全文获取类型
收费全文 | 637篇 |
免费 | 4篇 |
国内免费 | 6篇 |
专业分类
化学 | 408篇 |
晶体学 | 15篇 |
力学 | 5篇 |
数学 | 118篇 |
物理学 | 101篇 |
出版年
2021年 | 5篇 |
2020年 | 7篇 |
2019年 | 4篇 |
2018年 | 5篇 |
2017年 | 4篇 |
2016年 | 7篇 |
2015年 | 4篇 |
2014年 | 12篇 |
2013年 | 19篇 |
2012年 | 39篇 |
2011年 | 36篇 |
2010年 | 27篇 |
2009年 | 20篇 |
2008年 | 25篇 |
2007年 | 30篇 |
2006年 | 21篇 |
2005年 | 17篇 |
2004年 | 20篇 |
2003年 | 21篇 |
2002年 | 10篇 |
2001年 | 7篇 |
2000年 | 6篇 |
1999年 | 11篇 |
1998年 | 9篇 |
1997年 | 12篇 |
1996年 | 14篇 |
1995年 | 11篇 |
1994年 | 17篇 |
1993年 | 14篇 |
1992年 | 15篇 |
1991年 | 16篇 |
1990年 | 9篇 |
1989年 | 9篇 |
1988年 | 10篇 |
1987年 | 6篇 |
1986年 | 10篇 |
1985年 | 10篇 |
1984年 | 7篇 |
1983年 | 13篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1980年 | 5篇 |
1979年 | 11篇 |
1978年 | 14篇 |
1977年 | 5篇 |
1976年 | 9篇 |
1975年 | 4篇 |
1974年 | 6篇 |
1973年 | 12篇 |
1972年 | 7篇 |
排序方式: 共有647条查询结果,搜索用时 15 毫秒
101.
New LS Saha S Ong MM Boelsterli UA Chan EC 《Rapid communications in mass spectrometry : RCM》2007,21(6):982-988
A rapid and sensitive ultra-performance liquid chromatography/tandem mass spectrometry (UPLC/MS/MS) method was developed and validated for the determination of troglitazone in mouse plasma. Troglitazone and its internal standard (IS), rosiglitazone, were separated on an ACQUITY UPLC BEH C(18) column (1.7 microm particle size, 50 x 2.1 mm i.d.) by gradient elution with water and methanol at a flow rate of 0.5 mL/min. The cycle time of each analysis was 2.5 min. Rosiglitazone and troglitazone eluted at 1.13 and 1.57 min, respectively, and were chromatographically resolved from the ion suppression and enhancement zones due to the biological matrix effect. Quantitation of the analytes was performed in electrospray negative ionization mode (ESI -ve) using multiple reaction monitoring (MRM) experiments. The weighted (1/x) calibration curve was quadratic over the plasma concentration range 1-2500 ng/mL with a correlation coefficient (r(2)) of 0.9966. The limit of quantitation (LOQ) of troglitazone in mouse plasma was lower than 1 ng/mL. The inter- and intra-day variations of the assay were lower than 12.1%; the overall accuracy ranged from 86.4-110.2% and recovery from spiked plasma was more than 60%. The developed method was successfully applied to determine troglitazone in mouse plasma after intraperitoneal administration. 相似文献
102.
103.
Jzsef Deli Pter Molnr Zoltn Matus Gyula Tth Andrea Steck Urs A.Niggli Hanspeter Pfander 《Helvetica chimica acta》1998,81(10):1815-1820
From the pollens of Aesculus hippocastanum, a new apocarotenoid was isolated as the main carotenoid and, based on the spectroscopic data, identified as (all-E,3R)-3-hydroxy-6′-apo-β-caroten-6′-al ( 4 , aesculaxanthin). In addition, (all-E)-lutein ( 3 ) and (all-E)-β-citraurin ( 5 ) were isolated. Furthermore, 6 (aesculaxanthol) was prepared by reduction of 4 with NaBH4 and tentatively identified as natural carotenoid. 相似文献
104.
Roger Blum Edgardo Giovannini Urs Hengartner Gabriel Vallat 《Helvetica chimica acta》2002,85(6):1827-1840
On Rearrangements by Cyclialkylations of Arylpentanols to 2,3‐Dihydro‐1 H ‐indene Derivatives. Part 1. An Unexpected Rearrangement by the Acid‐Catalyzed Cyclialkylation of 4‐(2‐Chlorophenyl)‐2,4‐dimethyl pentan‐2‐ol under Formation of trans ‐4‐Chloro‐2,3‐dihydro‐1,1,2,3‐tetramethyl‐1 H ‐indene The acid‐catalyzed cyclialkylation of 2,4‐dimethyl‐4‐phenylpentan‐2‐ol led exclusively to the expected product, 2,3‐dihydro‐1,1,3,3‐tetramethyl‐1H‐indene. However, analogous cyclialkylation of 4‐(2‐chlorophenyl)‐2,4‐dimethylpentan‐2‐ol ( 1 ) gave a ca. 1 : 1 mixture of 4‐chloro‐2,3‐dihydro‐1,1,3,3‐tetramethyl‐1H‐indene ( 2 ) and of trans‐4‐chloro‐2,3‐dihydro‐1,1,2,3‐tetramethyl‐1H‐indene ( 3 ; Scheme 1). The specific action of the Cl substituent is investigated and a mechanism for this unexpected frame‐work transposition proposed. 相似文献
105.
Dieter Seebach Stefan G. Müller Urs Gysel Jürg Zimmermann 《Helvetica chimica acta》1988,71(5):1303-1318
Preparative Chromatographic Resolution of Synthetically Useful Cyclic Acetals Racemic cyclic acetals derived from aldehydes and glycine, glycolic acid, thioglycolic acid, formylacetic acid, and acetoacetic acid (oxazolidinones 4 – 13 , dioxolanones 14 , 15 , oxathiolanone 16 , dioxinones 17 – 23 ) are resolved by preparative high-pressure liquid chromatography on silica gel coated with the polymer from N-acryloylphenylalanine ethyl ester (Chiraspher®). The separation factors α range from1,1 to 2,4. Use of a Prepbar®-chromatography system allows injection of several grams at a time. The enantiomeric acetals thus obtained are fully characterized. First application to amino-acid synthesis are mentioned. 相似文献
106.
Graham Smith Urs D. Wermuth Jonathan M. White 《Acta Crystallographica. Section C, Structural Chemistry》2005,61(7):o464-o468
In the crystal structures of the proton‐transfer compounds of strychnine with 3,5‐dinitrosalicylic acid, namely strychninium 3,5‐dinitrosalicylate, C21H23N2O2+·C7H3N2O7−, (I), and 5‐nitrosalicylic acid, namely strychninium–5‐nitrosalicylate–5‐nitrosalicylic acid (1/1/2), C21H23N2O2+·C7H4NO5−·2C7H5NO5, (II), protonation of one of the N atoms of the strychnine molecule occurs and this group is subsequently involved in intermolecular hydrogen‐bonding interactions. In (I), this is four‐centred, the primary being with an adjacent strychninium carbonyl O‐atom acceptor in a side‐to‐side interaction giving linear chains. Other interactions are with the phenolate and nitro O‐atom acceptors of the anionic species, resulting in a one‐dimensional polymer structure. In (II), the N+—H interaction is three‐centred, the hydrogen bonding involving carboxyl O‐atom acceptors of the anion and both acid adduct species, giving unique discrete hetero‐tetramer units. The structure of (II) also features π‐bonding interactions between the two acid adduct molecules. 相似文献
107.
For the direct solution of quadratic eigenvalue problems of the form (λ2 M + P + Q ) x = 0 , a generalization of the Rayleigh quotient iteration is presented. Numerical simulations show good convergence for problems where the eigenvalues have nonzero imaginary part. The method is used to calculate eigenvalue paths of parameter dependent problems in structural dynamics. Bifurcations with double eigenvalues, which can occur in the path, are passed by using a perturbation of the velocity dependent matrix P . (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
108.
Huang D Lüthi U Kolb P Cecchini M Barberis A Caflisch A 《Journal of the American Chemical Society》2006,128(16):5436-5443
Alzheimer's disease, the most common amyloid-associated disorder, accounts for the majority of the dementia diagnosed after the age of 60. The cleavage of the beta-amyloid precursor protein is initiated by beta-secretase (BACE-1), a membrane-bound aspartic protease, which has emerged as an important but difficult protein target. Here, an in silico screening approach consisting of fragment-based docking, ligand conformational search by a genetic algorithm, and evaluation of free energy of binding was used to identify low-molecular-weight inhibitors of BACE-1. More than 300,000 small molecules were docked and about 15,000 prioritized according to a linear interaction energy model with evaluation of solvation by continuum electrostatics. Eighty-eight compounds were tested in vitro, and 10 of them showed an IC(50) value lower than 100 microM in a BACE-1 enzymatic assay. Interestingly, the 10 active compounds shared a triazine scaffold. Moreover, four of them were active in an assay with mammalian cells (EC(50) < 20 microM), indicating that they are cell-permeable. Therefore, these triazine derivatives are very promising lead candidates for BACE-1 inhibition. The discoveries of this series and two other series of nonpeptidic BACE-1 inhibitors demonstrate the usefulness of our in silico high-throughput screening approach. 相似文献
109.
Shahid Ullah Tomas Alsberg Robin Vestergren Urs Berger 《Analytical and bioanalytical chemistry》2012,404(8):2193-2201
A sensitive and accurate method was developed and validated for simultaneous analysis of perfluoroalkyl carboxylic acids, sulfonic acids, and phosphonic acids (PFPAs) at low picograms per gram concentrations in a variety of food matrices. The method employed extraction with acetonitrile/water and cleanup on a mixed-mode co-polymeric sorbent (C8?+?quaternary amine) using solid-phase extraction. High-performance liquid chromatographic separation was achieved on a C18 column using a mobile phase gradient containing 5?mM 1-methyl piperidine for optimal chromatographic resolution of PFPAs. A quadrupole time-of-flight high-resolution mass spectrometer operating in negative ion mode was used as detector. Method detection limits were in the range of 0.002 to 0.02?ng?g?1 for all analytes. Sample preparation (extraction and cleanup) recoveries at a spiking level of 0.1?ng?g?1 to a baby food composite were in the range of 59 to 98?%. A strong matrix effect was observed in the analysis of PFPAs in food extracts, which was tentatively assigned to sorption of PFPAs to the injection vial in the solvent-based calibration standard. The method was successfully applied to a range of different food matrices including duplicate diet samples, vegetables, meat, and fish samples. Figure
Extracted high-resolution mass chromatograms of a PFPAs spiked at 0.06 ng g?–1 to baby food, b PFSAs spiked at 0.02 ng g?–1 to baby food, and c PFCAs spiked at 0.02 ng g?–1 to baby food 相似文献
110.
J Streuff M Feurer P Bichovski G Frey U Gellrich 《Angewandte Chemie (International ed. in English)》2012,51(34):8661-8664
Reduction, please! The title reaction affords α-hydroxyketones, a common structural motif in biologically active natural products, in good yields and high enantioselectivities at room temperature. The commercially available ansa-titanocene 1 was found to be an efficient catalyst for this process, which presumably proceeds by addition of a ketyl radical to a nitrile. 相似文献