全文获取类型
收费全文 | 7087篇 |
免费 | 140篇 |
国内免费 | 14篇 |
专业分类
化学 | 4782篇 |
晶体学 | 30篇 |
力学 | 135篇 |
数学 | 789篇 |
物理学 | 1505篇 |
出版年
2021年 | 43篇 |
2020年 | 42篇 |
2019年 | 65篇 |
2016年 | 115篇 |
2015年 | 94篇 |
2014年 | 124篇 |
2013年 | 222篇 |
2012年 | 178篇 |
2011年 | 268篇 |
2010年 | 160篇 |
2009年 | 161篇 |
2008年 | 238篇 |
2007年 | 225篇 |
2006年 | 228篇 |
2005年 | 210篇 |
2004年 | 194篇 |
2003年 | 181篇 |
2002年 | 166篇 |
2001年 | 126篇 |
2000年 | 121篇 |
1999年 | 99篇 |
1998年 | 86篇 |
1997年 | 90篇 |
1996年 | 112篇 |
1995年 | 123篇 |
1994年 | 112篇 |
1993年 | 134篇 |
1992年 | 93篇 |
1991年 | 72篇 |
1990年 | 82篇 |
1989年 | 90篇 |
1988年 | 86篇 |
1987年 | 93篇 |
1986年 | 85篇 |
1985年 | 103篇 |
1984年 | 86篇 |
1983年 | 63篇 |
1982年 | 68篇 |
1981年 | 95篇 |
1980年 | 96篇 |
1979年 | 90篇 |
1978年 | 103篇 |
1977年 | 63篇 |
1976年 | 94篇 |
1975年 | 98篇 |
1974年 | 77篇 |
1973年 | 97篇 |
1972年 | 60篇 |
1971年 | 56篇 |
1967年 | 44篇 |
排序方式: 共有7241条查询结果,搜索用时 15 毫秒
91.
The mass spectrometric behaviour of 1-aryl-5-(2-dialkylaminovinyI)-lH-tetrazoles was studied, especially 1-phenyl-5-(2-dimethylaminovinyl)-l H-tetrazole as a typical example of D- and 15N-labelled derivatives revealed a rearrangement via a carbodiimide-like intermediate ion. 相似文献
92.
Michael Karas Hanno Ehring Eckhard Nordhoff Bernd Stahl Kerstin Strupat Franz Hillenkamp Matthias Grehl Bernt Krebs 《Journal of mass spectrometry : JMS》1993,28(12):1476-1481
Selected benzoic acid derivatives and related substances were used as additives to 2,5-dihydroxybenzoic acid (2,5DHB) and the performance of the mixtures in matrix-assisted laser desorption/ionization mass spectrometry was investigated. Using benzoic acid derivatives substituted at position 2 and/or 5 or related substances as a co-matrix in the 1–10% range with 2,5DHB results in improved ion yields and signal-to-noise ratio of analyte molecules, especially for the high-mass range. The enhanced performance is prominent for 2-hydroxy-5-methoxybenzoic acid and exists for both proteins and oligosaccharides. It is suggested that the improvement is caused by a disorder in the 2,5DHB crystal lattice allowing ‘softer’ desorption. Charge transfer from matrix ions to additive molecules at the expense of analyte ionization gives a simple explanation for the deteriorating effects of some tested additives. 相似文献
93.
By a comparison of the mass spectra of mono-, bis- and tris(trimethylsilyl)pyrazole isomers, characteristic influences of the different TMS-groups are revealed. An N-TMS-group forms an ion of mass 72 in the spectrum of N-TMS-pyrazole and with more than one TMS-group present, leads to the expulsion of a neutral fragment C3H8Si. Decomposition of the 4-TMS-group is more facile than the others, giving rise to a small molecular ion of intensity (approx.) 5%Σ40. In N-silylated pyrazoles without a 4-TMS-group, intensive elimination of HCN from the [M ? CH3˙]+ ion can be assigned to an unsubstituted C-3 position. In the case of unsymmetrically substituted pyrazoles without and N-TMS-group, silylation by an N-TMS acetamide may help to identify the predominating tautomer. 相似文献
94.
95.
Benchmarking of ruthenium initiators for the ROMP of a norbornenedicarboxylic acid ester 总被引:1,自引:0,他引:1
Sandra Demel Wolfgang Schoefberger Christian Slugovc Franz Stelzer 《Journal of molecular catalysis. A, Chemical》2003,200(1-2):11-19
The kinetic study of ring-opening metathesis polymerization (ROMP) of a diester functionalised norbornene derivative, (±)-exo,endo-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid diethyl ester, with a series of ruthenium benzylidene complexes revealed the applicability of these initiators for well defined polymerization reactions. Values for the rate of initiation as well as the rate of propagation of the initiators were determined and correlated to the molecular weight and polydispersity of the isolated polymers. As the only initiator providing an entry to virtually monodisperse polymers the classical “first generation Grubbs-catalyst” was identified, while N-heterocyclic carbene based initiators polymerized with a rate of propagation much higher than the rate of initiation yielding polymers with a broader molecular weight distribution. 相似文献
96.
Jochen Münchenberg Jens R. Goerlich Axel K. Fischer Peter G. Jones Reinhard Schmutzler 《无机化学与普通化学杂志》1996,622(2):348-354
Synthesis, Structure, and some Reactions of N-(N′,N′,N″,N″-tetramethyl)guanidinyl-substituted Phosphoryl Compounds The tetramethylguanidinyl-substituted phosphoryl compounds 1 – 10 were prepared in the reaction of the appropriate chlorophosphoryl compounds with either N′,N′,N″,N″-tetramethylguanidine (HTMG) or N-trimethylsilyl-(N′,N′,N″,N″-tetramethyl)guanidine (TMSTMG). With methyl iodide 1 reacted with N-alkylation to give the ammonium salt 11. 1 reacted with BF3 · Et2O at both imino nitrogen atoms with formation of the bis-BF3-adduct 12 . The X-ray structure determination of phenylphosphonic acid-bis(N′,N′,N″,N″-tetramethylguanidinide) 3 shows shortened PN-bonds and widened PNC-angles, consistent with the partial double bond character of the PN-bond. 相似文献
97.
When crystallized from appropriate solvents, the complex aqua-bis(dimethylglyoximato)nitrocobalt (III) may incorporate solvent molecules, thus forming a variety of mixed crystals. In the resulting host-guest crystals, the space groupP2l/m and the packing motif of the pure host compound are retained. Lattice constantsa andb remain essentially unaltered upon intercalation, whereasc and the monoclinic angle depend largely on the clathrated guest. Space filling and intermolecular contacts are discussed.Dedicated to Professor Peter Paetzold at the occasion of his 60th birthday. A preliminary account of this work has been given at the Spring Meeting of the British Crystallographic Association, Newcastle upon Tyne, 1994. 相似文献
98.
Chen Wang Amit Fischer Avner Ehrlich Yaakov Nahmias Itamar Willner 《Chemical science》2020,11(17):4516
The enzymes glucose oxidase (GOx), acetylcholine esterase (AchE) and urease that drive biocatalytic transformations to alter pH, are integrated into pH-responsive DNA-based hydrogels. A two-enzyme-loaded hydrogel composed of GOx/urease or AchE/urease and a three-enzyme-loaded hydrogel composed of GOx/AchE/urease are presented. The biocatalytic transformations within the hydrogels lead to the dictated reconfiguration of nucleic acid bridges and the switchable control over the stiffness of the respective hydrogels. The switchable stiffness features are used to develop biocatalytically guided shape-memory and self-healing matrices. In addition, loading of GOx/insulin in a pH-responsive DNA-based hydrogel yields a glucose-triggered matrix for the controlled release of insulin, acting as an artificial pancreas. The release of insulin is controlled by the concentrations of glucose, hence, the biocatalytic insulin-loaded hydrogel provides an interesting sense-and-treat carrier for controlling diabetes.Biocatalytic control over the stiffness of pH-responsive hydrogels is applied to develop shape-memory, self-healing and controlled release matrices. 相似文献
99.
100.
Using intermolecular SCF-perturbation theory, the controlling factors determining the regioselectivity in the acrolein dimerization are derived by a rigorous decomposition of the interaction energy. This partitioning of the interaction energy is performed up to a level where a comparison with frontier orbital models is possible. The regioselectivity is found to be determined by orbital interactions occuring via the terminal atoms and atoms involved in secondary Woodward-Hoffmann interactions. 相似文献