全文获取类型
收费全文 | 4373篇 |
免费 | 92篇 |
国内免费 | 11篇 |
专业分类
化学 | 3166篇 |
晶体学 | 10篇 |
力学 | 66篇 |
数学 | 739篇 |
物理学 | 495篇 |
出版年
2020年 | 32篇 |
2019年 | 31篇 |
2016年 | 62篇 |
2015年 | 71篇 |
2014年 | 62篇 |
2013年 | 160篇 |
2012年 | 134篇 |
2011年 | 171篇 |
2010年 | 101篇 |
2009年 | 109篇 |
2008年 | 132篇 |
2007年 | 137篇 |
2006年 | 157篇 |
2005年 | 151篇 |
2004年 | 173篇 |
2003年 | 128篇 |
2002年 | 137篇 |
2001年 | 72篇 |
2000年 | 70篇 |
1999年 | 71篇 |
1998年 | 55篇 |
1997年 | 67篇 |
1996年 | 73篇 |
1995年 | 69篇 |
1994年 | 72篇 |
1993年 | 66篇 |
1992年 | 81篇 |
1991年 | 52篇 |
1990年 | 50篇 |
1989年 | 48篇 |
1988年 | 60篇 |
1987年 | 43篇 |
1986年 | 64篇 |
1985年 | 68篇 |
1984年 | 87篇 |
1983年 | 66篇 |
1982年 | 65篇 |
1981年 | 85篇 |
1980年 | 88篇 |
1979年 | 69篇 |
1978年 | 89篇 |
1977年 | 60篇 |
1976年 | 71篇 |
1975年 | 56篇 |
1974年 | 53篇 |
1973年 | 64篇 |
1972年 | 44篇 |
1971年 | 40篇 |
1970年 | 39篇 |
1968年 | 25篇 |
排序方式: 共有4476条查询结果,搜索用时 15 毫秒
131.
Lumbierres M Palomo JM Kragol G Roehrs S Müller O Waldmann H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(24):7405-7415
A new flexible and efficient methodology for the solid-phase synthesis of lipidated peptides has been developed. The approach is based on the use of previously synthesized building blocks and overcomes the limitations of previously reported methods, since long doubly lipidated peptides can be synthesized by using this route. Furthermore, it was thus possible to prepare a large number of N- and H-Ras peptides bearing a wide range of reporter and/or linking groups--efficient tools for the investigation of biological processes. In terms of efficiency and flexibility this solid-phase method is superior to the solution-phase synthesis. It gives pure peptides in multimilligram amounts within a much shorter time and with superior overall yield. 相似文献
132.
Controlled anodic dissolution of copper in a separate generator cell yields well-defined concentrations of catalyst, depending on the voltage applied. This adjustable generation of copper catalyst makes it possible to determine iron over a wide range of concentration (10–1500 μg Fe3+ ml-1) via the iron(III)—thiosulphate reaction. By the copper(II)-catalyzed hydrogen peroxide—hydroquinone reaction, EDTA can be determined as an inhibitor (0.5–5 μg ml-1) and cadmium(II) as a reactivator (1–10 μg ml-1). As zinc(II) forms complexes with 2,2'-bipyridine, which activates copper in this reaction, it can be determined (5–50 μg Zn2+ ml-1) by measuring the decrease in activation. The electrogeneration of silver ion as a catalyst is also described. The sulphanilic acid—peroxodisulphate reaction is catalyzed by silver(I), which is again activated by 2,2'-bipyridine. Zinc(II) can be determined (0.29–2.9 mg Zn2+ ml-1) by the same principle as in the copper(II)-catalyzed reaction. 相似文献
133.
Herbert Alber und Maria von Renzenberg 《Fresenius' Journal of Analytical Chemistry》1931,86(1-4):114-127
Zusammenfassung Die Beobachtung von Schlieren mit dem unbewaffneten Auge, die sogenannte visuelle Methode, erweist sich als ein Verfahren, das seinen Hauptvorteil in der ausserordentlich grossen Einfachheit besitzt. Das Fehlen jeder besonderen optischen Einrichtung ist ein so erheblicher Vorteil, dass in den meisten Fällen der Nachteil einer etwas geringeren Empfindlichkeit (n =0,0001–0,0002) aufgehoben wird, wenn er nicht sogar erwünscht ist.Überall dort, wo man rasch und ohne spezielle Hilfsmittel Schlierenbeobachtungen vornehmen will, wird die visuelle Methode geeignet sein, während das Schlierenmikroskop für die Betrachtung von sehr schwachen Schlieren und besonders für das Studium von doppeltschattierten. Schlieren vorzuziehen ist.Mittels der visuellen Methode können wir auch zahlenmäßige Angaben über die Schlierenintensität machen, also Messungen der Schlierenstärke vornehmen, wobei der Messbereich des Schlierenmikroskops bei allerdings verminderter Empfindlichkeit bis zu höheren Unterschieden in den Brechungsindices von Fliess- und Standprobe (n bis 0,0250) ergänzt wird.Die praktische Verwendbarkeit der visuellen Methode ist in vielen Parallelversuchen mit dem Schlierenmikroskop und in einigen speziellen Versuchsreihen, über welche in einer der folgenden Mitteilungen berichtet werden soll, bewiesen worden.Wir haben schon an anderer Stelle bemerkt, dass es bei der hier geübten Art des Zusammenarbeitens oft schwer ist, dem Anteil gerecht zu werden, der dem einzelnen Mitarbeiter zukommt. Die vorliegende Mitteilung setzt sich aus Material zusammen, das teilweise der Doktordissertation von Frl. Maria von Rentenberg entnommen, teilweise von Dr. H. Alber selbst gesammelt worden ist. Letzterem bin ich auch für die Arbeit dankbar, die er bei der Zusammenstellung der Abhandlung geleistet hat. 相似文献
134.
Herbert Spohn 《Journal of statistical physics》1977,17(6):385-412
A single (nonrelativistic, spinless) electron subject to a constant external electric field interacts with impurities located on an infinitely extended lattice by a potential of random strength. The random strength is given by a field of Gaussian random variables. We show the existence of the averaged dynamics and prove that in the weak coupling limit, 0, 2
t= fixed, one obtains the usual transport equation for the velocity distribution.Work supported by a Max Kade Foundation fellowship.On leave of absence of the Fachbereich Physik der Universität München. 相似文献
135.
Zamudio-Rivera LS George-Tellez R López-Mendoza G Morales-Pacheco A Flores E Höpfl H Barba V Fernández FJ Cabirol N Beltrán HI 《Inorganic chemistry》2005,44(15):5370-5378
The one pot reaction of salicylaldehyde 1, beta-amino alcohols 2a-2c, and di-n-butyltin(IV) oxide 3a or diphenyltin(IV) oxide 3b produced five diorganotin(IV) compounds, 4a-4c, 5a, and 5c, in good yields. All compounds were characterized by IR, (1)H, (13)C, and (119)Sn NMR spectroscopy, and elemental analysis; furthermore, compounds 4b, 4c, 5a, and 5c were characterized by X-ray diffraction analysis. After the structural characterization, all of the compounds were tested in vitro against Bacillus subtilis (Gram-positive, strain ATCC 6633), Escherichia coli (Gram-negative, strain DH5alpha), Pseudomonas aeruginosa (Gram-negative, strain BH3), Desulfovibrio longus (strain DSM 6739), and Desulfomicrobium aspheronum (strain DSM 5918) to assess their antimicrobial activity. Compounds 4 and 5 demonstrated a wide range of bactericidal activities against the tested aerobic (one Gram-positive and two Gram-negative subtypes) and anaerobic bacteria (two sulfate-reducing bacteria, SRB). Compound 5 had better bactericidal performances than compound 4. For all of the compounds, the acute toxicity was measured using luminescent bacteria toxicity (LBT-Microtox) tests to track their further environmental impact. According to these results and in order to fulfill environmental regulations, the toxicity of the compounds studied herein can be modulated through the proper selection of the disubstituted tin(IV) moiety. 相似文献
136.
Burford N Herbert DE Ragogna PJ McDonald R Ferguson MJ 《Journal of the American Chemical Society》2004,126(51):17067-17073
Homoatomic P-P coordinate bonding is exploited to prepare the first examples of triphosphorus monocations and tetraphosphorus dications using dimethylphosphenium or diphenylphosphenium Lewis acceptors with diphosphinomethane, diphosphinoethane, diphosphinohexane, or diphosphinobenzene ligands. Solid-state structures and spectroscopic characterization data for complexes involving bis(diphenylphosphino)methane ligands show coordination of only one donor site of the diphosphine ligand in the monocations, and chelate complexation is not observed. Tetraphosphorus dications are observed with longer diphosphines, in which the ligand tethers two phosphenium acceptors. The structural preferences between monocations with pendant phosphines and tethered dications are dependent on intramolecular steric interactions and the flexibility of the tether. 相似文献
137.
The -ribofuranosylazide1 is transformed after usual derivatization by suitable protecting groups into the P–N-ylid2, which gives the corresponding N-Glykosyl-N-alkylcarbodiimides4 and a small amount of the glykosylisocyanatde-rivative3 by reaction with alkylisocyanates. The carbodiimides4 were reacted with hydrazoic acid to give the alkylaminotetrazolnucleosides5 and finally the free nucleosidanalogs6. In the case of5 c the 5-aziridinyltetrazolnucleosid5 h was formed by an usual neighbouring group reaction. In addition the compound1 is transformed into the 3,5-diprotected anchor derivative7 by reaction withTIPSCl2. The latter could be transformed by usual steps into the alkylaminotetra-zolnucleosides8 with a free 2-OH group. In the next step the 2-p-tolylthiocarbo-nates9 were prepared followed by transformation to the 2-desoxynucleosides10 by means of tributyltinhydride. Finally the free 2-desoxynucleosides11 were prepared. By reacting the carbodiimides4 with phenylisocyanate a mixture of the two possible regiouretidinonnucleosidderivatives12 and13 are formed. In the case of the N-glykosyl-N-allylcarbodiimide4 d only the one isomer13 d arises.
Herrn Prof. Dr.A. Neckel mit den besten Wünschen zum 60. Geburtstag gewidmet. 相似文献
138.
A dynamical extension of the "curvy-steps" approach to linear-scaling self-consistent field calculations is presented, which yields an extended-Lagrangian formulation of ab initio molecular dynamics. An exponential parametrization of the one-electron density matrix, expressed in terms of atom-centered Gaussian basis functions, facilitates propagation along the manifold of density matrices in a geometrically correct fashion that automatically enforces idempotency constraints. The extended Lagrangian itself is constraint free, thus neither density matrix purification nor expensive, iterative solution for Lagrange multipliers is required. Propagation is highly efficient, and time steps compare favorably to those used in Car-Parrinello molecular dynamics simulations. The behavior of the method, especially with regard to the maintenance of adiabatic decoupling of nuclei and electrons, is examined for a sequence of diatomic molecules, and comparison is made to trajectories propagated on the converged Born-Oppenheimer surface. Certain claims to the contrary notwithstanding, our results demonstrate that vibrational frequencies may depend on the value of the fictitious mass parameter, even in an atom-centered basis. Light-atom stretching frequencies can be significantly redshifted, even when the nuclear and electronic energy scales are well separated. With a sufficiently small fictitious mass and a short time step, accurate frequencies can be obtained; we characterize appropriate values of these parameters for a wide range of vibrational frequencies. 相似文献
139.
Monolithic capillary columns were prepared by copolymerization of styrene and divinylbenzene inside a 200 microm i.d. fused silica capillary using a mixture of tetrahydrofuran and decanol as porogen. Important chromatographic features of the synthesized columns were characterized and critically compared to the properties of columns packed with micropellicular, octadecylated poly(styrene-co-divinylbenzene) (PS-DVB-C18) particles. The permeability of a 60 mm long monolithic column was slightly higher than that of an equally dimensioned column packed with PS-DVB-C18 beads and was invariant up to at least 250 bar column inlet pressure, indicating the high-pressure stability of the monolithic columns. Interestingly, monolithic columns showed a 3.6 times better separation efficiency for oligonucleotides than granular columns. To study differences of the molecular diffusion processes between granular and monolithic columns, Van Deemter plots were measured. Due to the favorable pore structure of monolithic columns all kind of diffusional band broadening was reduced two to five times. Using inverse size-exclusion chromatography a total porosity of 70% was determined, which consisted of internodule porosity (20%) and internal porosity (50%). The observed fast mass transfer and the resulting high separation efficiency suggested that the surface of the monolithic stationary phase is rather rough and does not feature real pores accessible to macromolecular analytes such as polypeptides or oligonucleotides. The maximum analytical loading capacity of monolithic columns for oligonucleotides was found to be in the region of 500 fmol, which compared well to the loading capacity of the granular columns. Batch-to-batch reproducibility proved to be better with granular stationary phases compared to monolithic stationary phase, in which each column bed is the result of a unique column preparation process. 相似文献
140.
Using a mass spectrometric sampling method, we have observed the decomposition of CH4 in an rf plasma usedfor diamond deposition. The gas samples were extracted through an orifice located downstream of the plasma zone and analyzed online. For the experiments a dilute mixture of H2 and CH4 containing 0.1–3% CH4 has been used. CH4 is converted to C2H2 and C2H4 quantitatively. Small amounts of heavier hydrocarbons are formed. A comparison of the experimental results with a recent kinetic model treating a purely thermal environment is made and the differences between our experiment and the model are explained. The role of acetylene as a species formed in an atmosphere rich in atomic hydrogen is proposed. The electron impact dissociation process is suggested as the rare-determining step in the plasma-chemical decomposition of methane. 相似文献