全文获取类型
收费全文 | 948篇 |
免费 | 35篇 |
国内免费 | 4篇 |
专业分类
化学 | 693篇 |
晶体学 | 9篇 |
力学 | 11篇 |
数学 | 162篇 |
物理学 | 112篇 |
出版年
2023年 | 10篇 |
2022年 | 28篇 |
2021年 | 27篇 |
2020年 | 32篇 |
2019年 | 29篇 |
2018年 | 17篇 |
2017年 | 16篇 |
2016年 | 30篇 |
2015年 | 31篇 |
2014年 | 27篇 |
2013年 | 48篇 |
2012年 | 73篇 |
2011年 | 63篇 |
2010年 | 36篇 |
2009年 | 33篇 |
2008年 | 46篇 |
2007年 | 48篇 |
2006年 | 50篇 |
2005年 | 55篇 |
2004年 | 42篇 |
2003年 | 33篇 |
2002年 | 30篇 |
2001年 | 11篇 |
2000年 | 9篇 |
1999年 | 5篇 |
1998年 | 5篇 |
1997年 | 16篇 |
1996年 | 14篇 |
1995年 | 7篇 |
1994年 | 9篇 |
1993年 | 4篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1988年 | 3篇 |
1987年 | 4篇 |
1985年 | 6篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1978年 | 7篇 |
1976年 | 3篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1933年 | 2篇 |
1931年 | 2篇 |
1925年 | 2篇 |
1923年 | 2篇 |
1918年 | 3篇 |
1917年 | 2篇 |
1915年 | 2篇 |
排序方式: 共有987条查询结果,搜索用时 15 毫秒
931.
932.
Alice Hubenko 《Discrete Mathematics》2008,308(7):1018-1024
Let us call a digraph D cycle-connected if for every pair of vertices u,v∈V(D) there exists a cycle containing both u and v. In this paper we study the following open problem introduced by Ádám. Let D be a cycle-connected digraph. Does there exist a universal edge in D, i.e., an edge e∈E(D) such that for every w∈V(D) there exists a cycle C such that w∈V(C) and e∈E(C)?In his 2001 paper Hetyei conjectured that cycle-connectivity always implies the existence of a universal edge. In the present paper we prove the conjecture of Hetyei for bitournaments. 相似文献
933.
934.
Veronika olínov Petra Szelov Alice Mov Jií Jir
ek Vclav Kai
ka 《Molecules (Basel, Switzerland)》2021,26(18)
The problem of a growing resistance of bacteria and other microorganisms to conventional antibiotics gave rise to a search for new potent antimicrobial agents. Insect antimicrobial peptides (AMPs) seem to be promising novel potential anti-infective therapeutics. The dipeptide β-alanyl-tyrosine (β-Ala-Tyr) is one of the endogenous insect toxins exhibiting antibacterial activity against both Gram-negative and Gram-positive bacteria. Prior to testing its other antimicrobial activities, it has to be prepared in a pure form. In this study, we have developed a capillary zone electrophoresis (CZE) method for analysis of β-Ala-Tyr isolated from the extract of the hemolymph of larvae of the fleshfly Neobellieria bullata by reversed-phase high-performance liquid chromatography (RP-HPLC). Based on our previously described correlation between CZE and free-flow zone electrophoresis (FFZE), analytical CZE separation of β-Ala-Tyr and its admixtures have been converted into preparative purification of β-Ala-Tyr by FFZE with preparative capacity of 45.5 mg per hour. The high purity degree of the β-Ala-Tyr obtained by FFZE fractionation was confirmed by its subsequent CZE analysis. 相似文献
935.
Man SP Benoit DM Buchaca E Esan F Motevalli M Wilson J Sullivan A 《Inorganic chemistry》2006,45(14):5328-5337
The synthesis of diethyl-8-quinolinyloxymethylphosphonate 1 and 8-quinolinyloxymethylphosphonic acid hemihydroiodide 2 is reported along with their spectroscopic and analytical characteristics (NMR, infrared, mass spectra, and elemental analysis). The single-crystal X-ray structure of 2 is described. Solutions of the disodium phosphonate 3, from 2 and sodium hydroxide, were prepared and used in situ. The coordination complex between 1 and zinc chloride 4 is described according to its single-crystal X-ray structure. The spectrophotometric features (absorption and emission) of 1, 2, 3, and 4 are reported along with the fluorescence response and affinity of 3 to some metal cations. The origin of the fluorescence from compounds 1 and 4 is investigated using density functional theory and the oscillator strength for each transition is computed. Two deactivation pathways, one of which is only weakly emissive, are identified in 4. In contrast to 4, which is only moderately more emissive than 1 a strong zinc-activated fluorescence response is observed from aqueous solutions of 3. 相似文献
936.
Even though open-framework solids have been made in a variety of compositions such as silicates, phosphates, germanates, borates, and phosphites, few are known that are based on trigonal-pyramidal sulfite anions. We report here the first synthetic and structural studies of metal-organic framework materials in the zinc sulfite composition. It is demonstrated here that Zn2+ and SO32- can form various neutral inorganic subunits that can be 0-D clusters, 1-D chains, or 2-D sheets. These inorganic subunits of different dimensionality can subsequently be connected into extended frameworks of higher dimensionality through bifunctional ligands. In (ZnSO3)2en, infinite corrugated ZnSO3 layers are pillared by ethylenediamine (en) molecules into a 3-D network that can be classified as a (3,4)-connected net based on tetrahedral Zn nodes and trigonal-pyramidal S nodes. In (ZnSO3)pip, infinite ZnSO3 chains are cross-linked with piperazine molecules into a 3-D framework that can be classified as 4-connected net based on tetrahedral Zn nodes only. In (ZnSO3)2(TMDPy)2, (ZnSO3)2 dimers are doubly bridged by trimethylenedipyridine molecules into an infinite chain with a string of circles. Each circle along the chain is interlocked with another circle from a chain in the perpendicular direction, creating a 2-D pattern with an infinite-square array of catenane-like units. 相似文献
937.
Busacca CA Campbell S Saha A Lorenz JC Grozinger K Jones PJ Grinberg N Shen S Lee H Qiu F Granger A Yuabova Z Norwood D Senanayake CH 《Magnetic resonance in chemistry : MRC》2005,43(12):1032-1039
Four unknown trace impurities (7-10) were observed in the capsule formulation of the HIV drug Tipranavir after prolonged storage at 30 degrees C/70% RH. Extensive NMR and LC/MS analyses revealed the compounds to be covalent adducts between TRIS, an excipient of the formulation, and diastereomeric Tipranavir alcohols formed via slow air oxidation of the drug substance. The structures were ultimately confirmed by total synthesis with final purification by chiral, preparative supercritical fluid chromatography. A novel Favorskii rearrangement to furnish butyrolactones was also uncovered during the synthesis. 相似文献
938.
Kim HY Lurain AE García-García P Carroll PJ Walsh PJ 《Journal of the American Chemical Society》2005,127(38):13138-13139
Three highly enantio- and diastereoselective one-pot procedures for the synthesis of cyclopropyl and iodocyclopropyl alcohols with up to four contiguous stereocenters are reported. Route 1 involves asymmetric addition of an alkylzinc reagent to an enal followed by diastereoselective cyclopropanation. Route 2 parallels route 1, except that iodoform is used to generate the zinc carbenoid, and the products are iodocyclopropyl alcohols. Route 3 entails asymmetric vinylation of an aldehyde with divinylzinc reagents and subsequent diastereoselective cyclopropanation. 相似文献
939.
Probing dipeptide trans/cis stereochemistry using pH control of thiopeptide analogues, and application to the PepT1 transporter 总被引:2,自引:0,他引:2
Bailey PD Boyd CA Collier ID Kellett GL Meredith D Morgan KM Pettecrew R Price RA 《Organic & biomolecular chemistry》2005,3(22):4038-4039
The stereochemistry of thiodipeptides of proline [e.g. Ala-Psi[CS-N]-Pro] can be controlled using pH, allowing the trans-preference for substrates of the peptide transporter PepT1 to be confirmed. 相似文献
940.
Godt A Duda S Unsal O Thiel J Härter A Roos M Tschierske C Diele S 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(22):5094-5106
The synthesis of monodisperse gigantocycles with 63, 87, and 147 ring atoms on the gram scale is described. These molecules were assembled from terphenylene derivatives and long, flexible chains which were mainly aliphatic, with terminal alkyne groups. The latter allowed for ring formation through oxidative alkyne dimerization in high yield (80-87%). The combination of a rod-like and a banana-shaped mesogen connected by flexible chains within the backbone of a ring gives rise to nematic and smectic mesophases. 相似文献