全文获取类型
收费全文 | 31778篇 |
免费 | 1065篇 |
国内免费 | 27篇 |
专业分类
化学 | 22141篇 |
晶体学 | 234篇 |
力学 | 570篇 |
数学 | 4290篇 |
物理学 | 5635篇 |
出版年
2023年 | 257篇 |
2022年 | 226篇 |
2021年 | 395篇 |
2020年 | 532篇 |
2019年 | 482篇 |
2018年 | 678篇 |
2017年 | 629篇 |
2016年 | 1224篇 |
2015年 | 1005篇 |
2014年 | 991篇 |
2013年 | 2045篇 |
2012年 | 2299篇 |
2011年 | 2446篇 |
2010年 | 1421篇 |
2009年 | 1153篇 |
2008年 | 2057篇 |
2007年 | 2021篇 |
2006年 | 1781篇 |
2005年 | 1601篇 |
2004年 | 1303篇 |
2003年 | 993篇 |
2002年 | 898篇 |
2001年 | 668篇 |
2000年 | 570篇 |
1999年 | 427篇 |
1998年 | 307篇 |
1997年 | 227篇 |
1996年 | 340篇 |
1995年 | 236篇 |
1994年 | 220篇 |
1993年 | 268篇 |
1992年 | 237篇 |
1991年 | 162篇 |
1990年 | 147篇 |
1989年 | 122篇 |
1988年 | 125篇 |
1987年 | 126篇 |
1986年 | 120篇 |
1985年 | 186篇 |
1984年 | 145篇 |
1983年 | 123篇 |
1982年 | 109篇 |
1981年 | 99篇 |
1980年 | 77篇 |
1979年 | 91篇 |
1978年 | 80篇 |
1977年 | 74篇 |
1976年 | 76篇 |
1975年 | 71篇 |
1973年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Rodríguez JC Zeng Y Wilks A Rivera M 《Journal of the American Chemical Society》2007,129(38):11730-11742
Relaxation compensated Carr-Purcell-Meiboom-Gill (rc-CPMG) NMR experiments have been used to investigate micros-ms motions in heme oxygenase from Pseudomonas aeruginosa (pa-HO) in its ferric state, inhibited by CN- (pa-HO-CN) and N3- (pa-HO-N3), and in its ferrous state, inhibited by CO (pa-HO-CO). Comparative analysis of the data from the three forms indicates that the nature of the coordinated distal ligand affects the micros-ms conformational freedom of the polypeptide in regions of the enzyme far removed from the heme iron and distal ligand. Interpretation of the dynamical information in the context of the crystal structure of resting state pa-HO shows that residues involved in the network of structural hydrogen-bonded waters characteristic of HOs undergo micros-ms motions in pa-HO-CN, which was studied as a model of the highly paramagnetic S = 5/2 resting state form. In comparison, similar motions are suppressed in the pa-HO-CO and pa-HO-N3 complexes, which were studied as mimics of the obligatory oxyferrous and ferric hydroperoxide intermediates, respectively, in the catalytic cycle of heme degradation. These findings suggest that in addition to proton delivery to the nascent Fe(III)-OO(-) intermediate during catalysis, the hydrogen-bonding network serves two additional roles: (i) propagate the electronic state (reactive state) in each of the distinct steps of the catalytic cycle to key but remote sections of the polypeptide via small rearrangements in the network of hydrogen bonds and (ii) modulate the conformational freedom of the enzyme, thus allowing it to adapt to the demanding changes in axial coordination state and substrate transformations that take place during the catalytic cycle. This idea was probed by disrupting the hydrogen-bonding network in pa-HO by replacing R80 with L. NMR spectroscopic studies conducted with R80L-pa-HO-N3 and R80L-pa-HO-CO revealed that the mutant exhibits nearly global conformational disorder, which is absent in the equivalent complexes of the wild type enzyme. The "chaotic" disorder in the R80L mutant is likely related to its significantly lower efficiency to hydroxylate heme in the presence of H2O2, relative to the wild type enzyme. 相似文献
992.
A highly efficient Cu-catalyzed tandem C-N bond-forming reaction of 1,4-dihalo-1,3-dienes has been developed. The transformation allows the synthesis of pyrroles and heteroarylpyrroles with a wide variety of functional groups and substitution patterns from readily available precursors. 相似文献
993.
María Blasco Celia DomeÑo Karim Bentayeb 《International journal of environmental analytical chemistry》2013,93(12):833-846
A clean-up procedure based on a solid-phase extraction column was optimized for determination of polycyclic aromatic hydrocarbons (PAHs) in lichen extracts to remove co-extracted compounds from the matrix in the final extract. Several kinds of solid phases were evaluated: normal phase (-NH2 and alumina), strong anion exchange and reversed phase. The -NH2 columns were the most effective by using a packed solid bed of 500?mg. The lichen raw extract was loaded on the column previously conditioned with dichloromethane and hexane. Hexane (0.5?mL) was used as rinsing solvent, and PAHs were quantitatively eluted (80–97%) using 2?mL of hexane–dichloromethane (65–35) as eluting solvent. In these conditions, even the heaviest PAHs were quantitatively eluted. The optimized SPE method provides a short time and low-solvent-consumption sample clean-up compared with other conventional methods based on column chromatography. The analytical procedure, dynamic sonication-assisted extraction, followed by the optimized solid-phase extraction clean-up, was used to determine the 16 EPA priority PAHs from native lichens collected from the Aragon valley in central Pyrenees. The PAH concentrations in lichen samples ranged from 352 to 1654?ng?g?1, and the minimum concentration value was established as the regional reference PAH levels in the area. 相似文献
994.
The influence of an initial salt concentration, phi(0), on the gradient separation of proteins using hydrophobic interaction chromatography on Separon HEMA 1000 was investigated. The results obtained were compared with the retention times and peak widths calculated according to a mathematical model. 相似文献
995.
Esther Domínguez Concepción Laborra Aranzazu Linaza Ana Madoz Issa A. Katime 《Monatshefte für Chemie / Chemical Monthly》1989,120(8-9):743-748
Summary A series of mono and diesters of itaconic acid containing alkoxy and sulfide side chains of variable lengths has been synthesized. Their structures have been established on the basis of their spectral data.
Eine Serie von Mono- und Diestern von Itaconsäure: Synthese und Strukturbestimmung
Zusammenfassung Es wurde eine Serie von Mono- und Diestern von Itaconsäure mit Alkoxy- und Sulfidseitenketten variabler Länge synthetisiert. Die Strukturaufklärung erfolgte mittels spektroskopischer Daten.相似文献
996.
Klaus Bieger Miguel Tomás José Barluenga Rafael Santiago Santiago García-Granda 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-4)
Abstract The title compound is the main product of the 2:1 reaction of DMAD with diazaphosphinines. The supposed mechanisms with intermediates and related products will be presented. 相似文献
997.
Sáenz-Carbonell Luis Loyola-Vargas Víctor M. 《Applied biochemistry and biotechnology》1997,61(3):321-337
Hairy roots are less susceptible to manipulation by changes in medium composition than callus and cell suspension cultures.
However, when the individual components of the Gamborg’s B5 medium were modified one by one, several of them modified the yield of root cultures. Nitrogen (N) source changes, the absence
of phosphorus (P), calcium (Ca), and cobalt (Co), and the increase in the sucrose content of the medium increased the level
of alkaloids inDatura stramonium hairy root cultures. The use of ammoniun as N source provoked the alkaloid excretion into the medium. The addition of different
auxins inhibited the alkaloid accumulation, but the addition of abscisic acid (ABA) did not change the alkaloid content of
the cultures. The use of MeJa increased the content of hyoscyamine to 100%. 相似文献
998.
I. I. Chernobaev M. N. Yakubovich A. I. Tripol’skií N. V. Pavlenko V. L. Struzhko 《Theoretical and Experimental Chemistry》1997,33(1):38-40
Methane is formed in the Fischer-Tropsch synthesis under real conditions both through intermediates leading to chain propagation
and by hydrogenation of surface carbon.
L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, Prospekt Nauki 31, 252039 Kiev,
Ukraine. Translated from Teoreticheskaya i éksperimental’naya Khimiya, No. 1, pp. 41–43, January–February, 1996. 相似文献
999.
A mixture of five tetracycline (TC) derivatives: minocycline (MC), demeclocycline (DMCTC), doxycycline (DC), and sancycline (SC), as well as each TC derivative from its main degradation product were separated by capillary zone electrophoresis (CZE). The influence of the pH and the concentration and nature of the background electrolyte (BGE) on the separations was investigated. Ethylenediaminetetraacetic acid (EDTA; 1 mM) was used as additive in a 25 mM phosphate buffer (pH 2.3) because this BGE enabled the rapid separation of the TC derivatives and of each TC derivative from its respective degradation product in less than 6 min. After optimization of the separation conditions, the analytical characteristics of the method were investigated. The parameters involved were linearity, precision (repeatability and reproducibility), and limits of detection (LODs). LODs obtained for the five TC derivatives studied were about 3 microg/mL. Finally, the CZE method developed was applied to study the stability of TC derivatives and to analyze the TC derivative content in three different pharmaceutical preparations. 相似文献
1000.
Zuzana Hricovíniová Miloš Hricovíni Ladislav Petruš 《Monatshefte für Chemie / Chemical Monthly》2001,132(6):731-737
Summary. D-Glycero-D-gulo-heptose reacted with 2,2-dimethoxypropane to give its 2,3:6,7-di-O-isopropylidene derivative. Its base-catalyzed addition
to formaldehyde resulted in the formation of 2,3:6,7-di-O-isopropylidene-2-C-(hydroxymethyl)-D-glycero-D-gulo-heptofuranose. After acid hydrolysis of this aldolization product, a new branched-chain aldose, 2-C-(hydroxymethyl)-D-glycero-D-gulo-heptose, was obtained, which was stereospecifically rearranged under the catalytic action of molybdic acid to D-glycero-D-ido-oct-2-ulose.
Received October 17, 2000. Accepted December 4, 2000 相似文献