全文获取类型
收费全文 | 10083篇 |
免费 | 176篇 |
国内免费 | 38篇 |
专业分类
化学 | 5758篇 |
晶体学 | 85篇 |
力学 | 353篇 |
数学 | 1634篇 |
物理学 | 2467篇 |
出版年
2022年 | 95篇 |
2021年 | 111篇 |
2020年 | 122篇 |
2019年 | 123篇 |
2018年 | 117篇 |
2017年 | 102篇 |
2016年 | 178篇 |
2015年 | 159篇 |
2014年 | 165篇 |
2013年 | 566篇 |
2012年 | 353篇 |
2011年 | 438篇 |
2010年 | 317篇 |
2009年 | 273篇 |
2008年 | 398篇 |
2007年 | 356篇 |
2006年 | 348篇 |
2005年 | 316篇 |
2004年 | 336篇 |
2003年 | 273篇 |
2002年 | 275篇 |
2001年 | 187篇 |
2000年 | 181篇 |
1999年 | 150篇 |
1998年 | 115篇 |
1997年 | 131篇 |
1996年 | 162篇 |
1995年 | 122篇 |
1994年 | 127篇 |
1993年 | 156篇 |
1992年 | 155篇 |
1991年 | 158篇 |
1990年 | 116篇 |
1989年 | 119篇 |
1988年 | 131篇 |
1987年 | 180篇 |
1986年 | 149篇 |
1985年 | 198篇 |
1984年 | 184篇 |
1983年 | 144篇 |
1982年 | 166篇 |
1981年 | 176篇 |
1980年 | 171篇 |
1979年 | 170篇 |
1978年 | 164篇 |
1977年 | 162篇 |
1976年 | 141篇 |
1975年 | 158篇 |
1974年 | 126篇 |
1973年 | 123篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
231.
E. Vincent L. Verdonck L. Naessens G.P. Van Der K.Elen 《Journal of organometallic chemistry》1984,277(2):235-244
The infrared, Raman and (1H, 13C) NMR spectra of trivinyltin chloride, bromide and iodide have been analyzed and discussed. The vibrational assignment has been confirmed by an approximate normal coordinate analysis. Evidence has been found for a marked influence of the X substituent on the tin-carbon bond due to isovalent rehybridization. Variations in the π electron system of the vinyl group are hardly significant. 相似文献
232.
Hubert TD Vue C Bernardy JA Van Horsen DL Rossulek MI 《Journal of AOAC International》2001,84(2):392-398
3-Trifluoromethyl-4-nitrophenol (TFM) is a pesticide used for the selective control of sea lampreys (Petromyzon marinus) in stream and river tributaries of the Great Lakes. To determine concentrations of TFM and TFM glucuronide in the edible fillet tissue of fish during sea lamprey control treatments, an analytical method was developed to determine the concentrations of these residues in rainbow trout (Oncorhynchus mykiss; RBT) and channel catfish (Ictalurus punctatis; CCF). Homogenized fillets were extracted with methanol-water (80 + 20). TFM and TFM glucuronide were isolated from coextractives by C18 solid-phase extraction. TFM glucuronide was hydrolyzed to TFM by the addition of beta-glucuronidase to the TFM glucuronide extract. The extracts were analyzed separately by liquid chromatography with UV-visible detection. Recoveries from TFM-fortified CCF and RBT tissues were 84.1 and 96.1%, respectively. The method detection limits (MDLs) are 2.4 ng/g for TFM-fortified tissues of CCF and 3 ng/g for those of RBT. Recoveries were 78.8 and 77% from TFM glucuronide-fortified CCF and RBT tissues, respectively. The MDLs for TFM glucuronide-fortified tissues are 3.5 and 6.9 ng/g for CCF and RBT, respectively. 相似文献
233.
Kaval N Ermolat'ev D Appukkuttan P Dehaen W Kappe CO Van der Eycken E 《Journal of combinatorial chemistry》2005,7(3):490-502
The "click chemistry" approach has been explored on the 2-(1H)-pyrazinone scaffold for the generation of pharmacologically interesting heterocyclic moieties. Huisgen 1,3-dipolar cycloaddition has been evaluated as the key step for the construction of the 1,2,3-triazole ring at the C-3 position of 2-(1H)-pyrazinones. Two different pathways have been successfully evaluated: (1) via C-C or C-O linkage of the acetylenic part to the C-3 position of the 2-(1H)-pyrazinone scaffold or (2) via azide introduction in the C-3 position. The subsequent application of "click chemistry" resulted in the formation of hitherto unknown skeletons. Microwave irradiation has successfully been applied in different steps of the sequence. 相似文献
234.
E.J.M. Van Schaick F.C. Mijlhoff G. Renes H.J. Geise 《Journal of Molecular Structure》1974,21(1):17-21
The molecular structure of cis- and trans-1,2-difluoroethene was studied in the gas phase by electron diffraction, using the sector-microphotometer technique. The molecules are planar. For trans-difluoroethene the geometrical parameters are: C-F bond: 1.338(0.003) Å; CC bond: 1.320(0.009) Å; C-H bond: 1.088(0.004) Å, ∠CCF 119.8°(0.2°); ∠CCH: 125° (1.2°). For cis-difluoroethene: C-F bond: 1.332(0.003) Å; CC bond: 1.311(0.008) Å; C-H bond: 1.100(0.003) Å; ∠CCF: 122.5°(0.2°); ∠CCH:127.0 °(2.3°). 相似文献
235.
Summary The application of the laser microprobe mass analyser (LAMMA) to several acyclic quaternary ammonium salts with aliphatic and aromatic substituents was investigated. The LAMMA spectra share some typical features with the ones, obtained by mass spectrometric (MS) analysis using soft ionisation techniques, e. g. the intact cations are detected as well as the iminium ions, issued from their decomposition. However, as a result of the abundant fragmentation and because of the presence of additional characteristic peaks, LAMMA spectra become largely comparable to the ones, recorded on a conventional MS with electron impact-ionisation and sample introduction by direct probe, though it is well known that the latter method induces thermal degradation of these salts and consequently only allows for the detection of the corresponding decomposition products.
Analyse organischer quarternärer Ammoniumsalze mittels LAMMA
Zusammenfassung Die Anwendung des Laser-Mikrosonden-Massen-Analysators (LAMMA) zur Charakterisierung verschiedener acyklischer quarternärer Ammonium-salze mit aliphatischen und aromatischen Substituenten wurde untersucht. Die LAMMA-Spektren zeigen charakteristische Merkmale jener Spektren, die durch weiche Ionisierungstechniken in der Massenspektrometrie erhalten werden: z. B. werden intakte Kationen sowie Imin-Ionen detektiert. Als Folge der weitgehenden Fragmentierung und wegen zahlreicher zusätzlicher Peaks sind LAMMA-Spektren jenen Spektren sehr ähnlich, die durch Elektronenstoß-Ionisation und Direkteinführung der Probe erzeugt werden. Allerdings ist bekannt, daß die letztere Methode einen starken Abbau von Salzen bewirkt und daher nur den Nachweis von Zerfallsprodukten zuläßt.相似文献
236.
Flow injection analysis can be used for the determination of both iron(II) and iron(III) with an amperometric detector. The flow-through cell contains a glassy carbon electrode. Selection of the appropriate voltammetric technique, choice of the indication potentials, sample size, composition of the carrier stream, etc., are discussed. The limit of determination is about 10-6 M; the calibration curves are linear in the concentration ranges 10-3–10-5 M for iron(III) and 5 × 10-4–10-5 M for iron(II). To illustrate the potentialities of the proposed method, standard rocks have been analysed. 相似文献
237.
Multicomponent adsorption data of a fermentation broth containing adipoyl-7-amino-3-deacetoxycephalosporanic acid (adipoyl-7-ADCA), a cephalosporin precursor for 7-ADCA, and two key impurities, alpha-hydroxyadipoyl-7-ADCA and alpha-aminoadipoyl-7-ADCA were obtained from batch equilibrium and frontal chromatography tests. Amberlite XAD-1600 was chosen as the resin. A rate model was applied to simulate the chromatograms. An alkaline buffer, which by itself has no affinity for the resin, was used as the eluent. The widely used reversed-phase modulator model is inaccurate in explaining the stepwise elution data. A new model, the induced competition model, has been developed to account for apparent retention of the buffer in the presence of adsorbed species. Close agreement between the simulations and the data was achieved with the new model. 相似文献
238.
For trace analyses of environmental waters, spark-source mass-spectrometry has been combined with a preconcentration procedure involving chelation of the dissolved trace elements with oxine and subsequent adsorption of the oxinates and naturally occurring organic and colloidal metal species onto activated carbon. The activated carbon is filtered off and ashed at low temperature. The residue is dissolved, an internal standard and pure graphite are added and, after drying, the electrodes are prepared. The photographically recorded mass spectrum is evaluated by a suitable computer routine. The error of the procedure is around 30%. While this preconcentration and analysis procedure is capable of measuring about 40 elements quantitatively, in practice 10-25 trace elements are determined simultaneously above the 0.1-mug/l. detection limit, as is illustrated by analyses of drinking water, surface and ground water samples. Although a sophisticated technique, SSMS can be considered for regular panoramic survey analyses. 相似文献
239.
A procedure has been developed for estimating the information content of retrieval systems with binary-coded mass spectra, as well as mass spectra coded by other methods, from the statistical properties of a reference file. For a reference file, binary-coded with a threshold of 1% of the intensity of the base peak, this results typically in an estimated information content of about 50 bits for 200 selected values. It is shown that, because of errors occurring in the binary-coded spectra, the actual information content is only about 12 bits. This explains the poor performance observed for retrieval systems with binary-coded mass spectra. 相似文献
240.
Abstract— Kinetic studies of the hematoporphyrin–sensitized photooxidation of l -tryptophan and tryptamine at pH 10 in either homogeneous aqueous solutions or in aqueous dispersions of Triton X–100 and cetyltrimethylammonium bromide micelles indicate that the indole substrates are attacked via a mixed type I (electron transfer from triplet dye)/type II (1 O2 -involving) mechanism. Both reactive intermediates, generated by micelle-solubilized hematoporphyrin, can diffuse to attack substrate molecules located in either the bulk aqueous phase or a different micelle. In particular, incorporation of the substrate into a micelle has only minor effects on its reactivity toward1 O2 , although the 1 O2 —indole interaction appears to be more efficient in cationic micelles owing to a favourable orientation of the target with respect to the attacking species. On the other hand, the electron transfer from triplet porphyrin to a micellized substrate is virtually non-operative when the latter is located in an anionic micelle, whereas in neutral or cationic micelles, the efficiency of the process is again controlled by the substrate orientation. Studies of tryptamine photooxidation sensitized by meso-tetra-(4-sulfonato-phenyl) porphine in the presence of sodium dodecylsulphate micelles lend further support to the abovementioned conclusions. 相似文献