全文获取类型
收费全文 | 9401篇 |
免费 | 300篇 |
国内免费 | 19篇 |
专业分类
化学 | 6391篇 |
晶体学 | 70篇 |
力学 | 349篇 |
数学 | 1400篇 |
物理学 | 1510篇 |
出版年
2023年 | 53篇 |
2022年 | 79篇 |
2021年 | 129篇 |
2020年 | 168篇 |
2019年 | 163篇 |
2018年 | 185篇 |
2017年 | 148篇 |
2016年 | 315篇 |
2015年 | 255篇 |
2014年 | 284篇 |
2013年 | 591篇 |
2012年 | 596篇 |
2011年 | 674篇 |
2010年 | 367篇 |
2009年 | 351篇 |
2008年 | 593篇 |
2007年 | 619篇 |
2006年 | 558篇 |
2005年 | 541篇 |
2004年 | 417篇 |
2003年 | 380篇 |
2002年 | 301篇 |
2001年 | 145篇 |
2000年 | 135篇 |
1999年 | 121篇 |
1998年 | 89篇 |
1997年 | 75篇 |
1996年 | 116篇 |
1995年 | 96篇 |
1994年 | 84篇 |
1993年 | 73篇 |
1992年 | 67篇 |
1991年 | 49篇 |
1990年 | 49篇 |
1989年 | 45篇 |
1988年 | 30篇 |
1987年 | 32篇 |
1985年 | 61篇 |
1984年 | 45篇 |
1983年 | 34篇 |
1982年 | 53篇 |
1981年 | 45篇 |
1980年 | 54篇 |
1979年 | 45篇 |
1978年 | 40篇 |
1977年 | 38篇 |
1976年 | 47篇 |
1975年 | 46篇 |
1974年 | 30篇 |
1973年 | 30篇 |
排序方式: 共有9720条查询结果,搜索用时 15 毫秒
81.
Nathalie Laronze Jean-François Moisan Catherine Roch-Marchal Feng-Xian Liu Gilbert Hervé 《Journal of Cluster Science》2002,13(3):355-368
Transition metal salts of heteropolyacids have been prepared taking into account the strong acidic and cation exchanging properties of the solid heteropolyacids. The exchange between protons and the transition metal cation is carried out by stirring a suspension of the hydrated heteropolyacid in a solution of the metal acetylacetonate complex in toluene. The exchange occurs on the surface of the solid particles and diffusion of protons and metal cations into the hydrated lattice leads to the substitution of all the protons. The method can be utilized in order to prepare supported vanadyl and copper molybdophosphates from supported heteropolyacids and they have been studied in the catalysis of the oxidative dehydrogenation of isobutyric acid. The effect of vanadyl counter-ions on the catalytic behavior is the same as observed with bulk catalysts but, on the contrary, copper supported molybdophosphate shows an acid catalytic activity not observed with bulk catalysts. 相似文献
82.
An alternative route toward developing basis sets for post-Hartree-Fock calculations, the hybrid bond polarization function method, is investigated. Two new basis sets, denoted 6-31G(d, p)+ B and 6-31 + G(d,p)+B, are defined for the first-row hydrides. The dissociation energies of the first-row hydride species in their respective ground states are computed using full fourth-order Møller-Plesset theory, and compared with results obtained with large polarized basis sets containing no bond functions. It is shown that results are competitive even with basis sets as large as 6-311++G(3df,3pd), while computation times are reduced by a factor of 4 to 20. On empirical grounds, the basis set superposition error should be neglected entirely. 相似文献
83.
Zouhair Asfari Rym Abidi Françoise Arnaud Jacques Vicens 《Journal of inclusion phenomena and macrocyclic chemistry》1992,14(2):163-169
The template synthesis of copper(II) phthalocyanine by the condensation of dicyanobenzene within the supercages of Cu(II) exchanged X and Y type zeolites was studied using differential scanning calorimetry. The exotherms for intrazeolite and surface complexation have been resolved. These thermal events exhibit a dependence on the copper loading and hydration level which has been interpreted as a pressure effect. 相似文献
84.
Cantat T Jaroschik F Nief F Ricard L Mézailles N Le Floch P 《Chemical communications (Cambridge, England)》2005,(41):5178-5180
New samarium carbene complexes have been synthesized and characterized by X-ray diffraction analysis; the carbenic nature was assessed by reactivity studies. 相似文献
85.
A laboratory information management system (LIMS) can make a major contribution to the quality and therefore to the efficiency
and competitiveness of a laboratory. Since it can impact all aspects of a laboratory's organization it must be the key if
not the principal player of the laboratory's quality system. It should support the laboratory in establishing, maintaining
and applying quality procedures thereby enabling the laboratory to achieve its quality goals. As a tool, LIMS permits the
laboratory to input and use its own know-how and experience to optimize the total organization (internal and external) and
workflow of generated information. However, perceived "quality" in the context of an LIMS, can be viewed as being made up
of different facets such as the security, reliability and accessibility of information as well as its turn around time and
production cost. This paper reviews the role of a LIMS in the laboratory and the contribution that both system design and
functionality can have on "building quality ".
Received: 5 October 1998 · Accepted: 20 October 1998 相似文献
86.
Menezes PH Gonçalves SM Hallwass F Silva RO Bieber LW Simas AM 《Organic letters》2003,5(10):1601-1604
[reaction: see text] Several (77)Se NMR experiments were performed by titrating a sample of selenides with the chiral shift reagent methylbenzylamine (MBA), followed by acquisition of (77)Se NMR spectra. Eventually, we observed the appearance of two anisochronous resonances, with a relatively large separation, from 37 to 56 Hz, corresponding to the formation of the diastereomeric complexes. This methodology avoids derivatization processes, and the studied compound can be easily recovered from the NMR tube. 相似文献
87.
Rudolph Willem Ann Delmotte Inge De Borger Monique Biesemans Marcel Gielen Franois Kayser Edward R. T. Tiekink 《Journal of organometallic chemistry》1994,480(1-2):255-259
The synthesis of (Z)-2-methyl-3-triphenylstannyl-3-pentene-2-ol and its characterization by an X-ray diffraction study and multinuclear NMR are reported. The tin atom exhibits a distorted tetrahedral SnC4 geometry with the four Sn---C bond distances experimentally equivalent. The slight distortion from the ideal tetrahedral geometry is because of the presence of a weak intramolecular HO → Sn interaction of 3.012(3) Å which produces a loose four-membered ring. The title compound was characterized in solution by 1H, 13C and 119Sn NMR, and the persistence of the weak intramolecular HO → Sn coordination in solution was revealed by 13C and 119Sn secondary isotope multiplet of partially labelled entities (SIMPLE-NMR) experiments. 相似文献
88.
Hourdin M Gouhier G Gautier A Condamine E Piettre SR 《Journal of combinatorial chemistry》2005,7(2):285-297
Ten cross-linked polystyrene-supported, protected chiral amines featuring both a spacer, comprising from 5 to 15 atoms, and a fluorinated linker have been successfully prepared. The development of the monitoring technique by gel-phase 19F NMR spectrometry on cross-linked polystyrene derivatives proved to be of high value in four steps of the process, as shown by the comparison of data gathered from both a classic NMR spectrometer and elemental analysis. Gel-phase 19F NMR spectrometry, thus, constitutes a useful technique that complements IR and 13C NMR spectrometries for the qualitative monitoring of reactions. In addition, quantitative determination of the conversion in a given transformation is possible, provided that 19F chemical shifts of the substrate and the product be different enough (Deltadelta>base width), as illustrated by the Mitsunobu coupling process (16-->17). The technique is nondestructive, and the samples used to monitor the reactions may be returned to the reaction medium. Deprotection of the above amines was achieved and furnished eight of the final resins in good to acceptable purity for future applications. 相似文献
89.
Development of quantitative vitellogenin-ELISAs for fish test species used in endocrine disruptor screening 总被引:5,自引:0,他引:5
Nilsen BM Berg K Eidem JK Kristiansen SI Brion F Porcher JM Goksøyr A 《Analytical and bioanalytical chemistry》2004,378(3):621-633
The yolk protein precursor vitellogenin (Vtg) in plasma has proved to be a simple and sensitive biomarker for assessing exposure of fish to environmental estrogens. Within international bodies such as the Organization for Economic Cooperation and Development (OECD) work is ongoing to develop screening and testing programmes for endocrine disrupting effects of new chemicals, and in the focus of this development are the fish test species common carp (Cyprinus carpio), fathead minnow (Pimephales promelas), zebrafish (Danio rerio) and Japanese medaka (Oryzias latipes). In this study we have developed quantitative enzyme linked immunosorbent assays (ELISAs) for Vtg in common carp/fathead minnow, zebrafish and Japanese medaka. The assays were developed using a combination of monoclonal and polyclonal fish Vtg antibodies in a sandwich format, using stabilized Vtg from the test species as a standard. The carp Vtg ELISA has a working range of 1–63 ng/mL, a minimal detection limit of 0.6 ng/mL, and may also be used for quantification of Vtg in fathead minnow. In fathead minnow whole-body homogenate samples, the practical detection limit is 400 ng/mL due to the matrix effect. The zebrafish Vtg ELISA has a working range of 0.5–63 ng/mL, a minimal detection limit of 0.4 ng/mL, and a practical detection limit of 200 ng/mL in whole-body homogenate samples. The medaka Vtg ELISA has a working range of 0.25–16 ng/mL, a minimal detection limit of 0.1 ng/mL, and a practical detection limit of 125 ng/mL in whole-body homogenate samples. The intra- and inter-assay variations were below 20% for all assays. The assays were evaluated with sets of representative samples spanning the wide dynamic range of Vtg-levels found in fish exposed to environmental estrogens, and all three assays are currently undergoing international inter-laboratory validation. 相似文献
90.
Paolesse R Nardis S Venanzi M Mastroianni M Russo M Fronczek FR Vicente MG 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(5):1192-1197
5,10,15-Triphenylcorrole (1) reacts with the Vilsmeier reagent (POCl(3)/DMF) to give the corresponding 3-formyl derivative 3 as the major product. The regioselectivity of the reaction was proven by X-ray crystallography and only traces of the 2-formyl isomer were observed. A more polar product is also observed and this compound becomes the major product when an excess of DMF is used for the preparation of the Vilsmeier reagent, while the formation of the 3-formyl isomer is almost completely suppressed. X-ray crystallography allowed us to identify this compound as the fully substituted N-ethane bridged derivative 4, formed from the attack of the Vilsmeier reagent at the inner core of the macrocycle. This compound is unique among porphyrinoid macrocycles, and further confirms the peculiarity of corrole chemistry. 相似文献