全文获取类型
收费全文 | 15247篇 |
免费 | 679篇 |
国内免费 | 8篇 |
专业分类
化学 | 11275篇 |
晶体学 | 89篇 |
力学 | 277篇 |
数学 | 2089篇 |
物理学 | 2204篇 |
出版年
2023年 | 122篇 |
2022年 | 82篇 |
2021年 | 141篇 |
2020年 | 224篇 |
2019年 | 241篇 |
2018年 | 223篇 |
2017年 | 225篇 |
2016年 | 523篇 |
2015年 | 478篇 |
2014年 | 520篇 |
2013年 | 966篇 |
2012年 | 1173篇 |
2011年 | 1275篇 |
2010年 | 688篇 |
2009年 | 527篇 |
2008年 | 1118篇 |
2007年 | 1079篇 |
2006年 | 988篇 |
2005年 | 939篇 |
2004年 | 768篇 |
2003年 | 614篇 |
2002年 | 551篇 |
2001年 | 210篇 |
2000年 | 190篇 |
1999年 | 123篇 |
1998年 | 98篇 |
1997年 | 92篇 |
1996年 | 136篇 |
1995年 | 88篇 |
1994年 | 70篇 |
1993年 | 95篇 |
1992年 | 78篇 |
1991年 | 64篇 |
1990年 | 60篇 |
1989年 | 47篇 |
1988年 | 49篇 |
1987年 | 44篇 |
1986年 | 69篇 |
1985年 | 94篇 |
1984年 | 80篇 |
1983年 | 46篇 |
1982年 | 77篇 |
1981年 | 69篇 |
1980年 | 73篇 |
1979年 | 54篇 |
1978年 | 57篇 |
1977年 | 43篇 |
1976年 | 50篇 |
1975年 | 29篇 |
1974年 | 32篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
701.
Steffen Blaurock Thomas Koch Olaf Kühl Peter Lönnecke Felicite Majoumo René Sommer 《Phosphorus, sulfur, and silicon and the related elements》2013,188(6-7):1451-1456
Substituted bifunctional phosphorus-based ligands HX(CRR') n PR"H (or -PR" 2 ) [where X = O, S, NR', (substituted) cyclopentadienyl; n = 1, 2, 3; R, R', R" = alkyl, aryl, H] were employed as bridging ligands in the synthesis of early/late bridged transition metal complexes. Synthetic routes to the bifunctional ligands were also developed. First, mononuclear complexes, such as [TpZr(OCH 2 PPh 2 ) 3 ] (Tp = trispyrazolylborato), [Cp 2 Zr(1-O-2-PHR-C 6 H 10 )(Me)] (R = 2,4,6-Pr i 3 C 6 H 2 (Tipp)), [Cp 2 Zr(SCH 2 CH 2 PHR) 2 ] (R = Ph, Mes, Tipp), and phosphinoferrocene derivatives, were prepared. These complexes are suitable precursors for the introduction of a second metal (as in, for example, [TpZr( w -OCH 2 PPh 2 ) 3 Mo(CO) 3 ]). 相似文献
702.
José Barluenga Fernando López 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3-4)
Abstract Whereas several procedures have been developed for synthesizing λ3-phospholes and their oxides and sulphides1, access to 2H-λ5-Phospholes is limited to the reaction of phosphines2 with Dimethyl Acetylen-dicarboxilate (DMAD) and only unstable 3H-λ5-Phospholes were detected2 when diphenylvinylphosphine were used. 相似文献
703.
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
704.
Jelem Restrepo José Salazar Simón E. López 《Phosphorus, sulfur, and silicon and the related elements》2013,188(12):2311-2320
Abstract We describe a rapid and convenient methodology for the preparation of diverse 3-aryl and 3-trifluoromethyl 4H-1,2,4-benzothiadiazine-1,1-dioxides in a one-pot microwave-promoted reaction between 2-aminobenzenesulfonamides and benzaldehydes, trifluoroacetic acid or benzoic acids. GRAPHICAL ABSTRACT 相似文献
705.
Pascal Gosselin Serge Masson Monique Saquet André Thuillier 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-2):103-104
Abstract A new synthesis of dithioesters, starting from phenylisothiocyanate is described (scheme I) and this method is convenient for the preparation of β-olefinic dithioesters (e. g. 1, scheme II). 相似文献
706.
Yves Gareau André Beauchemin 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1):393-394
Abstract The reaction of diisopropyl xanthogen and thio-diisopropyl xanthogen disulfide with alkynes furnished 1,3-dithiol-2-one and 1,3-dithiol-2-thione. 相似文献
707.
Microwave assisted free radical copolymerization of diisopropyl fumarate (DIPF) and benzyl acrylate (BzA) with different copolymer compositions was performed using benzoyl peroxide as initiator. The effect of the reaction conditions on the macromolecular characteristics, monomer reactivity ratio and copolymer properties were studied. The monomer conversion and average molecular weights increase with the content of BzA units in the copolymer. The copolymers were characterized by IR, 1H and 13C-NMR spectroscopies and the molecular weights were analyzed with size exclusion chromatography (SEC). The reactivity ratios obtained from an extended Kelen-Tüdös method under microwave irradiations are a factor which is double than those obtained by thermal copolymerization. The product r1r2 = 0.152 suggests a preference of both propagating macroradicals toward consecutive homopolymerization. The hydrodynamic and polydispersity size were measured in ethylacetate, tetrahydrofuran and methylethylketone with a quasi-elastic light scattering (QELS) technique showing that the quality of the solvents increases in the order: EA < THF < MEK. 相似文献
708.
Amit Das René Jurk Klaus Werner Stöckelhuber Thomas Engelhardt Juliane Fritzsche Manfred Klüppel 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(2):144-150
A novel method is described for the preparation of nanocomposites comprising a high performance rubber for tire application and layered silicates clay. In this work nanocomposites of solution‐styrene butadiene rubber (S‐SBR) with montmorillonite layered silicate were prepared with carboxylated nitrile rubber (XNBR), a polar rubber, as a compatibilizer. A sufficient amount of organomodified layered silicate was loaded in carboxylated nitrile rubber (XNBR) and this compound was blended as a master batch in the S‐SBR. Mixed intercalated/exfoliated morphologies in the nanocomposite are evinced by X‐ray diffraction measurements and transmission electron microscopy. Dynamic mechanical analysis also supports the compatibility of the composites. A good dispersion of the layered silicate in the S‐SBR matrix was reflected from the physical properties of the nanocomposites, especially in terms of tensile strength and high elongation properties. 相似文献
709.
Isabelle Déléris Anne Saint‐Eve Etienne Sémon Hervé Guillemin Elisabeth Guichard Isabelle Souchon Jean‐Luc Le Quéré 《Journal of mass spectrometry : JMS》2013,48(5):594-607
For the on‐line monitoring of flavour compound release, atmospheric pressure chemical ionization (APCI) and proton transfer reaction (PTR) combined to mass spectrometry (MS) are the most often used ionization technologies. APCI‐MS was questioned for the quantification of volatiles in complex mixtures, but direct comparisons of APCI and PTR techniques applied on the same samples remain scarce. The aim of this work was to compare the potentialities of both techniques for the study of in vitro and in vivo flavour release. Aroma release from flavoured aqueous solutions (in vitro measurements in Teflon bags and glass vials) or flavoured candies (in vivo measurements on six panellists) was studied using APCI‐ and PTR‐MS. Very similar results were obtained with both techniques. Their sensitivities, expressed as limit of detection of 2,5‐dimethylpyrazine, were found equivalent at 12 ng/l air. Analyses of Teflon bag headspace revealed a poor repeatability and important ionization competitions with both APCI‐ and PTR‐MS, particularly between an ester and a secondary alcohol. These phenomena were attributed to dependency on moisture content, gas/liquid volume ratio, proton affinities and product ion distribution, together with inherent drawbacks of Teflon bags (adsorption, condensation of water and polar molecules). Concerning the analyses of vial headspace and in vivo analyses, similar results were obtained with both techniques, revealing no competition phenomena. This study highlighted the equivalent performances of APCI‐MS and PTR‐MS for in vitro and in vivo flavour release investigations and provided useful data on the problematic use of sample bags for headspace analyses. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
710.
Job Tchoumtchoua Dieudonné Njamen Jean Claude Mbanya Alexios‐Leandros Skaltsounis Maria Halabalaki 《Journal of mass spectrometry : JMS》2013,48(5):561-575
Hyphenated techniques and especially ultra‐high performance liquid chromatography‐mass spectrometry (UHPLC‐MS) are nowadays widely employed in natural products research. However, the complex nature of plant extracts complicates considerably the analysis and the identification of their constituents. Nevertheless, new MS analyzers with increased resolving power and accuracy such as the orbital trap (Orbitrap) could facilitate drastically this process. The objective of this study is the development of a new structure‐oriented approach based on fast UHPLC‐high‐resolution (HR)MS and HRMS/MS methodologies for the identification of isoflavonoids in crude extracts. In addition, aims to assist dereplication procedures, to decrease the laborious isolation steps and orient the focused isolation of compounds of interest. As a proof of concept, the methanol extract of the stem bark of Amphimas pterocarpoides (Leguminosae) was selected. Based on chromatographic (retention time, polarity) and spectrometric features (ultraviolet spectra, accurate m/z, proposed elemental composition, ring double bond equivalent, and relative isotopic abundance) as well as HRMS/MS spectra, several isoflavonoids were identified. In order to verify the proposed structures, 11 isoflavonoids were selectively isolated and unambiguously identified using 1&2D nuclear magnetic resonance techniques. Moreover, the isolated isoflavonoids were studied in HRMS/MS level, employing electrospray ionization and atmospheric pressure chemical ionization sources, in both modes. Useful information regarding their fragmentation patterns was obtained, and characteristic diagnostic ions were defined for the identification of methoxylated isoflavones, dihydroisoflavones and 5‐hydroxylated isoflavonoids. Based on the current results, the proposed dereplication strategy was verified and could comprise a novel approach for the analysis of crude extracts in the future not only for isoflavonoids but also for other chemical classes of natural products. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献