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排序方式: 共有395条查询结果,搜索用时 15 毫秒
301.
Adam T Ferge T Mitschke S Streibel T Baker RR Zimmermann R 《Analytical and bioanalytical chemistry》2005,381(2):487-499
Pyrolysis single-photon ionisation (SPI)–time-of-flight mass spectrometry (TOFMS) and statistical analysis techniques have been applied to differentiate three major tobacco types, Burley, Virginia and Oriental, by means of the gas phase. SPI is known as a soft ionisation technique that allows fast and comprehensive on-line monitoring of a large variety of aliphatic and aromatic substances without fragmentation of the molecule ions. The tobacco samples were pyrolysed at 800°C in a nitrogen atmosphere. The resulting pyrolysis gas contained signals from more than 70 masses between m/z 5 and 170. Mass spectra obtained were analysed by principal component analysis (PCA) and linear discriminant analysis (LDA) to distinguish between different tobacco types. Prior variable reduction of the data set was carried out by calculation of the Fisher ratios. Results achieved give information about chemical composition and characteristics of the smoke derived from each tobacco type and enable conclusions on plant cultivation to be drawn. Based on LDA, a model for tobacco type recognition of unknown samples was established, which was cross-checked by additional measurements of each tobacco type. Furthermore, first results on the recognition of tobacco mixtures based on principal component regression (PCR) are presented. 相似文献
302.
Fedor A. Zhuravlev 《Tetrahedron letters》2006,47(17):2929-2932
Pd-catalyzed C-2 arylation of oxazolo[4,5-b]pyridine proceeds efficiently at 30 °C and tolerates a variety of aryl halides, including derivatized amino acids for which no racemization was observed during the reaction. Experimental evidence for facile deprotonation of oxazolo[4,5-b]pyridine under the reaction conditions is presented and the nature of the anionic intermediates is computationally examined. 相似文献
303.
Michail N. Elinson Alexey I. Ilovaisky Valentina M. Merkulova Nikita O. Stepanov Fedor M. Miloserdov Yuri N. Ogibin Gennady I. Nikishin 《Tetrahedron》2007,63(42):10543-10548
An electrochemically induced catalytic multicomponent transformation of cyclic 1,3-diketones, isatins, and malononitrile in alcohols in an undivided cell in the presence of sodium bromide as an electrolyte results in the formation of spirooxindoles with fused functionalized 5,6,7,8-tetrahydro-4H-chromene system in 83-98% yields. The application of this efficient electrocatalytic method to the formation of medicinally relevant spirocyclic (4H-chromene)-4,3′-oxindoles is beneficial from the viewpoint of diversity-oriented large-scale processes and represents novel, facile, and environmentally benign synthetic concept for multicomponent reaction strategy. 相似文献
304.
A sub-monolayer of atomic sodium was deposited on a LiF(001) surface at 40 K. The adsorbed sodium exists at the surface as single atoms and clusters. The surface was dosed with 1 L of HF, to form adsorbed (HF)2...Na(n) (n=1,2,3,...) complexes, which were then irradiated by 640 nm laser light, to induce charge-transfer reaction. The reaction-product atomic H(g) was observed leaving the surface by two-color Rydberg-atom time-of-flight (TOF) spectroscopy. The TOF spectrum of the desorbed H atoms contained two components; a "fast" component with a maximum at approximately 0.85 eV, and a "slow" component with a maximum at 0.45 eV. These two components were attributed to photoreaction on adsorbed single atoms and clusters of sodium, respectively. The fast component exhibited a structure (48+/-17 meV spacing) near the high-energy end of spectrum. This structure was attributed to vibration of NaFHF photoproduct residing on the surface. The cross section of the harpooning event in the Na...(HF)2 adsorbed complex was determined as (9.1+/-2.0)x10(-19) cm(2). To interpret the experimental vibrational structure and the relative energies of the fast and slow components of the TOF spectrum, high-level ab initio calculations were performed for reactants Na(n)...(HF)(m) (n,m=1,2) and reaction products Na(n)F(m)H(m-1). The calculated NaF-HF and Na-Na(HF)(2) bond dissociation energies indicated that photoexcitation of the precursor complexes led not only to ejection of H atoms, but also to dissociation of the Na(n)...(HF)(2) (n=1,2) species through cleavage of the NaF-HF and Na-Na(HF)(2) bonds. 相似文献
305.
Wolfgang P. Schleich Kedar S. Ranade Christian Anton Markus Arndt Markus Aspelmeyer Manfred Bayer Gunnar Berg Tommaso Calarco Harald Fuchs Elisabeth Giacobino Markus Grassl Peter Hänggi Wolfgang M. Heckl Ingolf-Volker Hertel Susana Huelga Fedor Jelezko Bernhard Keimer Jörg P. Kotthaus Gerd Leuchs Norbert Lütkenhaus Ueli Maurer Tilman Pfau Martin B. Plenio Ernst Maria Rasel Ortwin Renn Christine Silberhorn Jörg Schiedmayer Doris Schmitt-Landsiedel Kurt Schönhammer Alexey Ustinov Philip Walther Harald Weinfurter Emo Welzl Roland Wiesendanger Stefan Wolf Anton Zeilinger Peter Zoller 《Applied physics. B, Lasers and optics》2016,122(5):130
306.
Fedor V. Ryzhkov Michail N. Elinson Yuliya E. Ryzhkova Anatoly N. Vereshchagin Victor A. Korolev Mikhail P. Egorov 《Journal of heterocyclic chemistry》2021,58(3):793-804
Multicomponent reactions (MCRs) are important processes, in which more than three different reactants directly get converted into one new structure bearing most of the atoms of these reactants. It is a very powerful tool in drug discovery and combinational chemistry. A new pseudo-four-component synthetic approach to 5-(5-hydroxy-3-methyl-1H-pyrazol-4-yl)-substituted 5H-chromeno[2,3-b]pyridines with 68%–95% yields is reported. This MCR opens an efficient and convenient way to substituted 5H-chromeno[2,3-b]pyridines, which are promising compounds in medicinal chemistry and for the treatment of lung cancer through inhibition of aldo-keto reductase 1B10. A new consensus approach of molecular docking and molecular dynamics was applied for the investigation of interaction of synthesized 5H-chromeno[2,3-b]pyridines and aldo-keto reductase 1B10. 相似文献
307.
Resonance-enhanced multiphoton ionization photoelectron spectroscopy has been applied to study the electronic spectroscopy and relaxation pathways among the 3p and 3s Rydberg states of trimethylamine. The experiments used femtosecond and picosecond duration laser pulses at wavelengths of 416, 266, and 208 nm and employed two-photon and three-photon ionization schemes. The binding energy of the 3s Rydberg state was found to be 3.087 +/- 0.005 eV. The degenerate 3p x, y states have binding energies of 2.251 +/- 0.005 eV, and 3p z is at 2.204 +/- 0.005 eV. Using picosecond and femtosecond time-resolved experiments we spectrally and temporally resolved an intricate sequence of energy relaxation pathways leading from the 3p states to the 3s state. With excitation at 5.96 eV, trimethylamine is found to decay from the 3p z state to 3p x, y in 539 fs. The decay to 3s from all the 3p states takes place with a 2.9 ps time constant. On these time scales, trimethylamine does not fragment at the given internal energies, which range from 0.42 to 1.54 eV depending on the excitation wavelength and electronic state. 相似文献
308.
Subach OM Gundorov IS Yoshimura M Subach FV Zhang J Grüenwald D Souslova EA Chudakov DM Verkhusha VV 《Chemistry & biology》2008,15(10):1116-1124
We used a red chromophore formation pathway, in which the anionic red chromophore is formed from the neutral blue intermediate, to suggest a rational design strategy to develop blue fluorescent proteins with a tyrosine-based chromophore. The strategy was applied to red fluorescent proteins of the different genetic backgrounds, such as TagRFP, mCherry, HcRed1, M355NA, and mKeima, which all were converted into blue probes. Further improvement of the blue variant of TagRFP by random mutagenesis resulted in an enhanced monomeric protein, mTagBFP, characterized by the substantially higher brightness, the faster chromophore maturation, and the higher pH stability than blue fluorescent proteins with a histidine in the chromophore. The detailed biochemical and photochemical analysis indicates that mTagBFP is the true monomeric protein tag for multicolor and lifetime imaging, as well as the outstanding donor for green fluorescent proteins in F?rster resonance energy transfer applications. 相似文献
309.
Fedor Pakovich 《Geometric And Functional Analysis》2008,18(1):163-183
In this paper we give a solution of the following problem: under what conditions on infinite compact sets and polynomials f
1, f
2 do the preimages f
1−1{K
1} and f
2−1{K
2} coincide. Besides, we investigate some related questions. In particular, we show that polynomials sharing an invariant compact
set distinct from a point have equal Julia sets.
Received: May 2006, Accepted: June 2006 相似文献
310.
Aytan R. Asgarova Ali N. Khalilov Ivan Brito Abel M. Maharramov Namiq G. Shikhaliyev Jonathan Cisterna Alejandro Crdenas Atash V. Gurbanov Fedor I. Zubkov Kamran T. Mahmudov 《Acta Crystallographica. Section C, Structural Chemistry》2019,75(3):342-347
Cooperative action of hydrogen and halogen bonding in the reaction of 3‐(3,5‐di‐tert‐butyl‐4‐hydroxyphenyl)‐1‐phenylprop‐2‐en‐1‐one with HCl or HBr in alcohol medium under microwave irradiation (20 W, 80 °C, 10 min) allows the isolation of the haloetherification products (2S,3S)‐3‐(3‐tert‐butyl‐5‐chloro‐4‐hydroxyphenyl)‐2‐chloro‐3‐ethoxy‐1‐phenylpropan‐1‐one, C21H24Cl2O3, (2S,3S)‐2‐bromo‐3‐(3‐tert‐butyl‐5‐bromo‐4‐hydroxyphenyl)‐3‐methoxy‐1‐phenylpropan‐1‐one, C20H22Br2O3, and (2S,3S)‐2‐bromo‐3‐(3‐tert‐butyl‐5‐bromo‐4‐hydroxyphenyl)‐3‐ethoxy‐1‐phenylpropan‐1‐one, C21H24Br2O3, in good yields. Both types of noncovalent interactions, e.g. hydrogen and halogen bonds, are formed to stabilize the obtained products in the solid state. 相似文献