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排序方式: 共有321条查询结果,搜索用时 5 毫秒
121.
Carlos Afonso Jean‐Claude Tabet Gianluca Giorgi František Tureček 《Journal of mass spectrometry : JMS》2012,47(2):208-220
Copper complexes with a cyclic D‐His‐β‐Ala‐L‐His‐L‐Lys and all‐L‐His‐β‐Ala‐His‐Lys peptides were generated by electrospray which were doubly charged ions that had different formal oxidation states of Cu(I), Cu(II) and Cu(III) and different protonation states of the peptide ligands. Electron capture dissociation showed no substantial differences between the D‐His and L‐His complexes. All complexes underwent peptide cross‐ring cleavages upon electron capture. The modes of ring cleavage depended on the formal oxidation state of the Cu ion and peptide protonation. Density functional theory (DFT) calculations, using the B3LYP with an effective core potential at Cu and M06‐2X functionals, identified several precursor ion structures in which the Cu ion was threecoordinated to pentacoordinated by the His and Lys side‐chain groups and the peptide amide or enolimine groups. The electronic structure of the formally Cu(III) complexes pointed to an effective Cu(I) oxidation state with the other charge residing in the peptide ligand. The relative energies of isomeric complexes of the [Cu(c‐HAHK + H)]2+ and [Cu(c‐HAHK ? H)]2+ type with closed electronic shells followed similar orders when treated by the B3LYP and M06‐2X functionals. Large differences between relative energies calculated by these methods were obtained for open‐shell complexes of the [Cu(c‐HAHK)]2+ type. Charge reduction resulted in lowering the coordination numbers for some Cu complexes that depended on the singlet or triplet spin state being formed. For [Cu(c‐HAHK ? H)]2+ complexes, solution H/D exchange involved only the N–H protons, resulting in the exchange of up to seven protons, as established by ultra‐high mass resolution measurements. Contrasting the experiments, DFT calculations found the lowest energy structures for the gas‐phase ions that were deprotonated at the peptide Cα positions. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
122.
Ionic liquids (ILs) are a class of ionic, nonmolecular solvents which remain in liquid state at temperatures below 100 °C. ILs possess a variety of properties including low to negligible vapor pressure, high thermal stability, miscibility with water or a variety of organic solvents, and variable viscosity. IL-modified silica as novel high-performance liquid chromatography (HPLC) stationary phases have attracted considerable attention for their differential behavior and low free-silanol activity. Indeed, around 21 surface-confined ionic liquids (SCIL) stationary phases have been developed in the last six years. Their chromatographic behavior has been studied, and, despite the presence of a positive charge on the stationary phase, they showed considerable promise for the separation of neutral solutes (not only basic analytes), when operated in reversed phase mode. This aspect points to the potential for truly multimodal stationary phases. This review attempts to summarize the state-of-the-art about SCIL phases including their preparation, chromatographic behavior, and analytical performance. 相似文献
123.
Mateus de Freitas Virgílio Pereira José Manoel Balthazar Davi Antônio dos Santos Angelo Marcelo Tusset Davi Ferreira de Castro Igor Afonso Acampora Prado 《Nonlinear dynamics》2017,87(3):1653-1666
This paper presents a polynomial chaos-based framework for designing optimal linear feedback control laws for nonlinear systems with stochastic parametric uncertainty. The spectral decomposition of the original stochastic dynamical model in an orthogonal polynomial basis, prescribed by the Wiener–Askey scheme, provides a deterministic model from which the optimal linear control law is designed. Optimality of the proposed control law is proved by solving the Hamilton–Jacobi–Bellman equation, and asymptotic stability of the controlled nonlinear systems is guaranteed in the Lyapunov sense. We are especially interested in synchronization of chaotic systems. For this reason, the control strategy is applied in the trajectory tracking of periodic orbits for the Duffing oscillator and the Rössler system with uncertain stochastic parameters and initial conditions. The results are verified with Monte Carlo simulations. 相似文献
124.
A.S. Gouveia-Neto L.A. Bueno A.C.M. Afonso J.F. Nascimento E.B. Costa Y. Messaddeq S.J.L. Ribeiro 《Journal of Non》2008,354(2-9):509-514
Energy-transfer excited upconversion luminescence in Ho3+/Yb3+- and Tb3+/Yb3+-codoped PbGeO3–PbF2–CdF2 glass and glass–ceramic under infrared excitation is investigated. In Ho3+/Yb3+-codoped samples, green (545 nm), red (652 nm), and near-infrared (754 nm) upconversion emission corresponding to the 5S2(5F4) → 5I8, 5F5 → 5I8, and 5S2(5F4) → 5I7 transitions, respectively, was observed. Blue (490 nm) emission assigned to the 5F2,3 → 5I8 transition was also detected. In the Tb3+/Yb3+-codoped system, bright UV–visible emission around 384, 415, 438, 473–490, 545, 587, and 623 nm, identified as due to the 5D3(5G6) → 7FJ(J = 6, 5, 4) and 5D4 → 7FJ(J = 6, 5, 4, 3) transitions, was measured. The comparison of the upconversion process in glass ceramic and its glassy precursor revealed that the former samples present much higher upconversion efficiencies. The dependence of the upconversion emission upon pump power, and doping contents was also examined. The results indicated that successive energy-transfer between ytterbium and holmium ions and cooperative energy-transfer between ytterbium and terbium ions followed by excited-state absorption are the dominant upconversion excitation mechanisms herein involved. The viability of using the samples for three-dimensional solid-state color displays is also discussed. 相似文献
125.
Lídia A. S. Cavaca Rafael F. A. Gomes Carlos A. M. Afonso 《Molecules (Basel, Switzerland)》2022,27(5)
The presence of sulfur–carbon bonds is transversal to several areas of chemistry, e.g., drug discovery, materials, and chemical biology. However, a lack of efficient and sustainable procedures for the preparation of thioaminals, the N,S-analogues of O,O-acetals, contributes to this functional group often being overlooked by the scientific community. In this work is described the formation of thioaminals in water promoted by copper(II) triflate. 相似文献
126.
127.
Ana Carolina P. Afonso Patricia Gonzlez-Berdullas Joaquim C. G. Esteves da Silva Luís Pinto da Silva 《Molecules (Basel, Switzerland)》2022,27(24)
Marine Coelenterazine is one of the most well-known chemi-/bioluminescent systems, and in which reaction the chemi-/bioluminophore (Coelenteramide) is generated and chemiexcited to singlet excited states (leading to light emission). Recent studies have shown that the bromination of compounds associated with the marine Coelenterazine system can provide them with new properties, such as anticancer activity and enhanced emission. Given this, our objective is to characterize the photophysical properties of a previously reported brominated Coelenteramide analog, by employing a combined experimental and theoretical approach. To better analyze the potential halogen effect, we have also synthesized and characterized, for the first time, two new fluorinated and chlorinated Coelenteramide analogs. These compounds show similar emission spectra in aqueous solution, but with different fluorescence quantum yields, in a trend that can be correlated with the heavy-atom effect (F > Cl > Br). A blue shift in emission in other solvents is also verified with the F–Cl–Br trend. More relevantly, the fluorescence quantum yield of the brominated analog is particularly sensitive to changes in solvent, which indicates that this compound has potential use as a microenvironment fluorescence probe. Theoretical calculations indicate that the observed excited state transitions result from local excitations involving the pyrazine ring. The obtained information should be useful for the further exploration of halogenated Coelenteramides and their luminescent properties. 相似文献
128.
129.
Atmospheric solid analysis probe mass spectrometry vs electrospray tandem mass spectrometry of polydimethylsiloxanes in positive and negative ionization modes 下载免费PDF全文
130.