全文获取类型
收费全文 | 4036篇 |
免费 | 225篇 |
国内免费 | 16篇 |
专业分类
化学 | 2591篇 |
晶体学 | 61篇 |
力学 | 78篇 |
数学 | 505篇 |
物理学 | 1042篇 |
出版年
2023年 | 40篇 |
2022年 | 58篇 |
2021年 | 79篇 |
2020年 | 135篇 |
2019年 | 125篇 |
2018年 | 45篇 |
2017年 | 53篇 |
2016年 | 189篇 |
2015年 | 152篇 |
2014年 | 133篇 |
2013年 | 179篇 |
2012年 | 225篇 |
2011年 | 279篇 |
2010年 | 147篇 |
2009年 | 130篇 |
2008年 | 211篇 |
2007年 | 206篇 |
2006年 | 161篇 |
2005年 | 168篇 |
2004年 | 151篇 |
2003年 | 85篇 |
2002年 | 117篇 |
2001年 | 52篇 |
2000年 | 58篇 |
1999年 | 46篇 |
1998年 | 38篇 |
1997年 | 40篇 |
1996年 | 46篇 |
1995年 | 37篇 |
1994年 | 48篇 |
1993年 | 35篇 |
1992年 | 28篇 |
1991年 | 41篇 |
1990年 | 36篇 |
1989年 | 19篇 |
1988年 | 41篇 |
1987年 | 49篇 |
1986年 | 24篇 |
1985年 | 43篇 |
1984年 | 31篇 |
1983年 | 31篇 |
1982年 | 27篇 |
1981年 | 40篇 |
1980年 | 31篇 |
1979年 | 30篇 |
1978年 | 27篇 |
1976年 | 24篇 |
1975年 | 26篇 |
1974年 | 19篇 |
1973年 | 19篇 |
排序方式: 共有4277条查询结果,搜索用时 0 毫秒
21.
Traditional O.R. systems are compared with problem solving in Artificial Intelligence via Expert Systems. The discussion centers on explicit knowledge representation. The general aspects are illustrated by two planning systems:
- LESP 2: A learning system for inspection plan generation
- IDA: A system for finding functions and solutions in construction
22.
23.
W. Fleischhacker K. Florkova B. Richter 《Monatshefte für Chemie / Chemical Monthly》1994,125(3):345-353
Summary Following our concept of rather new structural variations in the nitrogen region of morphine alkaloids we describe the first synthesis of N/C-10-bridged tertiary bases. 相似文献
24.
Summary The capillary gas chromatographic retention behavior of -pinene and tricyclene has been investigated on stationary phases of different polarities. On all but one of the columns employed, tricyclene eluted before -pinene; only permethylated -cyclodextrins dissolved in moderately polar polisiloxanes gave a reversed elution order. The intermolecular interactions which caused the unexpected retention behavior were investigated in detail, applying methods of computer simulation. To achieve this, we have developed a calculation algorithm on the basis of molecular mechanical optimizations and programmed it in a macro. This makes it possible to systematically investigate a given configuration space in which all the possible interactions can take place. It was shown that permethylated -cyclodextrin as host molecule for both guest molecules offers an optimum cavity size. As a result the number of energetically favorable contacts between host and guest molecules as well as the strength of the interactions in this stationary phase were larger. As a consequence the elution order, normally only influenced by the vapor pressure of the compounds at a given temperature, was changed. Nonspecific interactions played an especially important role for these kinds of substances. 相似文献
25.
A. Weisz E. Iberkleid A. Mandelbaum W. Blum W. J. Richter 《Journal of mass spectrometry : JMS》1987,22(1):3-5
Low energy CID mass spectra of m/z 173, C8H13O4+, obtained from the diethyl ester of cis,syn,cis-l,2,3,3a,4,5,5a,6,7,8-decahydroindacene-4,5-dicarboxylic acid and cis,syn,cis-l,2,3,4,5,6,7,8,8a,9,10,10a-dodecahydrophenanthrene-9,10-dicarboxylic acid indicate they have the structure of protonated diethyl maleate. This finding together with previous deuterium labelling results suggest that the formation of this ion from both precursors takes place by migration of a hydrogen atom from an allylic δ-position followed by the concerted cleavage of two allylic C? C bonds in analogy to the ground state retro-Diels–Alder fragmentation. 相似文献
26.
Klein SR Himel TM Abrams G Amidei D Baden AR Barklow T Boyarski AM Boyer J Burchat PR Burke DL Butler F Dorfan JM Feldman GJ Gidal G Gladney L Gold MS Goldhaber G Golding L Haggerty J Hanson G Hayes K Herrup D Hollebeek RJ Innes WR Jaros JA Juricic I Kadyk JA Karlen D Lankford AJ Larsen RR LeClaire BW Levi M Lockyer NS Lüth V Matteuzzi C Nelson ME Ong RA Perl ML Petersen A Richter B Riles K Rowson PC Schaad T Schellman H Schmidke WB Sheldon PD Trilling GH de la Vaissiere C Wood DR Yelton JM 《Physical review letters》1987,58(7):644-647
27.
The phase-conjugate reflectivity obtainable by degenerate four-wave mixing in silicon at a 1.06 m is calculated including free-carrier absorption. A maximum reflectivity of more than 100% is possible. The dependence of the reflectivity on the signal and pump energy densities up to 70 mJ/cm2 is measured and found to agree with theory. The wave-front-correction property of DFWM is demonstrated with a lens in the signal beam. 相似文献
28.
Zepeck F Gräwert T Kaiser J Schramek N Eisenreich W Bacher A Rohdich F 《The Journal of organic chemistry》2005,70(23):9168-9174
[Structure: see text]. The IspG protein is known to catalyze the transformation of 2-C-methyl-d-erythritol 2,4-cyclodiphosphate into 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate in the nonmevalonate pathway of isoprenoid biosynthesis. We have found that the apparent IspG activity in the cell extracts of recombinant Escherichia coli cells as observed by a radiochemical assay can be enhanced severalfold by coexpression of the isc operon which is involved in the assembly of iron-sulfur clusters. The recombinant protein was isolated by affinity chromatography under anaerobic conditions. With a mixture of flavodoxin, flavodoxin reductase, and NADPH as the reducing agent, stringent assay methods based on photometry or on 13C NMR detection of multiply 13C-labeled substrate/product ratios afforded catalytic activities greater than 60 nmol mg(-1) min(-1) for the protein "as isolated" (i.e., without reconstitution of any kind). Lower apparent activities were found using photoreduced deazaflavin as an artifactual electron donor, whereas dithionite was unable to serve as an artificial electron donor. The apparent Michaelis constant for 2-C-methyl-D-erythritol 2,4-cyclodiphosphate was 700 microM. The enzyme was inactivated by EDTA and could be reactivated by Mn2+. The pH optimum was at 9.0. The protein contained 2.4 iron ions and 4.4 sulfide ions per subunit. The replacement of any of the three conserved cysteine residues afforded mutant proteins which were devoid of catalytic activity and contained less than 6% of Fe2+ and less than 23% of S2- as compared to the wild-type protein. Sequence comparison indicates that putative IspG proteins of plants, the apicomplexan protozoan Plasmodium falciparum, and bacteria from the Bacteroidetes/Chlorobi group contain an insert of about 170-320 amino acid residues as compared with eubacterial enzymes. 相似文献
29.
Langer P Armbrust H Eckardt T Magull J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(6):1443-1455
The Lewis acid mediated cyclization of epoxides with 1,3-bis(trimethylsilyloxy)-1,3-butadienes, electroneutral equivalents of 1,3-dicarbonyl dianions, results in the formation of 2-alkylidenetetrahydrofurans with a great variety of substitution patterns and functional groups. This includes the synthesis of 2,3'-bifuranylidenes and 7-oxabicyclo[4.3.0]nonanes. The cyclization of dienes with functionalized epoxides containing base-labile groups proceeds with good chemoselectivity. In all reactions, good regio- and E diastereoselectivities are observed. Based on the stereoselectivities observed for reactions of 1,2-disubstituted epoxides, a working hypothesis for the mechanism of the reaction is suggested. 相似文献
30.
Alagia M Biondini F Brunetti BG Candori P Falcinelli S Moix Teixidor M Pirani F Richter R Stranges S Vecchiocattivi F 《The Journal of chemical physics》2004,121(21):10508-10512
The double photoionization of HCl molecules by synchrotron radiation has been studied in the energy range between 30 and 50 eV. The HCl(2+) and Cl(2+) product ions have been detected by a photoelectron-photoion-coincidence technique, while the H(+)+Cl(+) formation, which follows the double ionization of HCl, has been studied by photoelectron-photoion-photoion coincidence. The photon energy threshold for the production of HCl(2+) ions has been found to be 35.4+/-0.6 eV, while for the dissociative channel leading to H(+)+Cl(+), it has been measured a threshold at 36.4+/-0.6 eV and a change in the slope of the cross-section energy dependence at 38.7+/-0.7 eV. The production of H+Cl(2+) occurs with a threshold photon energy of 42.8+/-1.1 eV. These results appear to be in a good agreement with previous data by different experimental techniques and recent theoretical calculations performed by our laboratory. 相似文献