全文获取类型
收费全文 | 1008篇 |
免费 | 14篇 |
国内免费 | 3篇 |
专业分类
化学 | 588篇 |
晶体学 | 4篇 |
力学 | 23篇 |
数学 | 144篇 |
物理学 | 266篇 |
出版年
2020年 | 8篇 |
2019年 | 7篇 |
2018年 | 8篇 |
2017年 | 6篇 |
2016年 | 17篇 |
2015年 | 13篇 |
2014年 | 11篇 |
2013年 | 57篇 |
2012年 | 41篇 |
2011年 | 53篇 |
2010年 | 20篇 |
2009年 | 24篇 |
2008年 | 21篇 |
2007年 | 33篇 |
2006年 | 33篇 |
2005年 | 28篇 |
2004年 | 27篇 |
2003年 | 24篇 |
2002年 | 35篇 |
2001年 | 16篇 |
2000年 | 28篇 |
1999年 | 21篇 |
1998年 | 13篇 |
1997年 | 8篇 |
1996年 | 18篇 |
1995年 | 24篇 |
1994年 | 20篇 |
1993年 | 15篇 |
1992年 | 14篇 |
1991年 | 7篇 |
1990年 | 24篇 |
1989年 | 15篇 |
1988年 | 19篇 |
1987年 | 8篇 |
1986年 | 23篇 |
1985年 | 22篇 |
1984年 | 16篇 |
1983年 | 9篇 |
1982年 | 12篇 |
1981年 | 15篇 |
1980年 | 15篇 |
1979年 | 16篇 |
1978年 | 16篇 |
1977年 | 15篇 |
1976年 | 12篇 |
1975年 | 22篇 |
1974年 | 20篇 |
1973年 | 21篇 |
1971年 | 8篇 |
1968年 | 6篇 |
排序方式: 共有1025条查询结果,搜索用时 15 毫秒
991.
Dr. Ellen F. Reid Dr. Vernon C. Cook Dr. David J. D. Wilson Dr. Conor F. Hogan 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(47):15907-15917
The photophysical and related properties of platinum(II) Schiff base complexes can be finely and predictably tuned over a wide range of wavelengths by small and easily implemented changes to ligand structure. A series of such complexes, differing only in the number and positioning of methoxy substituents on the phenoxy ring, were synthesised and their photophysical, electrochemical and electrochemiluminescent (ECL) properties investigated. Theoretical calculations were performed in order to gain further insight into the relationship between structure and properties in these materials. By positioning methoxy groups para and/or ortho to either the imine or the oxygen group on the ligand, electron density could be directed selectively toward the LUMO or HOMO as required. This allowed the emission colour (both photoluminescent and electrochemiluminescent) to be tuned over a wide range between 587 and 739 nm. The variation in orbital energies was also manifested in the positions of the absorption bands and the redox properties of the complexes, as well as in the NMR shifts for the uncoordinated ligands. All reported complexes displayed intense electrochemiluminescence (ECL), which could be initiated either by annihilation or co‐reactant pathways. The relationship between the electrochemical and photophysical properties and the efficiency of the ECL is discussed. For two of the complexes solid‐state ECL could be generated from electrodeposited layers of the complex. 相似文献
992.
Although ortho-esters constitute a well-known class of organic compounds, the orthoacrylates were unknown prior to a report from these laboratories in 1972. To date, this novel class of acrylic esters has not been investigated as polymerization monomers. We report here polymerization and copolymerization studies of representative members of this class of compounds. Although the orthoacrylates studied did not polymerize in the presence of conventional cationic initiators nor radical initiators, they did respond to radical-initiated copolymerization with a variety of conventional vinyl monomers. Their corresponding reactivity ratios were determined, as well as the Q and e parameters. Monomer syntheses and characterization are reported, as well as structural and properties studies on the copolymers. 相似文献
993.
Field desorption mass spectra of oligomers show peaks of mass up to 2400 amu. 相似文献
994.
Identification of ligands for RNA targets via structure-based virtual screening: HIV-1 TAR 总被引:4,自引:0,他引:4
Filikov AV Mohan V Vickers TA Griffey RH Cook PD Abagyan RA James TL 《Journal of computer-aided molecular design》2000,14(6):593-610
Binding of the Tat protein to TAR RNA is necessary for viral replication of HIV-1. We screened the Available Chemicals Directory (ACD) to identify ligands to bind to a TAR RNA structure using a four-step docking procedure: rigid docking first, followed by three steps of flexible docking using a pseudobrownian Monte Carlo minimization in torsion angle space with progressively more detailed conformational sampling on a progressively smaller list of top-ranking compounds. To validate the procedure, we successfully docked ligands for five RNA complexes of known structure. For ranking ligands according to binding avidity, an empirical binding free energy function was developed which accounts, in particular, for solvation, isomerization free energy, and changes in conformational entropy. System-specific parameters for the function were derived on a training set of RNA/ligand complexes with known structure and affinity. To validate the free energy function, we screened the entire ACD for ligands for an RNA aptamer which binds l-arginine tightly. The native ligand ranked 17 out of ca. 153,000 compounds screened, i.e., the procedure is able to filter out >99.98% of the database and still retain the native ligand. Screening of the ACD for TAR ligands yielded a high rank for all known TAR ligands contained in the ACD and suggested several other potential TAR ligands. Eight of the highest ranking compounds not previously known to be ligands were assayed for inhibition of the Tat-TAR interaction, and two exhibited a CD50 of ca. 1 M. 相似文献
995.
A new set of generalized hybrid atomic orbitals is proposed for use in the theory of molecular electronic structure. The hybrids have conceptual and (when expanded in terms of Gaussian functions) computational advantages over conventional hybrids which are essentially adapted to spherically-symmetric environments. Formaldehyde is used to illustrate the use of these orbitals. 相似文献
996.
Abe K Abt I Ahn CJ Akagi T Allen NJ Ash WW Aston D Bacchetta N Baird KG Baltay C Band HR Barakat MB Baranko G Bardon O Barklow T Bazarko AO Ben-David R Benvenuti AC Bienz T Bilei GM Bisello D Blaylock G Bogart JR Bolton T Bower GR Brau JE Breidenbach M Bugg WM Burke D Burnett TH Burrows PN Busza W Calcaterra A Caldwell DO Calloway D Camanzi B Carpinelli M Cassell R Castaldi R Castro A Cavalli-Sforza M Church E Cohn HO Coller JA Cook V Cotton R Cowan RF Coyne DG D'Oliveira A Damerell CJ 《Physical review D: Particles and fields》1996,53(5):R2271-R2275
997.
Recently, microwave studies of the rotational spectra of water and its various isotopic species have been reported. These studies provide rotational constants and among others the quartic distortion constants, which depend on the quadratic part of the vibrational potential function. These data are collected and discussed, and the molecular force field and structure of water is considered in light of this recent microwave data. The quartic distortion data gives force constants which are very reasonable considering the difficulties in the distortion analysis of these light molecules, where as many as 22 parameters are being evaluated to fit the observed spectrum. The infrared and microwave data are combined within the theoretical framework of the small oscillations model and the results compare favorably with the true harmonic force field. The infrared and microwave valence bond force constants of H2O are (mdyn/Å): The results further confirm the usefulness of rotation-vibration data in the determination of force constans, and show that even for water with extremely large anharmonicity effects, a very representative force field can be obtained by combining ground state infrared and microwave data.Various molecular structures have been evaluated, and the average structures in the ground vibrational state for H2O, D2O and T2O are found to be: A one-dimensional approximation to the anharmonicity effects is applied to determine the equilibrium molecular structure of H2O from the average structure data. The result is as follows: 相似文献
998.
The complex phase relationships near the BaO-poor region of the quaternary Ba-Sm-Y-Cu-O oxide system prepared in pure air (O2p=22 kPa, 950 °C) and in 0.1% O2 (O2p=100 Pa, 810 °C) have been determined. This investigation also included the subsolidus compatibilities in ten subsystems (Ba-Sm-Y-O, Ba-Sm-Cu-O, Ba-Y-Cu-O, Sm-Y-Cu-O, Ba-Sm-O, Ba-Y-O, Ba-Cu-O, Sm-Y-O, Sm-Cu-O, and Y-Cu-O), and the homogeneity range of five solid solutions (Ba(SmxY2−x)CuO5, (Sm,Y)2O3, (Sm,Y)2CuO4, (Y,Sm)2Cu2O5, and Ba(Sm,Y)2O4). The single phase range of the superconductor solid solution, (Ba2−xSmx)(Sm1−yYy)Cu3O6+z, and the phase compatibilities in its vicinity, which are particularly important for processing, are described in detail. The phase equilibrium data of the Ba-Sm-Y-Cu-O system will enable the improvement of the intrinsic superconducting properties of second-generation wires, and facilitate the flux-pinning process. 相似文献
999.
A method for the determination of the major components of (methoxymethyl)melamine resins, with quantitative analysis of unreacted melamine by capillary zone electrophoresis (CZE) using electrospray ionization-mass spectrometry (ESI-MS) is presented. Using a low background electrolyte (BGE) pH, components are separated according to their charge/ionic radius ratio with a distinctly different separation selectivity compared to the HPLC methods commonly employed in melamine-resin analysis. The use of a time-of-flight mass spectrometer (TOF-MS) was concluded to be necessary, as the complex samples studied required maximum sensitivity and resolution, which is clearly superior for TOF-MS detectors over their quadrupole counterparts. A standard curve of free melamine was determined with an R(2) = 0.999 over a concentration range of an order of magnitude. This method offers the unique separation selectivity of CZE as well as a quicker analysis time, especially for dimers compared to the HPLC methods used to date. 相似文献
1000.
Bernas T Asem EK Robinson JP Cook PR Dobrucki JW 《Photochemistry and photobiology》2005,81(4):960-969
The double-stranded helical structure of DNA is maintained in part by hydrogen bonds between strands and by stacking interactions between adjacent purine and pyrimidine bases in one strand. The transition (denaturation) from a double-stranded (ds) to a single-stranded (ss) form can be induced in isolated DNA or fixed cells by exposure to elevated temperatures, alkali or acids, aprotic or nonpolar solvents or some drugs. We report here that DNA denaturation can occur in situ in cell nuclei as a result of interaction between light and an intercalated dye, acridine orange or ethidium bromide. This DNA photodenaturation was probed using metachromatic properties of acridine orange and imaged by fluorescence confocal microscopy. Furthermore, an empirical kinetic model was developed to separate changes of acridine orange luminescence intensities caused by photobleaching from those that were a result of DNA denaturation. We investigated the influence of oxygen on these phenomena and propose a mechanism by which photodenaturation may occur. 相似文献