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51.
Zhengbo Zhu Dr. Minami Odagi Dr. Chenfei Zhao Dr. Khalil A. Abboud Helmi Ulrika Kirm Jaan Saame Dr. Märt Lõkov Prof. Dr. Ivo Leito Prof. Dr. Daniel Seidel 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(5):2044-2048
Acyclic ketone-derived oxocarbenium ions are involved as intermediates in numerous reactions that provide valuable products, however, they have thus far eluded efforts aimed at asymmetric catalysis. We report that a readily accessible chiral carboxylic acid catalyst exerts control over asymmetric cyclizations of acyclic ketone-derived trisubstituted oxocarbenium ions, thereby providing access to highly enantioenriched dihydropyran products containing a tetrasubstituted stereogenic center. The high acidity of the carboxylic acid catalyst, which exceeds that of the well-known chiral phosphoric acid catalyst TRIP, is largely derived from stabilization of the carboxylate conjugate base through intramolecular anion-binding to a thiourea site. 相似文献
52.
Inga Leinemann Weihao Zhang Tiit Kaljuvee Kaia Tõnsuaadu Rainer Traksmaa Jaan Raudoja Maarja Grossberg Mare Altosaar Dieter Meissner 《Journal of Thermal Analysis and Calorimetry》2014,118(2):1313-1321
The present study deals with chemical reactions and enthalpies during the synthesis of Cu2ZnSnSe4 (CZTSe) from CuSe, SnSe, and ZnSe in molten NaI as flux material in closed degassed ampoules. Differential thermal analysis (DTA) at heating rates 5 °C min?1 and cooling rates 10 °C min?1 were used for the determination of temperatures of phase transitions and/or chemical reactions. XRD and Raman analyses confirmed that the formation of CZTSe starts already at 380 °C after the melting of Se that deliberates from the transformation of CuSe to Cu1.8Se, and the CZTSe formation process impedes to a great extent due to the presence of solid NaI. After the melting of NaI, the formation of CZTSe is completed. For the determination of enthalpy values, the calibration with pure NaI was performed. The thermal effects and enthalpies were compared with the available known thermodynamical values. The specific enthalpy of exothermic Cu2ZnSnSe4 formation at 661 °C in NaI ?36 ± 3 kJ mol?1 was determined experimentally for the first time. Ternary compound Na2SnSe3 was formed during the synthesis process. NaI·2H2O, if present in NaI, was found to be a critical issue in the synthesis process of CZTSe monograin powders in molten NaI—it gave rise to the formation of oxygen-containing by-products Na2SeO4 and Na2Cu(OH)4. The complete dehydration of NaI·2H2O at T ≤ 70 °C in vacuum is necessary to avoid the formation of oxygen-containing compounds. 相似文献
53.
High yields of several thiolesters of 2-[3-(4-cyanophenyl)-4,5-dihydro-5-isoxazolyl]acetic acid are easily prepared from the corresponding oxoester by the reaction with trimethyl silylated mercaptan and aluminum chloride. The selection of mercaptans expands the previous scope of this synthetic method. 相似文献
54.
Wong A Howes AP Yates JR Watts A Anupõld T Past J Samoson A Dupree R Smith ME 《Physical chemistry chemical physics : PCCP》2011,13(26):12213-12224
Monosodium L-glutamate monohydrate, a multiple oxygen site (eight) compound, is used to demonstrate that a combination of high-resolution solid-state NMR spectroscopic techniques opens up new possibilities for (17)O as a nuclear probe of biomolecules. Eight oxygen sites have been resolved by double rotation (DOR) and multiple quantum (MQ) NMR experiments, despite the (17)O chemical shifts lying within a narrow shift range of <50 ppm. (17)O DOR NMR not only provides high sensitivity and spectral resolution, but also allows a complete set of the NMR parameters (chemical shift anisotropy and electric-field gradient) to be determined from the DOR spinning-sideband manifold. These (17)O NMR parameters provide an important multi-parameter comparison with the results from the quantum chemical NMR calculations, and enable unambiguous oxygen-site assignment and allow the hydrogen positions to be refined in the crystal lattice. The difference in sensitivity between DOR and MQ NMR experiments of oxygen in bio/organic molecules is also discussed. The data presented here clearly illustrates that a high resolution (17)O solid-state NMR methodology is now available for the study of biomolecules, offering new opportunities for resolving structural information and hence new molecular insights. 相似文献
55.
Indrek?MandreEmail author Jaan?Kalda 《The European Physical Journal B - Condensed Matter and Complex Systems》2013,86(2):56
Monte-Carlo simulations are routinely used for estimating the scaling exponents of complex systems. However, due to finite-size effects, determining the exponent values is often difficult and not reliable. Here, we present a novel technique of dealing with the problem of finite-size scaling. This new method allows not only to decrease the uncertainties of the scaling exponents, but makes it also possible to determine the exponents of the asymptotic corrections to the scaling laws. The efficiency of the technique is demonstrated by finding the scaling exponent of uncorrelated percolation cluster hulls. 相似文献
56.
Harry Alles Jaan Aarik Aleks Aidla Aurelien Fay Jekaterina Kozlova Ahti Niilisk Martti Pärs Mihkel Rähn Maciej Wiesner Pertti Hakonen Väino Sammelselg 《Central European Journal of Physics》2011,9(2):319-324
Atomic layer deposition of HfO2 on unmodified graphene from HfCl4 and H2O was investigated. Surface RMS roughness down to 0.5 nm was obtained for amorphous, 30 nm thick hafnia film grown at 180°C. HfO2 was also deposited in a two-step temperature process where the initial growth of about 1 nm at 170°C was continued up to 10–30 nm at 300°C. This process yielded uniform, monoclinic HfO2 films with RMS roughness of 1.7 nm for 10–12 nm thick films and 2.5 nm for 30 nm thick films. Raman spectroscopy studies revealed that the deposition process caused compressive biaxial strain in graphene, whereas no extra defects were generated. An 11 nm thick HfO2 film deposited onto bilayer graphene reduced the electron mobility by less than 10% at the Dirac point and by 30–40% far away from it. 相似文献
57.
58.
Ahti Niilisk Mart Moppel Martti Pärs Ilmo Sildos Taavi Jantson Tea Avarmaa Raivo Jaaniso Jaan Aarik 《Central European Journal of Physics》2006,4(1):105-116
The Raman spectroscopy method was used for structural characterization of TiO2 thin films prepared by atomic layer deposition (ALD) and pulsed laser deposition (PLD) on fused silica and single-crystal
silicon and sapphire substrates. Using ALD, anatase thin films were grown on silica and silicon substrates at temperatures
125–425 °C. At higher deposition temperatures, mixed anatase and rutile phases grew on these substrates. Post-growth annealing
resulted in anatase-to-rutile phase transitions at 750 °C in the case of pure anatase films. The films that contained chlorine
residues and were amorphous in their as-grown stage transformed into anatase phase at 400 °C and retained this phase even
after annealing at 900 °C. On single crystal sapphire substrates, phase-pure rutile films were obtained by ALD at 425 °C and
higher temperatures without additional annealing. Thin films that predominantly contained brookite phase were grown by PLD
on silica substrates using rutile as a starting material. 相似文献
59.
Kentgens AP van Eck ER Ajithkumar TG Anupõld T Past J Reinhold A Samoson A 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2006,178(2):212-219
A combined approach is presented which expands the applicability of double rotation (DOR) by overcoming its most prominent disadvantages: spinning stability and sensitivity. A new design using air-bearings for the inner rotor and a computer-assisted start-up procedure allows DOR operation over in principle unlimited time at outer rotor speeds of up to 2000Hz. Sensitivity enhancement of the DOR experiment is achieved by applying amplitude-modulated adiabatic pulses such as the double frequency sweep (DFS) before pulse excitation. Repeating the DFS enhancement and signal readout several times without allowing for spin-lattice relaxation leads to sensitivity enhancements of a factor 3 for (27)Al in various minerals. As a result, it becomes possible to study low sensitivity quadrupolar nuclei and various long duration 2D measurements can be performed routinely. Spinning is adequate to suppress residual homonuclear dipolar couplings in the spectral dimension of typical quadrupolar spin systems. In 2D-exchange spectroscopy, however, homonuclear correlation can still be established through dipolar-quadrupolar cross-terms. 相似文献
60.
Aivar Tarre Jaan Aarik Hugo Mndar Ahti Niilisk Rainer Prna Raul Rammula Teet Uustare Arnold Rosental Vino Sammelselg 《Applied Surface Science》2008,254(16):5149-5156
Atomic layer deposition of Cr2O3 thin films from CrO2Cl2 and CH3OH on amorphous SiO2 and crystalline Si(1 0 0) and -Al2O3() substrates was investigated, and properties of the films were ascertained. Self-limited growth with a rate of 0.05–0.1 nm/cycle was obtained at substrate temperatures of 330–420 °C. In this temperature range epitaxial eskolaite was formed on the -Al2O3() substrates. The predominant crystallographic orientation in the epitaxial films depended, however, on the growth temperature and film thickness. Sufficiently thick films grown on the SiO2 and Si(1 0 0) substrates contained also the eskolaite phase, but thinner films deposited at 330–375 °C on these substrates were amorphous. The growth rate data of films with different phase composition allowed a conclusion that the crystalline phase grew markedly faster than the amorphous phase did. The amorphous, polycrystalline and epitaxial films had densities of 4.9, 5.1 and 5.1–5.3 g/cm3, respectively. 相似文献