and Theoretical Studies of Dissociative Electron Attachment to Metabolites Oxaloacetic and Citric Acids

In this contribution the dissociative electron attachment to metabolites found in aerobic organisms, namely oxaloacetic and citric acids, was studied both experimentally by means of a crossed-beam setup and theoretically through density functional theory calculations. Prominent negative ion resonances from both compounds are observed peaking below 0.5 eV resulting in intense formation of fragment anions associated with a decomposition of the carboxyl groups. In addition, resonances at higher energies (3–9 eV) are observed exclusively from the decomposition of the oxaloacetic acid. These fragments are generated with considerably smaller intensities. The striking findings of our calculations indicate the different mechanism by which the near 0 eV electron is trapped by the precursor molecule to form the transitory negative ion prior to dissociation. For the oxaloacetic acid, the transitory anion arises from the capture of the electron directly into some valence states, while, for the citric acid, dipole- or multipole-bound states mediate the transition into the valence states. What is also of high importance is that both compounds while undergoing DEA reactions generate highly reactive neutral species that can lead to severe cell damage in a biological environment.     

Early structural evolution of native cytochrome c after solvent removal

Electrospray ionization transfers thermally labile biomolecules, such as proteins, from solution into the gas phase, where they can be studied by mass spectrometry. Covalent bonds are generally preserved during and after the phase transition, but it is less clear to what extent noncovalent interactions are affected by the new gaseous environment. Here, we present atomic-level computational data on the structural rearrangement of native cytochrome c immediately after solvent removal. The first structural changes after desolvation occur surprisingly early, on a timescale of picoseconds. For the time segment of up to 4.2 ns investigated here, we observed no significant breaking of native noncovalent bonds; instead, we found formation of new noncovalent bonds. This generally involves charged residues on the protein surface, resulting in transiently stabilized intermediate structures with a global fold that is essentially the same as that in solution. Comparison with data from native electron capture dissociation experiments corroborates both its mechanistic postulations and our computational predictions, and suggests that global structural changes take place on a millisecond timescale not covered by our simulations.     

团簇Co4P的自旋布居数及磁学性质研究

以团簇Co4P作为非晶态合金Co-P体系的局域结构,对其进行全参数优化计算,最终得到6种优化构型.通过对团簇Co4P 6种优化构型的Mulliken自旋布居数(包括团簇原子各轨道自旋布居数、团簇轨道自旋布居数及团簇原子自旋布居数)进行分析,对团簇Co4P各优化构型的成单电子进行研究,发现Co原子的d轨道成单电子数最多;...     

High yield one-step synthesis of carbon spheres produced by dissociating individual hydrocarbons at their autogenic pressure at low temperatures

Hydrocarbons containing 5–14 carbon atoms (pentane, cyclohexane, camphorquinone, xylene, mesitylene, camphene, decahydronaphthalene, diphenylmethane, and anthracene) are individually dissociated under their autogenic pressure developed at 700 °C to produce pure carbon moieties from the respective hydrocarbon precursor. From all of the hydrocarbons, more than 99% pure carbon is obtained in spherical, filament- or egg-like microstructures. One of the key peculiarities in the thermal dissociation of various hydrocarbons, followed by solidification under autogenic pressure, is the formation of products in micrometer dimensions. It is in contrast to previous work with organometallic precursors, which yield nanometric products via a similar method. These results are compared with those obtained for the thermal dissociation of the same hydrocarbons under flow conditions. Specific systematic morphological, structural, and compositional analysis is presented for the products obtained from the thermal dissociation of diphenylmethane. A possible formation mechanism for the obtained products is provided.     

利用13C NMR技术探究叔胺溶液中HCO3-的生成

利用¹³C NMR技术对CO₂捕获叔胺溶剂进行了碳元素的定量研究,主要考察了对胺溶剂解吸热影响较大的HCO₃^-的生成规律。重点对叔胺分子结构中羟基官能团(-OH)、羟烷基数目、烷基支链及氮原子(N)所连接烷基链大小对胺溶剂生成HCO₃^-的影响。在20℃条件下分别对1 mol·L^-1具有不同CO₂负载的1-二甲基氨基-2-丙醇(1DMA2P)、N-甲基二乙醇胺(MDEA)、3-二甲氨基-1-丙醇(3DMA1P)、二乙氨基-2-丙醇(1DEA2P)、N,N-二乙基乙醇胺(DEEA)、N,N-二甲基乙醇胺(DMMEA)及三乙醇胺(TEA)溶液进行了¹³C NMR测试研究。实验结果显示:在相同浓度的叔胺水溶液中,同一CO₂负载下的叔胺-CO₂-水体系中HCO₃^-的含量顺序为:DMMEA > MDEA>3DMA1P > 1DMA2P > TEA > DEEA > 1DEA2P。通过对各叔胺分子结构中N原子的电子云密度大小及空间位阻效应分析,得出如下结论:3DMA1P分子中-OH官能团离N原子的距离大于其在DMMEA分子中的距离,导致其生成了较少的HCO₃^-;DMMEA分子中N原子上连接的烷基链小于DEEA分子中N原子上的烷基链,导致DMMEA溶液中生成了更多的HCO₃^-;MDEA分子中羟烷基数目少于TEA分子中的羟烷基数目,且MDEA比TEA多了一个甲基,导致MDEA溶液中含有更多的HCO₃^-;3DMA1P相比1DMA2P、DEEA相比1DEA2P分子中都少了一个甲基支链,导致3DMA1P溶液相比1DMA2P溶液、DEEA溶液相比1DEA2P溶液生成了更多的HCO₃^-。     

The Role of Low-Energy Electron Interactions in cis-Pt(CO)2Br2 Fragmentation

Platinum coordination complexes have found wide applications as chemotherapeutic anticancer drugs in synchronous combination with radiation (chemoradiation) as well as precursors in focused electron beam induced deposition (FEBID) for nano-scale fabrication. In both applications, low-energy electrons (LEE) play an important role with regard to the fragmentation pathways. In the former case, the high-energy radiation applied creates an abundance of reactive photo- and secondary electrons that determine the reaction paths of the respective radiation sensitizers. In the latter case, low-energy secondary electrons determine the deposition chemistry. In this contribution, we present a combined experimental and theoretical study on the role of LEE interactions in the fragmentation of the Pt(II) coordination compound cis-PtBr₂(CO)₂. We discuss our results in conjunction with the widely used cancer therapeutic Pt(II) coordination compound cis-Pt(NH₃)₂Cl₂ (cisplatin) and the carbonyl analog Pt(CO)₂Cl₂, and we show that efficient CO loss through dissociative electron attachment dominates the reactivity of these carbonyl complexes with low-energy electrons, while halogen loss through DEA dominates the reactivity of cis-Pt(NH₃)₂Cl₂.     

Tuning the electronic band structure of PCBM by electron irradiation

Tuning the electronic band structures such as band-edge position and bandgap of organic semiconductors is crucial to maximize the performance of organic photovoltaic devices. We present a simple yet effective electron irradiation approach to tune the band structure of [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) that is the most widely used organic acceptor material. We have found that the lowest unoccupied molecular orbital (LUMO) level of PCBM up-shifts toward the vacuum energy level, while the highest occupied molecular orbital (HOMO) level down-shifts when PCBM is electron-irradiated. The shift of the HOMO and the LUMO levels increases as the irradiated electron fluence increases. Accordingly, the band-edge position and the bandgap of PCBM can be controlled by adjusting the electron fluence. Characterization of electron-irradiated PCBM reveals that the variation of the band structure is attributed to the molecular structural change of PCBM by electron irradiation.     

Tailoring in thermomechanical properties of ethylene propylene diene monomer elastomer with silane functionalized multiwalled carbon nanotubes

The incorporation of silane treated multiwalled carbon nanotubes (S‐MWCNTs) is used as an effective path for tailoring thermomechanical properties of ethylene propylene diene monomer (EPDM). In this study, S‐MWCNTs were introduced into EPDM using internal dispersion kneader and two roller mixing mill. By altering the mass ratio of S‐MWCNTs from 0 to 1, thermal conductivity, thermal stability and phase transition temperatures and their respective enthalpies are discussed of the fabricated nanocomposites. It is observed that silane modification improves their dispersion and increases the interfacial bonding between MWCNTs and polymer matrix. Scanning electron microscopy along energy dispersive spectroscopy analysis is performed to confirm the silane functionalized MWCNTs are selectively distributed in the host polymer. More importantly, an important increase in mechanical properties like ultimate tensile strength and hardness is achieved through introducing silane functionalized MWCNTs. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43221.     

Microstructural and Chemical Influences of Silicate Grain-Boundary Phases in Yttria-Stabilized Zirconia

The microstructure and chemistry of grain-boundary phases in silicate-doped Y₂O₃─ZrO₂ ceramics were evaluated by analytical electron microscopy. Two different silicate compositions were used: one an aluminosilicate and the other a borosilicate glass. These grain-boundary phases had a significant impact on the grain morphology, the chemical composition of the grains, and the crystallization of second phases. These results indicate that controlled additions of specific glass phases may provide a means for tailoring the microstructure and physical properties of zirconia ceramics.     

Electron-Induced Decomposition of Uracil-5-yl O-(N,N-dimethylsulfamate): Role of Methylation in Molecular Stability

The incorporation of modified uracil derivatives into DNA leads to the formation of radical species that induce DNA damage. Molecules of this class have been suggested as radiosensitizers and are still under investigation. In this study, we present the results of dissociative electron attachment to uracil-5-yl O-(N,N-dimethylsulfamate) in the gas phase. We observed the formation of 10 fragment anions in the studied range of electron energies from 0–12 eV. Most of the anions were predominantly formed at the electron energy of about 0 eV. The fragmentation paths were analogous to those observed in uracil-5-yl O-sulfamate, i.e., the methylation did not affect certain bond cleavages (O-C, S-O and S-N), although relative intensities differed. The experimental results are supported by quantum chemical calculations performed at the M06-2X/aug-cc-pVTZ level of theory. Furthermore, a resonance stabilization method was used to theoretically predict the resonance positions of the fragment anions O⁻ and CH₃⁻.     

Boron and Nitrogen Doped Single walled Carbon Nanotubes as Possible Dilute Magnetic Semiconductors

The structure of single walled armchair and zig-zag carbon nanotubes having 70 atoms and two carbons replaced by boron or nitrogen is obtained at minium energy using HF/6-31G* molecular orbital theory. The calculations show that the ground state of the zig-zag tubes is a triplet state while for the armchair tubes it is a singlet. In the zig-zag tubes the density of states at the Fermi level is greater for the spin down states compared to the spin up state indicating that the doped tubes could be ferromagnetic.     

Theoretical Studies on the Structure of 1,2,4,5-Tetranitroimidazole

We have investigated the structure of 1,2,4,5-tetranitroimidazole ( I ) by using various levels of theories. We have optimized the geometry of I fully at AM1, PM3, HF/3-21G, HF/6-31G**, HF/6-311++G**, MP2/6-31G**, and BP86/6-31G**. A significant difference in optimized geometries is observed depending upon the calculational level, especially by including electron correlation. However, the geometry optimized by the PM3 method may be of practical use to derive molecular properties of nitroimidazole derivatives, including heat of formation and molar volume. All the theories utilized in this study agree that I is stable in the potential energy surface.     

Highly efficient electrically conductive networks in carbon‐black‐filled ternary blends through the formation of thermodynamically induced self‐assembled hierarchical structures

Highly efficient electrical conductive networks were constructed in carbon‐black (CB)‐filled polyoxymethylene (POM)–thermoplastic polyurethane (TPU)–polyamide 6 (PA6) ternary blends through the formation of a hierarchical structure composed of a minor PA6 phase as droplets inside one major phase (TPU) and CB particles localized at the TPU–PA6 interface by thermodynamically induced self‐assembly. The hierarchical structure was thermodynamically predicted on the basis of the minimization of total interfacial energies and confirmed by electron microscopy. The degrees of the TPU phase continuity before and after the addition of PA6 were determined by solvent‐extraction experiments. The percolation threshold of CB decreased by 50% compared to that in the POM–TPU binary blend because of the more efficient formation of a CB conductive network through CB‐covered PA6 domains inside the TPU phase. The hierarchical structure not only increased the electrical conductivity of the composites but also improved their thermal stability in comparison with the simple structure formed by the homogeneously dispersed CB particles in POM. The method reported in this article can offer possibilities for improving the comprehensive properties of the conductive composites and the widening of their applications. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45877.     

Morphology and micromechanical deformation behavior of styrene–butadiene block copolymers. IV. Structure–property correlation in binary block copolymer blends

The structure–property correlation in blends consisting of styrene/butadiene block copolymers forming alternating polystyrene (PS) and polybutadiene (PB) lamellae, and PS domains in rubbery matrix was investigated by different microscopic techniques (transmission electron microscopy, scanning force microscopy, and scanning electron microscopy), uniaxial tensile testing, and dynamic mechanical analysis. Unlike the pure lamellar block copolymer, the blends showed predominantly disordered wormlike morphology formed by the intermolecular mixing. These structures allowed a precise control of stiffness/toughness ratio of the blends over a wide range. The blends showed a gradual transition from predominantly viscoplastic to elastomeric behavior with increasing triblock copolymer content. The results demonstrated that the binary block copolymer blends provide the unique possibility of tailoring mechanical properties on the basis of nanostructured polymeric materials. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1219–1230, 2004     

Experimental methods in chemical engineering: Transmission electron microscopy—TEM

Thanks to an accelerating voltage in the range of 30 to 300 kV, an electron beam can pass through a thin specimen and form an image with sub-Ångström spatial resolution. When impinging on a thin crystalline specimen, the fast electrons scatter and diffract. The transmitted electron pattern depends on the local thickness, density, crystal structure, and chemical nature of the sample. The transmission electron microscope (TEM) shapes the incoming electron beam using magnetic lenses onto the specimen and, using a different set of magnetic lenses, focuses the projected electron pattern to a camera. The final image magnification and contrast are controlled using the parameters from the electron gun, apertures positioned along the optical path, and magnetic lenses. With this combination of lens and aperture, TEM offers two possible modes of operation: (a) imaging, including high-resolution electron microscopy to reveal the size, shape, crystallinity, and morphology of materials; and (b) diffraction, to determine the crystalline nature of a region of interest of a thin film, particle, or collection of particles. Chemical engineers have taken advantage of both of these modes to analyze their samples and inform their research. A bibliometric study conducted using the WoS database places TEM as one of the preferred microscopy tools to study advanced materials such as thin films, nanomaterials, and composites used in particular for the development of applications related to energy storage and conversion (catalysis, photocatalysis, electrochemistry, and batteries) and environment (adsorption, waste-water treatment, and filtration).     

Porous Si layers—Preparation, characterization and morphology

Porous silicon layers (PSL) of nano- and micro-structures were prepared by metal-assisted electroless etching of silicon in HF-oxidizing agent aqueous solutions. The effect of oxidizing agent and HF content on the characteristics of the formed porous layers was investigated. A thin Pt film was electroless deposited on p-Si〈1 0 0〉 prior to immersion in the etching solution. The properties and morphology of the PSL formed by this method were investigated by electrochemical impedance spectroscopy (EIS) scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) technique. The characteristics of the PSL were found to be affected by the constituents of the etching medium and also, the etching time. Potassium bromate (KBrO₃), potassium iodate (KIO₃), and potassium dichromate (K₂Cr₂O₇) have been used as oxidizing agents. Pt-assisted etching of p-Si for 1 h in an etching solution consisting of 22.0 M HF and 0.05 M of KBrO₃, results in the formation of nano- and micro-pores on the Si surface. The use of 0.05 M KIO₃ or K₂Cr₂O₇ as oxidizing agent has led to the formation of a deposit on the silicon surface. At relatively higher concentration [>0.05 M] of K₂Cr₂O₇ the surface deposit becomes clear and was found to consist of an insoluble passive solid-phase of K₂SiF₆ which increases the film impedance and blocks the porous structure formation. The use of higher concentration [>22 M] of HF in the etching electrolyte is accompanied by an increase in the dissolution rate of the insoluble K₂SiF₆ layer and a decrease in the PSL passivity. The experimental impedance data were fitted to theoretical data according to a proposed equivalent circuit model which accounts for the mechanism of the porous film formation at the Si/electrolyte interface.     

Different Orbitals for Different Spins: A Study of Bond Breaking in (H-Li-H)−

Calculations of electronic structure in which electrons of different spin are assigned to different orbitals have always been handicapped by difficulties arising from orbital nonorthogonality. A very simple and general way of meeting such difficulties, within the framework of classical valence bond theory, is outlined and then used in an ab initio study of the dissociation of (H-Li-H)⁻, a system recently studied by more standard methods. Just two VB structures are found to be dominant over the whole range of geometries and to offer a simple and physically transparent interpretation of the bond-breaking process.     

Solvothermal synthesis of microsphere ZnO nanostructures in DEA media

Microsphere ZnO nanostructures (ZnO-MNs) were synthesized via solvothermal method in diethanolamine (DEA) media. DEA was utilized to terminate the growth of ZnO nanoparticles which forms the ZnO-MNs. The ZnO-MNs were characterized by a number of techniques, including X-ray diffraction analysis (XRD) and field emission scanning electron microscopy (SEM). The ZnO-MNs prepared by solvothermal process at the temperature of 150 °C for 6, 12, 18, and 24 h exhibited a hexagonal (wurtzite) structure with sizes ranging from 2 to 4 μm. The growth mechanism and morphology of the ZnO-MNs were also investigated, and it was found that the ZnO-MNs were formed by ZnO nanoparticles with average particle size of 25 ± 5 nm. To show role of DEA in the formation of Zn-MNs, effect of MEA (monoethanolamine) and TEA (triethanolamine) on morphology of the final product are also investigated. The results showed that DEA is a good polymerization agent that can be used as a stabilizer in the solvothermal technique for preparing fine ZnO powder.