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31.
Oh Kuen Kwon Jun Ha Lee Jungchul Park Ki-Sub Kim Jeong Won Kang 《Current Applied Physics》2013,13(2):360-365
A tunable graphene-nanoribbon (GNR)-resonator was investigated via classical molecular dynamics simulations. Resonance frequencies increased with increasing externally applied gate-force and axial-strain, and could be tuned above several hundred GHz. Tunable resonance frequencies achieved from the gate force were higher than those achieved from the axial-strain. The operating frequencies of GNR-resonators without axial-strain or with small axial-strains were most widely tuned by the gate, and almost linearly increased with increasing mean deflection. As the axial strain increased, the tunable ranges of the GNR-resonators were exponentially decreased, although the operating frequencies increased. GNR-resonators without axial-strain could be applied to wide-range-tuners, whereas GNR-resonators with high axial-strain could be applied to high-frequency-fine-tuners. 相似文献
32.
Sang Sub Kim Sun-Woo Choi Han Gil Na Dong Sub Kwak Yong Jung Kwon Hyoun Woo Kim 《Current Applied Physics》2013,13(3):526-532
ZnO–SnO2 branch–stem nanostructures were realized on a basis of a two-step process. In step 1, SnO2-stem nanowires were synthesized. In step 2, ZnO-branch nanowires were successfully grown on the SnO2-stem nanowires through a simple evaporation technique. We have pre-deposited thin Au layers on the surface of SnO2 nanowire stems and subsequently evaporated Zn powders on the nanowires. The ZnO branches, which sprouted from the SnO2 stems, had diameters in a range of 30–35 nm. As-synthesized branches were of single crystalline hexagonal ZnO structures. Since the branch tips were comprised of Au-containing nanoparticles, the Au-catalyzed vapor–liquid–solid growth mechanism was more likely to control the growth process of the ZnO branches. To test a potential use of ZnO–SnO2 branch–stem nanostructures in chemical gas sensors, their sensing performances with respect to NO2 gas were investigated, showing the promising potential in chemical gas sensors. 相似文献
33.
Isovaleroyl oxokadsurane, a novel dibenzocyclooctadiene lignan pos-sessing a spirobenzofuranoid skeleton was isolated from the stems of Kadsura coccinea.Its structure and relative configuration were determined by X-ray diffraction analysis. 相似文献
34.
35.
Sungchil Kang Alexander Efremov Sun Jin Yun Jinyoung Son Kwang-Ho Kwon 《Plasma Chemistry and Plasma Processing》2013,33(2):527-538
An investigation of etching behaviors for Mo and Al2O3 thin films in O2/Cl2/Ar inductively coupled plasmas at constant gas pressure (6 mTorr), input power (700 W) and bias power (200 W) was carried out. It was found that an increase in Ar mixing ratio for Cl2/Ar plasma results in non-monotonic etching rates with the maximums of 160 nm/min at 60 % Ar for Mo and 27 nm/min at 20 % Ar for Al2O3. The addition of O2 in the Cl2/Ar plasma causes the non-monotonic Mo etching rate (max. 320 nm/min at 40–45 % O2) while the Al2O3 etching rate decreases monotonically. The model-based analysis of etching kinetics allows one to relate the non-monotonic etching rates in Cl2/Ar plasma to the change in the etching regime from the ion-flux-limited mode (at low Ar mixing ratios) to the neutral-flux-limited mode (for high Ar mixing ratios). In the Cl2/O2/Ar plasma, the non-monotonic Mo etching rate is probably due to the change in reaction probability. 相似文献
36.
Chang Jun Zou Quan Wu Tang Gui Hong Lan Qiang Tian Tai Yu Wang 《Journal of inclusion phenomena and macrocyclic chemistry》2013,76(1-2):61-68
For the first time, hydroxypropyl-β-cyclodextrin (HP-β-CD) has been brought in to include 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) in order to enhance inhibition efficiency of PBTCA, which leads a new approach to study oil–gas field corrosion inhibition in the process of acid treatment. Based on the host–guest inclusion reaction, the inclusion complex of PBTCA with HP-β-CD has been prepared in the laboratory. UV–Vis absorption spectrum was applied to study the inclusion behavior of PBTCA with HP-β-CD. The results revealed that PBTCA with HP-β-CD can form a 1:1 stoichiometry inclusion complex. The 1:1 inclusion complex synthesized by using lyophilization was further characterized by Fourier transform infrared spectroscopy. Besides, inhibition effect of the inclusion complex on the corrosion inhibition of Q235 carbon steel has been investigated in 0.1 M sulfuric acid (H2SO4) solution using potentiodynamic polarization, electrochemical impedance spectroscopy techniques and scanning electron microscopy (SEM). It was found that the presence of the inclusion complex better achieved the anti-corrosion property in aggressive medium than was the case with alone PBTCA and the highest inhibition efficiency of the inclusion complex over 90 % was obtained, which are suggestive of the active effect of the inclusion complex for improving inhibition efficiency of PBTCA. Meanwhile, the results obtained from SEM further showed that the inclusion complex acts as a more efficient corrosion inhibitor for Q235 carbon steel in H2SO4 medium. 相似文献
37.
Qiang Fu Xianjie Wang Jiecai Han Jun Zhong Tongrui Zhang Tai Yao Chengyan Xu Tangling Gao Shibo Xi Ce Liang Lingling Xu Ping Xu Bo Song 《Angewandte Chemie (International ed. in English)》2021,60(1):259-267
To ensure sustainable hydrogen production by water electrolysis, robust, earth‐abundant, and high‐efficient electrocatalysts are required. Constructing a hybrid system could lead to further improvement in electrocatalytic activity. Interface engineering in composite catalysts is thus critical to determine the performance, and the phase‐junction interface should improve the catalytic activity. Here, we show that nickel diphosphide phase junction (c‐NiP2/m‐NiP2) is an effective electrocatalyst for hydrogen production in alkaline media. The overpotential (at 10 mA cm?2) for NiP2‐650 (c/m) in alkaline media could be significantly reduced by 26 % and 96 % compared with c‐NiP2 and m‐NiP2, respectively. The enhancement of catalytic activity should be attributed to the strong water dissociation ability and the rearrangement of electrons around the phase junction, which markedly improved the Volmer step and benefited the reduction process of adsorbed protons. 相似文献
38.
Eun Kee Cho Phong K. Quach Yunfei Zhang Jae Hun Sim Tristan H. Lambert 《Chemical science》2022,13(8):2418
The use of hydrazine-catalyzed ring-closing carbonyl–olefin metathesis (RCCOM) to synthesize polycyclic heteroaromatic (PHA) compounds is described. In particular, substrates bearing Lewis basic functionalities such as pyridine rings and amines, which strongly inhibit acid catalyzed RCCOM reactions, are shown to be compatible with this reaction. Using 5 mol% catalyst loadings, a variety of PHA structures can be synthesized from biaryl alkenyl aldehydes, which themselves are readily prepared by cross-coupling.Hydrazine catalysis enables the ring-closing carbonyl–olefin metathesis (RCCOM) to form polycyclic heteroaromatics, especially those with basic functionality.Polycyclic heteroaromatic (PHA) structures comprise the core framework of many valuable compounds with a diverse range of applications (Fig. 1A).1 For example, polycyclic azines (e.g. quinolines) are embedded in many alkaloid natural products, including diplamine2 and eupolauramine3 to name just a few. These types of structures are also of interest for their biological activity, such as with the inhibitor of the Src-SH3 protein–protein interaction shown in Fig. 1A.4 Many nitrogenous PHAs are also useful as ligands for transition metal catalysis, as exemplified by the widely used ligand 1,10-phenanthroline.5 Meanwhile, chalcogenoarenes6 such as dinaphthofuran7 and benzodithiophene8 have attracted high interest for both their medicinal properties9 and especially for their potential use as organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), and organic field-effect transistors (OFETs).10 These and numerous other examples have inspired the development of a wide variety of strategies to construct PHAs.1,11–14 Although these approaches are as varied as the structures they target, the wide range of molecular configurations within PHA chemical space and the challenges inherent in exerting control over heteroatom position and global structure make novel syntheses of these structures a topic of continuing interest.Open in a separate windowFig. 1(A) Examples of PHAs. (B) RCCOM strategy for PHA synthesis. (C) Lewis base inhibition for Lewis acid vs. hydrazine catalyzed RCCOM. (D) Hydrazine-catalyzed RCCOM for PHA synthesis.One potentially advantageous strategy for PHA synthesis is the use of ring-closing carbonyl–olefin metathesis15 (RCCOM) to forge one of the PHA rings, starting from a suitably disposed alkenyl aldehyde precursor 2 that can be easily assembled by cross-coupling (Fig. 1B). In related work, the application of RCCOM to form polycyclic aromatic hydrocarbons (PAHs) was reported by Schindler in 2017.16 In this case, 5 mol% FeCl3 catalyzed the metathesis of substrates to form phenanthrenes and related compounds in high yields at room temperature. This method was highly attractive for its efficiency, its use of an earth-abundant metal catalyst, and the production of benign acetone as the only by-product. Nevertheless, one obvious drawback to the use of Lewis acid activation is that the presence of any functionality that is significantly more Lewis basic than the carbonyl group can be expected to strongly inhibit these reactions (Fig. 1C). Such a limitation thus renders this method incompatible with a wide swath of complex molecules, especially PHAs comprised of azine rings. This logic argues for a mechanistically orthogonal RCCOM approach that allows for the synthesis of PHA products with a broader range of ring systems and functional groups.We have developed an alternative approach to catalytic carbonyl–olefin metathesis that makes use of the condensation of 1,2-dialkylhydrazines 5 with aldehydes to form hydrazonium ions 6 as the key catalyst–substrate association step.17–19 This interaction has a much broader chemoorthogonality profile than Lewis acid–base interactions and should thus be much less prone to substrate inhibition than acid-catalyzed approaches. In this Communication, we demonstrate that hydrazine-catalyzed RCCOM enables the rapid assembly of PHAs bearing basic functionality (Fig. 1D).For our optimization studies, we chose biaryl pyridine aldehyde 7 as the substrate (20 salt 11 was also productive (entry 2), albeit somewhat less so. Notably, iron(iii) chloride generated no conversion at either ambient or elevated temperatures (entries 3 and 4). Trifluoroacetic acid (TFA) was similarly ineffective (entry 5). Meanwhile, a screen of various solvents revealed that, while the transformation could occur in a range of media (entries 6–9), THF was optimal. Finally, by raising the temperature to 90 °C (entry 10) or 100 °C (entry 11), up to 96% NMR yield (85% isolated yield) of adduct 8 could be obtained in the same time period.Optimization studiesa
Open in a separate windowaConditions: substrate 8 (0.2 mmol) and 5 mol% catalyst in 0.4 mL of solvent (0.5 M) in a 5 mL sealed tube were heated to the temperature indicated for 15 h. Yields were determined by 1H NMR using CH2Br2 as an internal standard.b2 equiv. of TFA was used.c85% isolated yield.Using the optimized conditions, we explored the synthesis of various PHAs (Fig. 2). In addition to benzo[h]isoquinoline (8), products 12 and 13 with fluorine substitution at various positions could be generated in good yields. Similarly, benzoisoquinolines 14 and 15 bearing electron-donating methoxy groups and the dioxole-fused product 16 were also accessed efficiently. Furthermore, a phenolic ether product 17 with a potentially acid-labile N-Boc group was generated in modest yield. We found that an even more electron-donating dimethylamino group was also compatible with this chemistry, allowing for the production of 18 in 68% yield. On the other hand, adduct 19 bearing a strongly electron-withdrawing trifluoromethyl group was isolated in only modest yield. The naphtho-fused isoquinoline 20 could be generated as well; however, 20 mol% catalyst was required to realize a 35% yield. The thiophene-fused product 21 was furnished in much better yield, also with the higher catalyst loading. Although not a heterocyclic system, we found that the reaction to form phenanthrene (22) was well-behaved, providing that compound in 83% yield. In addition, an amino-substituted phenanthrene 23 was also formed in good yield. Other thiophene-containing PAHs such as 24–26 were produced efficiently. On the other hand, adduct 27 was generated only in low yield. Naphthofuran (28), which is known to have antitumor and oestrogenic properties,21 was synthesized in good yield. Finally, pharmaceutically important structures such as benzocarbazole2229 and naphthoimidazole2330 could be accessed in moderate yields with increased catalyst loading.Open in a separate windowFig. 2Substrate scope studies for hydrazine 1-catalyzed RCCOM synthesis of polycyclic heteroaromatics. a Conditions: substrate and catalyst 1·(TFA)2 (5 mol%) in THF (0.5 M) were heated to 100 °C in a 5 mL sealed tube for 15 h. Yields were determined on purified products. b 20 mol% catalyst.We also examined the scope of the olefin substitution pattern ( Entry Substrate Time (h) Yield (%) 1 15 96 2 48 5 3b 48 27 4 48 54 5 48 64
Entry | Catalyst | Solvent | Temp. (°C) | 8 yield (%) |
---|---|---|---|---|
1 | 10 | THF | 80 | 67 |
2 | 11 | THF | 80 | 53 |
3 | FeCl3 | DCE | rt | 0 |
4 | FeCl3 | DCE | 80 | 0 |
5 | TFA | THF | 80 | 0b |
6 | 10 | i-PrOH | 80 | 31 |
7 | 10 | CH3CN | 80 | 28 |
8 | 10 | EtOAc | 80 | 26 |
9 | 10 | Toluene | 80 | 24 |
10 | 10 | THF | 90 | 87 |
11 | 10 | THF | 100 | 96c |