全文获取类型
收费全文 | 1317篇 |
免费 | 96篇 |
国内免费 | 58篇 |
专业分类
化学 | 752篇 |
晶体学 | 12篇 |
力学 | 59篇 |
综合类 | 1篇 |
数学 | 202篇 |
物理学 | 445篇 |
出版年
2023年 | 6篇 |
2022年 | 8篇 |
2021年 | 20篇 |
2020年 | 21篇 |
2019年 | 13篇 |
2018年 | 11篇 |
2017年 | 8篇 |
2016年 | 30篇 |
2015年 | 27篇 |
2014年 | 47篇 |
2013年 | 83篇 |
2012年 | 66篇 |
2011年 | 90篇 |
2010年 | 56篇 |
2009年 | 65篇 |
2008年 | 78篇 |
2007年 | 73篇 |
2006年 | 75篇 |
2005年 | 54篇 |
2004年 | 53篇 |
2003年 | 51篇 |
2002年 | 78篇 |
2001年 | 48篇 |
2000年 | 50篇 |
1999年 | 16篇 |
1998年 | 14篇 |
1997年 | 19篇 |
1996年 | 30篇 |
1995年 | 17篇 |
1994年 | 23篇 |
1993年 | 23篇 |
1992年 | 18篇 |
1991年 | 15篇 |
1990年 | 8篇 |
1989年 | 20篇 |
1988年 | 10篇 |
1987年 | 10篇 |
1985年 | 11篇 |
1984年 | 9篇 |
1983年 | 6篇 |
1982年 | 8篇 |
1980年 | 9篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1976年 | 10篇 |
1975年 | 14篇 |
1974年 | 4篇 |
1972年 | 5篇 |
1965年 | 6篇 |
排序方式: 共有1471条查询结果,搜索用时 15 毫秒
941.
Cheung FK Lin C Minissi F Crivillé AL Graham MA Fox DJ Wills M 《Organic letters》2007,9(22):4659-4662
A series of Ru(II) catalysts were prepared and tested in the asymmetric transfer hydrogenation of ketones. The catalyst containing a "4-carbon" tether gave the fastest rates of ketone reduction. This is due to both increased rate of regeneration of hydride "Ru-H" and increased rate of ketone reduction. Several classes of ketone were reduced in enantiomeric excesses of up to 97%. Substituents on the arene ring of the catalyst influence the reaction rate and enantioselectivity. 相似文献
942.
Wong WL Cheung KC Chan PH Zhou ZY Lee KH Wong KY 《Chemical communications (Cambridge, England)》2007,(21):2175-2177
A novel Re(I) complex covalently anchored with a pyrrolidinium moiety was successfully synthesized and used as an efficient and recyclable catalyst in the cycloaddition of CO2 with epoxides under mild reaction conditions to give excellent isolated yield and selectivity of cyclic carbonates in pyrrolidinium ionic liquid. 相似文献
943.
Cheah MH Tard C Borg SJ Liu X Ibrahim SK Pickett CJ Best SP 《Journal of the American Chemical Society》2007,129(36):11085-11092
IR spectroelectrochemistry of Fe4{Me(CH2S)3}2(CO)8 (4Fe6S) in the nu(CO) region shows that the neutral and anion forms have all their CO groups terminally bound to the Fe atoms; however, for the dianion there is a switch of the coordination mode of at least one of the CO groups. The available structural and nu(CO) spectra are closely reproduced by density-functional theory calculations. The calculated structure of 4Fe6S2- closely mirrors that of the diiron subsite of the [Fe-Fe] hydrogenase H cluster with a bridging CO group and an open coordination site on the outer Fe atom of pairs of dithiolate-bridged Fe0FeII subunits connected by two bridging thiolates. Geometry optimization based on the all-terminal CO isomer of 4Fe6S2- does not give a stable structure but reveals a second-order saddle point ca. 11.53 kcal mol(-1) higher in energy than the CO-bridged form. Spectroelectrochemical studies of electrocatalytic proton reduction by 4Fe6S show that slow turnover from the primary reduction process (E1/2'=-0.71 V vs Ag/AgCl) involves rate-limiting protonation of 4Fe6S- followed by reduction to H:4Fe6S-. Rapid electrocatalytic proton reduction is obtained at potentials sufficient to access 4Fe6S2-, where the rate of dihydrogen elimination from the FeIIFeII core of 4Fe6S is ca. 500 times faster than that from the FeIFeI core of Fe2(mu-S(CH2)3S)(CO)6. The dramatically increased rate of electrocatalysis obtained from 4Fe6S over all previously identified model compounds appears to be related to the features uniquely common between it and the H-cluster, namely, that turnover involves the same formal redox states of the diiron unit (FeIFeII and Fe0FeII), the presence of an open site on the outer Fe atom of the Fe0FeII unit, and the thiolate-bridge to a second one-electron redox unit. 相似文献
944.
Cheran LE Cheung S Al Chawaf A Ellis JS Belsham DD MacKay WA Lovejoy D Thompson M 《The Analyst》2007,132(3):242-255
Kelvin and acoustic fields of high-frequency have been employed in the non-invasive investigation of immortalized hypothalamic neurons, in order to assess their response to different concentrations of specific drugs, toxins, a stress-reducing hormone and neurotrophic factors. In an analytical systems biology approach, this work constitutes a first study of living neuron cultures by scanning Kelvin nanoprobe (SKN) and thickness shear mode (TSM) acoustic wave techniques. N-38 hypothalamic mouse neurons were immobilized on the gold electrode of 9 MHz TSM acoustic wave devices and gold-coated slides for study by SKN. The neurons were exposed to the neurochemicals betaseron, forskolin, TCAP, and cerebrolysin. Signals were collected with the TSM in real-time mode, and with the SKN in scanning and real-time modes, as the drugs were applied at biologically significant concentrations. With the TSM, for all drugs, some frequency and resistance shifts were in the same direction, contrary to normal functioning for this type of instrument. Possible mechanisms are presented to explain this behaviour. An oscillatory signal with periodicity of approximately 2 min was observed for some neuron-coated surfaces, where the amplitude of these oscillations was altered upon application of certain neurotrophic factors. These two new techniques present novel and non-invasive electrodeless methods for detecting changes at the cellular level caused by a variety of neuroactive compounds, without killing or destroying the neurons. 相似文献
945.
Chan CP Tzang LC Sin KK Ji SL Cheung KY Tam TK Yang MM Renneberg R Seydack M 《Analytica chimica acta》2007,584(1):7-11
Advances in nanotechnology have had significant impacts in the field of biodiagnostics. In this study, we describe the novel application of dissolvable, organic and biofunctional nanocrystals for the quantitative detection of a PCR product. Fluorescein diacetate (FDA), a fluorogenic precursor of fluorescein, was milled in a solution of a polymeric surfactant to create a stable, nanosized colloid with an interface for coupling streptavidin molecules. The application of these particulate labels for the quantitative detection of biotinylated human papillomavirus (HPV) DNA, amplified in a standard PCR procedure, was demonstrated. After the affinity reaction, the FDA molecules were dissolved and concomitantly converted into fluorescein. This approach resulted in a high selectivity, short incubation times and a sensitivity up to 147 times greater than obtained from state-of-the-art, directly fluorescent-labeled streptavidins. This innovative method offers rapid detection of small amounts of nucleic acids because less target material and thus fewer PCR cycles are required. 相似文献
946.
We report on a novel method for the encapsulation of highly water soluble materials by using layer-by-layer (LbL) polyelectrolyte self-assembly. State of the art polyelectrolyte self-assembly LbL coating and encapsulation methods are only applicable to insoluble or poorly water soluble template materials, because the process is performed in water causing dissolution of the solid template. Our method extends the material spectrum to highly water soluble template materials by using non-ionized polyelectrolytes in an organic phase (reverse-phase) instead of polyelectrolyte salts in an aqueous environment. By using the reverse-phase layer-by-layer (RP-LbL) technique, we have demonstrated the direct encapsulation of proteins, glucose, vitamin C, and inorganic salts in the solid state. Multilayer deposition was proven, layer thickness was determined by AFM, and the advantage of the method to prepare powders of encapsulated materials was demonstrated. The method is simple, robust, and applicable to a broad range of substances with potential applications in several industries. 相似文献
947.
Han FS Osajima H Cheung M Tokuyama H Fukuyama T 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(11):3026-3038
The facile synthesis of linear and cyclic chiral oligo(4-alpha/beta-methyl)thiazolines is described. Linear oligothiazolines have been efficiently synthesized by the iterative formation of thiazoline rings and two-directional block condensation. The construction of 24- to 36-membered cyclic oligothiazolines was achieved through the head-to-tail cyclo-oligomerization of doubly deprotected linear fragments. Studies of the interactions of both the linear and cyclic oligomers with chiral compounds revealed that cyclic oligomers displayed a strong binding affinity towards mandelic acid, whereas linear oligomers showed a poor affinity. Linear oligomers have been proven to inhibit the cell growth of the cancer cell lines HPAC, PC-3, and HCT-116. Studies of the structure-activity relationships showed that the IC50 values are clearly dependent on both the length and the terminal functionalities of the linear oligomers. Longer derivatives showed more potent activity (e.g., hexi- and octithiazolines exhibit IC50<1 microM) against all three cancer cell lines. In sharp contrast, cyclic oligomers were inactive to all three cell lines. 相似文献
948.
Zheng Z Wong KW Lau WC Kwok RW Lau WM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(11):3187-3192
Unconventional reaction-design strategies have been developed to exploit the intriguing kinematics that occur when adsorbed organic molecules are bombarded by a beam of hyperthermal protons: kinematic energy transfer is only effective in H-->H collisions and thus only C-H bonds are cleaved. This process yields a cross-linked molecular film with its chemistry governed by the selection of appropriate precursor molecules. Unlike the conventional wet-chemistry synthesis of cross-linked polymeric films, this new route uses no chemical initiators, additives, nor catalysts, and only requires a proton beam with a kinetic energy of a few electron volts in a dry-process mode compatible with molecular-device fabrication. The reaction designs are expressed unconventionally: reaction energy is tuned by the kinetic energy of the proton beam and reactant supply is controlled precisely by the proton fluence. However, conventional considerations such as bond-strength effects on kinematic outcomes and branching-ratio statistics are also important and they can extend the reaction applicability of the kinematics concept. For example, taking advantage of the fact that COO-H bonds are stronger than C-H bonds, we show, with practical reaction conditions, synthesis results, and surface analysis using X-ray photoelectron spectroscopy and atomic force microscopy, that we can break C-H bonds without breaking COO-H and other bonds, in the production of cross-linked molecular layers with any desirable COOH concentration and with no ester nor other chemical contaminations. The new reaction-design strategies are also applicable to the synthesis of molecular layers with other functionalities such as OH, and to the synthesis of a mixture of functionalities, such as OH/COOH, with a controllable concentration ratio. 相似文献
949.
Shih-Hsien Hsu Yi-Wen Chiu Yu-Ling Lien Hon Man Lee Cheu-Pyeng Cheng 《Journal of organometallic chemistry》2007,692(24):5421-5428
A series of Cu(I) and Cu(II) complexes containing substituted ketiminate ligands was synthesized. Reaction of CuCl2 with 2 equiv. of Li[OC(Me)CHC(Me)N(Ar)] in toluene generated dark green solid of Cu[OC(Me)CHC(Me)N(Ar)]2 (1). Similarly, Cu(I) complex, {Cu[OC(Me)CHC(Me)N(Ar)]Li[OC(Me)CHC(Me)N(Ar)]}2 (2) was synthesized by reacting 2 equiv. of Li[OC(Me)CHC(Me)N(Ar)] with CuCl in toluene at room temperature for 12 h. While the reaction of CuCl with Li[OC(Me)CHC(Me)N(Ar)] in the presence of triphenylphosphine in THF solution at room temperature, a three-coordinated Cu[OC(Me)CHC(Me)N(Ar)](PPh3) (3) can be isolated in high yield. Replacing the PPh3 of 3 with N-heterocarbene (NHC) generates Cu[OC(Me)CHC(Me)N(Ar)](NHC) (4) in low yield. Complexes 2, 3, and 4 were characterized by 1H and 13C NMR spectroscopies and all molecules were structurally characterized by X-ray diffractometry. Two coordination modes of ketiminate ligands were found in the molecular structure of 2, one of which is copper-coordinated terminal ketiminates and the other is lithium-copper-coordinated bridging ketiminates. 相似文献
950.
M. J. Holowach L. E. Hochreiter F. B. Cheung 《International Journal of Heat and Fluid Flow》2002,23(6):807-822
The ability to accurately predict droplet entrainment in annular two-phase flow is required to effectively calculate the interfacial mass, momentum, and energy transfer, which characterizes nuclear reactor safety, system design, analysis, and performance. Most annular flow entrainment models in the open literature are formulated in terms of dimensionless groups, which do not directly account for interfacial instabilities. However, many researchers agree that there is a clear presence of interfacial instability phenomena having a direct impact on droplet entrainment. The present study proposes a model for droplet entrainment, based on the underlying physics of droplet entrainment from upward co-current annular film flow that is characteristic to light water reactor safety analysis. The model is developed based on a force balance and stability analysis that can be implemented into a transient three-field (continuous liquid, droplet, and vapor) two-phase heat transfer and fluid flow systems analysis computer code. 相似文献