全文获取类型
收费全文 | 4617篇 |
免费 | 563篇 |
国内免费 | 165篇 |
专业分类
化学 | 4433篇 |
晶体学 | 3篇 |
力学 | 25篇 |
综合类 | 18篇 |
数学 | 625篇 |
物理学 | 241篇 |
出版年
2024年 | 5篇 |
2023年 | 60篇 |
2022年 | 233篇 |
2021年 | 204篇 |
2020年 | 208篇 |
2019年 | 152篇 |
2018年 | 170篇 |
2017年 | 131篇 |
2016年 | 214篇 |
2015年 | 240篇 |
2014年 | 277篇 |
2013年 | 387篇 |
2012年 | 245篇 |
2011年 | 309篇 |
2010年 | 235篇 |
2009年 | 279篇 |
2008年 | 256篇 |
2007年 | 268篇 |
2006年 | 227篇 |
2005年 | 204篇 |
2004年 | 164篇 |
2003年 | 151篇 |
2002年 | 202篇 |
2001年 | 58篇 |
2000年 | 64篇 |
1999年 | 62篇 |
1998年 | 54篇 |
1997年 | 43篇 |
1996年 | 38篇 |
1995年 | 27篇 |
1994年 | 29篇 |
1993年 | 23篇 |
1992年 | 9篇 |
1991年 | 12篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1988年 | 9篇 |
1987年 | 12篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1969年 | 2篇 |
1968年 | 2篇 |
排序方式: 共有5345条查询结果,搜索用时 15 毫秒
41.
A new pulse sequence is proposed for the determination of scalar coupling correlation in small- and medium-sized organic compounds. The method uses a combination of the double pulsed field gradient spin-echo (DPFGSE) and the selective population transfer (SPT) techniques and is shown to be useful in the analysis of complex spectra with many overlapped signals. The usefulness of this method in the structural elucidation of natural substances is demonstrated using strychnine and digitoxin as examples. 相似文献
42.
Danielle Giron S. Monnier M. Mutz P. Piechon T. Buser F. Stowasser K. Schulze M. Bellus 《Journal of Thermal Analysis and Calorimetry》2007,89(3):729-743
Adequate very sensitive quantification methods are needed for the development and are also now required for the monitoring
of undesirable solid form(s) as routine tests. The pre-requisite for quantitation are selectivity, sensitivity and most important
the purity of standards and their proper storage, what is a challenge for metastable forms.
Several analytical techniques are available such as X-ray diffraction, spectroscopy, thermal analysis and microcalorimetry.
The different steps of the validation of the analytical methods and problems to be solved are discussed. Examples illustrate
the different techniques and compare their possible advantages and limits. The relative standard deviation of measurements
should allow for checking the homogenization procedure of mixtures for calibration. The validation should be carried out following
ICH guidelines for validation of analytical methods. Comparison of different techniques in adequate concentration range add
confidence in the analytical results. 相似文献
43.
The reversed-phase high-performance liquid chromatography (RP-HPLC) method was developed for determination of lercanidipine hydrochloride and its synthetic impurities, degradation and oxidative products in Carmen® tablets. The best separation was performed on Zorbax SB C18 column, 250 x 4.6 mm, particle size 5 m. Acetonitrile-water-triethylamine 55:44.8:0.2 (v/v/v) was used as a mobile phase with flow rate 1 mL min–1. pH was adjusted to 3.0 with orthophosphoric acid. UV detection was performed at 240 nm. Duration of chromatographic run was about 12 min for six examined compounds. The chromatographic conditions for the determination of lercanidipine hydrochloride and its related substances were the same, but the concentration of lercanidipine hydrochloride was 0.03 mg mL–1 for assay and 0.3 mg mL–1 for related substances. The validation of the method performance characteristics (figures of merits, quality of parameters) was established to be adequate for the intended use. The evaluation of number of parameters, such as selectivity, linearity, accuracy, specificity, precision (repetability and reproducibility), sensitivity and detection and determination limits is entailed.Acknowledgements This work was supported by the Institute of Pharmacy of Serbia, Belgrade and by the Ministry for Science, Technology and Development of Serbia, Contact: 1458 相似文献
44.
Résumé L'analyse de coupes lourdes (huile de distillation sous vide et huile issue du séparateur à chaud) provenant de l'hydroliquéfaction
catalytique du charbon a été réalisée par mise en ouvre de différentes techniques chromatographiques. L'identification des
principales structures résulte de l'application de la chromatographie par couplage de transfert de charge en mode HPLC et
des spectroscopies UV et de masse.
Donotor-acceptor complex chromatography —Application to analysis of coal liquefaction products by HPLC
Summary A Vacuum gas oil and the corresponding residue of a coal liquefaction product were investigated by means of HPLC and MS. Nearly a complete identification was performed by chromatographic, UV-spectroscopic methods and MS.
相似文献
45.
Wolfgang Oppolzer 《Angewandte Chemie (International ed. in English)》1989,28(1):38-52
Metallo-ene reactions, hardly recognized until very recently, have experienced a breathtaking development when applied in an intramolecular sense. Efficient regio- and stereoselective magnesium-ene cyclizations have served as a cornerstone for numerous syntheses of structurally diverse natural products (e.g., sesquiterpenes of marine or plant origin, alkaloids, fragrances, insect defense compounds, and a fungitoxin). A brilliant example is the synthesis of the elusive odorant (+)-khusimone which outshines 20 years of work in the field of tricyclovetivane synthesis. Palladium-, platinum-, and nickel-catalyzed versions of the metallo-ene reaction are in a comparatively early stage of exploration, but, nevertheless, reveal intriguing potential. Hence an almost 100% stereospecific C? O→C? ;Pd-→ C? C chirality transfer permits simple and selective, cis- or trans-annelation processes. The mild cyclization conditions are compatible with various functional groups, such as nitrogen moieties, which offer interesting perspectives for the preparation of heterocycles (e.g., alkaloids) difficult to obtain by other methods. Carbon monoxide insertion reactions of the cyclized σ-metal intermediates were shown to afford annelated cyclopentanones and cyclopentenones with concomitant stereocontrolled formation of four carbon–carbon bonds. These and other observations, highlighted in this article, provide a platform for further extensions and applications of this powerful method in organic synthesis. 相似文献
46.
Tonino CarusoAldo Spinella 《Tetrahedron》2003,59(39):7787-7790
An improved methodology for the preparation of copper(I) alkynides has been developed using cesium carbonate as base. In the presence of cesium carbonate and CuI various 1-alkynes were converted smoothly to the corresponding copper alkynides which in situ reacted with propargylic halides in order to synthesize skipped diynes. 相似文献
47.
Marimganti S Yasmeen S Fischer D Maier ME 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(22):6687-6700
The synthesis of the omega-amino acid 4 is described utilizing a two-dimensional synthesis strategy combined with an enzymatic differentiation of homotopic ester groups. The amino acid 4 features two non-bonded interactions that result in conformational constraints on a cyclic construct. This amino acid was incorporated into the four macrolactams 17, 22, 31, and 37. The ring in 17 and 22 is 18-membered, whereas 31 and 37 have a 19-membered ring. The pairs with the same ring size differ in a N-methyl group. For the larger macrolactams (31 and 37) conformational analysis showed that the macrocyclic rings are somewhat more rigid than in the natural lead, the depsipeptide jasplakinolide. Nevertheless, their conformations are comparable to the natural product. There are no intramolecular hydrogen bonds, neither is the cis-rotamer populated in the N-methyl compound 37. Due to the increased flexibility of the smaller macrolactams 17 and 22 and signal overlap, a distinct solution structure could not be obtained for these compounds. The amino acid 4 should be useful for restricting the conformation of other small peptides. 相似文献
48.
Starting from maleanilic and maleamic acids, a facile general approach to kinetically controlled isomaleimides has been described for the first time using cyanuric chloride as a dehydrating agent with 85-98% yields. The effect of a variety of substituents present on the aromatic ring in amine and maleic anhydride moiety, on these kinetic/thermodynamic dehydration processes of anilic acids has been also described. Under the same set of reaction conditions the phthalanilic acid gave kinetically controlled product, isophtalimide in 91% yield, while the corresponding succinanilic acid furnished the thermodynamically more stable succinimide in high yield. 相似文献
49.
T. Raju K. Kulangiappar M. Anbu Kulandainathan A. Muthukumaran 《Tetrahedron letters》2005,46(41):7047-7050
Electrochemical bromination of toluene and substituted toluenes by two-phase electrolysis yields the corresponding α-brominated products. The reaction has been carried out in a single compartment cell with platinum electrodes at 0 °C in chloroform using an aqueous sodium bromide solution (60%) containing a catalytic amount of HBr. Two-phase electrolysis results in high yields (60-95%) of monobromo compounds with very high regioselectivity (>95%). 相似文献
50.
Francisco Sarabia 《Tetrahedron letters》2003,44(41):7671-7675
A synthetic approach to the [15]-membered stevastelins, a novel class of immunosuppressant agents, is reported based on a macrolactamization route to the 2,3-epoxy derivative 6. The synthesis of this compound was achieved via a stereoselective epoxidation of the allylic alcohol 13, followed by a coupling reaction with a variety of peptide derivatives to give the epoxy peptides 7-10. After an extensive study of cyclizations with these precursors, the best result was achieved with the macrolactamization of 8 in the presence of DEPC, to obtain the epoxy cyclic depsipeptide 6 in 42% yield. From this product, an epoxy analogue of stevastelin B, compound 27, was prepared. Finally, the synthesis of the natural product was attempted through the opening of the oxirane ring contained in 6 and 26, with a variety of methyl cuprate reagents, but without success. 相似文献