全文获取类型
收费全文 | 47707篇 |
免费 | 1195篇 |
国内免费 | 238篇 |
专业分类
化学 | 30015篇 |
晶体学 | 636篇 |
力学 | 1111篇 |
综合类 | 7篇 |
数学 | 5129篇 |
物理学 | 12242篇 |
出版年
2021年 | 453篇 |
2020年 | 623篇 |
2019年 | 615篇 |
2018年 | 463篇 |
2017年 | 433篇 |
2016年 | 1002篇 |
2015年 | 815篇 |
2014年 | 1029篇 |
2013年 | 2123篇 |
2012年 | 2239篇 |
2011年 | 2485篇 |
2010年 | 1529篇 |
2009年 | 1364篇 |
2008年 | 2169篇 |
2007年 | 2124篇 |
2006年 | 2073篇 |
2005年 | 1939篇 |
2004年 | 1678篇 |
2003年 | 1361篇 |
2002年 | 1401篇 |
2001年 | 1165篇 |
2000年 | 908篇 |
1999年 | 828篇 |
1998年 | 729篇 |
1997年 | 689篇 |
1996年 | 675篇 |
1995年 | 691篇 |
1994年 | 689篇 |
1993年 | 637篇 |
1992年 | 640篇 |
1991年 | 597篇 |
1990年 | 588篇 |
1989年 | 569篇 |
1988年 | 547篇 |
1987年 | 517篇 |
1986年 | 518篇 |
1985年 | 694篇 |
1984年 | 696篇 |
1983年 | 518篇 |
1982年 | 536篇 |
1981年 | 550篇 |
1980年 | 525篇 |
1979年 | 516篇 |
1978年 | 549篇 |
1977年 | 508篇 |
1976年 | 490篇 |
1975年 | 442篇 |
1974年 | 456篇 |
1973年 | 441篇 |
1967年 | 380篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
991.
A sensitive flotation—spectrophotometric method, based on the ion associate formed by the anionic thiocyanate complex of osmium with the basic dye methylene blue (MB) is described. The ion associate precipitates when the aqueous solution is shaken with toluene, and the separated and washed compound is dissolved in acetone. The molar absorptivity is 2.2 × 105 l mol-1 cm-1 at 655 nm. Beer's law is obeyed. The molar ratio of Os:SCN:MB in the separated and washed ion associate is 1:6:3. Ruthenium reacts similarly. The method is applied to the determination of traces of osmium in crucible platinum after separation of osmium by distillation as tetroxide. 相似文献
992.
Attempts to obtain a molecular ion from the polyene macrolide antibiotic pimaricin by EI and FD mass spectrometry were unsuccessful The loss of carbon dioxide and a varying number of water molecules from the molecular ion made a molecular-weight determination impossible 13C NMR spectroscopy of N-acetylpimaricin, its dodecahydroderivative, and of HH-2, a hydrogenation-hydrogenolysis product of N-acetylpimaricin, confirmed that the antibiotic has structure 3 containing a hemi-ketalic ring and lacking an OH group at C-8. The value of the anomeric coupling constant, Jc-1-h-1, indicates that the mycosamine moiety is β-glycosidically bound to the aglycone.The structure and configuration of the antibiotic have been studied by analysis of the 270 MHz 1H-NMR spectra of N acetylpimaricin and HH-2. By comparison of the two spectra and by extensive decoupling experiments, all signals in the spectrum of N-acetylpimaricin have been assigned to the protons in structure 3. From chemical-shift, coupling constant, and integral values, it was deduced that the mycosamine ring is pyranoid with a chair conformation, the hemi-ketal is 6 membered and occupies a chair conformation with the substituents in equatorial positions, the epoxy protons as well as the olefinic protons are trans to each other, and the antibiotic is diastereomerically pure. 相似文献
993.
A titration unit for automatic analysis systems is described. The titrator performs different titrations specified by strings of digital parameters. It has 4 independent titration stations with individual electrode systems. A multiburette with 20 cylinders provides all stations with the necessary reagents. End-point titrations, and incremental and equilibrium titrations are controlled by a microcomputer. In combination with a sample transport and a desk calculator, the titrator can be used to process automatically samples of different natures which require different treatment. 相似文献
994.
In order to assign the structural changes of the protein, observed in the Fourier transform infrared (FT-IR) difference spectra of the rhodopsin-metarhodopsin-II transition, to specific regions of the protein, rhodopsin was treated by proteases. Nonilluminated and bleached rhodopsin was treated with protease K and papain. Rhodopsin digested in the bleached state was subsequently regenerated with 11-cis-retinal. From these modified samples the rhodopsin-metarhodopsin-II FT-IR difference spectra were measured. Comparing the difference spectra with that of unmodified rhodopsin, clear deviations in the amide-I and amide-II spectral range are observed. This indicates that in the unmodified pigment conformational changes of those parts of the cytosolic surface take place which are susceptible to the proteases. From the larger spectral changes obtained with samples digested in the bleached state it is concluded that the extent of modification is larger. The difference spectra of rhodopsin modified with 10 mM dithiothreitol support the existence of the 4th loop which also undergoes conformational changes. The spectral changes are interpreted in terms of a transition of an ordered structure of the loops in rhodopsin to a more random structure in metarhodopsin-II. The results demonstrate that by combining FT-IR spectroscopy with protein modification by specific proteases, conformational changes of the protein can be localized to specific regions. 相似文献
995.
Graham Smith Colin H.L. Kennard Thomas C.W. Mak 《Journal of organometallic chemistry》1985,290(1):c7-c10
The crystal structure of trimethyl(thiomethyl)platmum(IV), [Pt(CH3)3(SCH3)]4, has been determined using X-ray diffraction. The compound has three independent but isostructural tetrameric units in the asymmetric unit. The four metal atoms in each tetramer are linked by bridging thiomethyl sulphurs, with three methyl groups completing the octahedral coordination around each platinum. Mean interatomic distances within the tetramer are PtS, 2.48(2), PtC, 2.00(4), PtPt, 3.80(2) Å, with a mean bridge angle SPtS, 79.1(7)°. 相似文献
996.
Selzer Y Cabassi MA Mayer TS Allara DL 《Journal of the American Chemical Society》2004,126(13):4052-4053
We report temperature dependence measurements on the conductance of individual molecular wires. The results show for the first time in a molecular junction the theoretically predicted transition from coherent superexchange tunneling conductance to an activated hopping mechanism as temperature is increased. 相似文献
997.
Spin-exchange experiments are useful for improving the resolution and establishment of sequential assignments in solid-state NMR spectra of uniformly (15)N-labeled proteins oriented macroscopically in phospholipid bilayers. To exploit this advantage fully, it is crucial that the diagonal peaks in the two-dimensional exchange spectra are suppressed. This may be accomplished using the recent pure-exchange (PUREX) experiments, which, however, suffer from up to a threefold reduction of the cross-peak intensity relative to experiments without diagonal-peak suppression. This loss in sensitivity may severely hamper the applicability for the study of membrane proteins. In this paper, we present a two-dimensional exchange experiment (iPUREX) which improves the PUREX sensitivity by 50%. The performance of iPUREX is demonstrated experimentally by proton-mediated (15)N-(15)N spin-exchange experiments for a (15)N-labeled N-acetyl-L-valyl-L-leucine dipeptide. The relevance of exchange experiments with diagonal-peak suppression for large, uniformly (15)N-labeled membrane proteins in oriented phospholipid bilayers is demonstrated numerically for the G-protein coupled receptor rhodopsin. 相似文献
998.
G. U. Tillaeva 《Chemistry of Natural Compounds》1998,34(4):429-430
The compositions of two samples of hydrogenated cottonseed oil recommended as fatty bases for suppositories have been studied.
Their physicochemical indices have been determined.
Tashkent Pharmaceutical Institute, fax (3712) 56 45 04. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 459–460,
July–August, 1998. 相似文献
999.
Summary In this study a new approach is presented for on-line radiometric detection in reversed phase LC of medium to low polarity
compounds labelled with14C.
The test compounds,14C-carbaryl and14C-parathion, are extracted post-column into a non-water miscible liquid scintillation cocktail. The segmented two-phase system
formed is introduced into the beta-detector without phase separation and collected in a capillary storage tube.
After completion of the LC separation and detection process, the direction of the flow in the storage system is reversed and
the segmented contents of the loop led at lower flow-rates through the beta-detector again. An enhanced signal, corresponding
to the increase in counting time is obtained without measurable peak broadening.
The lowest possible detection limit of the system is 9 counts per peak corresponding to subnanogram quantities of tested pesticides.
Calibration curves are linear over at least 2 orders of magnitude and have the expected theoretical slopes.
The reproducibility of the system is better than 4 % rel. S.D. An application to a recovery study of parathion shows the practical
potential of this technique.
Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984 相似文献
1000.
Contributions to the Chemistry of Phosphorus. 144. Synthesis and Properties of the Hexaphospha-3-germaspiro[2.4]heptane (t-BuP)2Ge(t-BuP)4 The cyclocondensation of K(t-Bu)P? P(t-Bu)K with germanium tetrachloride in the molar ratio of 2:1 yields the novel spirocyclic compound 1,2,4,5,6,7-hexa-tert-butyl-1,2,4,5,6,7-hexaphospha-3-germaspiro[2.4]heptane, (t-BuP)2Ge(t-BuP)4 ( 1 ). Besides considerable amounts of (t-BuP)4 are formed and occasionally some (t-BuP)3 can be found. 1 could be isolated in the pure state and has been NMR-spectroscopically characterized as a spirocyclic compound with a P2GeP4 skeleton. 相似文献