全文获取类型
收费全文 | 1053篇 |
免费 | 19篇 |
国内免费 | 6篇 |
专业分类
化学 | 708篇 |
晶体学 | 34篇 |
力学 | 24篇 |
数学 | 184篇 |
物理学 | 128篇 |
出版年
2023年 | 4篇 |
2022年 | 7篇 |
2021年 | 20篇 |
2020年 | 12篇 |
2019年 | 16篇 |
2018年 | 10篇 |
2017年 | 20篇 |
2016年 | 24篇 |
2015年 | 16篇 |
2014年 | 20篇 |
2013年 | 56篇 |
2012年 | 50篇 |
2011年 | 66篇 |
2010年 | 44篇 |
2009年 | 46篇 |
2008年 | 56篇 |
2007年 | 57篇 |
2006年 | 57篇 |
2005年 | 54篇 |
2004年 | 58篇 |
2003年 | 42篇 |
2002年 | 40篇 |
2001年 | 15篇 |
2000年 | 21篇 |
1999年 | 15篇 |
1998年 | 15篇 |
1997年 | 12篇 |
1996年 | 20篇 |
1995年 | 15篇 |
1994年 | 11篇 |
1993年 | 15篇 |
1992年 | 11篇 |
1991年 | 10篇 |
1990年 | 7篇 |
1989年 | 13篇 |
1988年 | 13篇 |
1987年 | 5篇 |
1986年 | 14篇 |
1985年 | 16篇 |
1984年 | 11篇 |
1983年 | 6篇 |
1982年 | 7篇 |
1981年 | 11篇 |
1980年 | 6篇 |
1977年 | 3篇 |
1976年 | 6篇 |
1918年 | 2篇 |
1914年 | 2篇 |
1913年 | 2篇 |
1907年 | 2篇 |
排序方式: 共有1078条查询结果,搜索用时 15 毫秒
21.
Andrzej Machocki Tadeusz Borowiecki Andrezej Denis 《Reaction Kinetics and Catalysis Letters》1993,51(2):541-545
The studies of the influence of the kind of natural calcium mineral and the kind and amount of sodium promoter on the conversion degree of methane and oxygen, selectivity and yield of the oxidative conversion of methane have shown the usability of limestone as a basic component of the catalyst for this process. 相似文献
22.
A new liquid nitrogen (LN2) jet-based thermal modulator for performing comprehensive two-dimensional (2D) gas chromatographic (GC x GC) separations has been designed and constructed. Temperature measurements of the trapping zone, a segment of uncoated fused silica capillary, show that it can be cooled to -196 degrees C in about 300 ms. A film of liquid nitrogen develops on the outside of the trapping capillary even when the oven temperature is in excess of 200 degrees C. Compounds as volatile as propane can be trapped by the modulator and held for periods of at least 1 min without breakthrough. The peak widths for n-alkanes are on the order of 80 ms at half height after passing through an 80cm second dimension column. Repeated analysis of gasoline demonstrated excellent run-to-run reproducibility of the system. 相似文献
23.
Witold Danikiewicz Tadeusz Jaworski Stefan Kwiatkowski 《Monatshefte für Chemie / Chemical Monthly》1986,117(10):1177-1184
Starting from methyl 7-oxo-7-(1-cyclopentene)-heptanoate, a simple synthesis of the methyl esters of 7-oxo- and 7-hydroxy-9,11-dideoxy-PGF
1 as a model for 7-substituted prostaglandin analogues is described.
7-Substituierte Prostaglandin-Analoge — ein neuer Syntheseweg
Zusammenfassung Ausgehend von Methyl-7-oxo-7-(1-cyclopenten)-heptanoat wird eine einfache Synthese der Methylester von 7-Oxo- und 7-Hydroxy-9,11-dideoxy-PGF 1 als Modellverbindungen für 7-substituierte Prostaglandinanaloge beschrieben.相似文献
24.
Jerzy Lisowski Marta Paluch Tadeusz Lis 《Journal of inclusion phenomena and macrocyclic chemistry》2006,55(1-2):123-129
The template condensation of (R)-2,2′-diamino-1,1′-binaphthyl and 2,6-diformylpyridine leads to lanthanide(III) complexes of the new chiral hexaaza macrocycle L that adopts highly twisted conformation in [LnL](NO3)3 complexes. The complexes have been characterised by ESI MS spectrometry and NMR spectroscopy. The analogous N2O4 chiral crown ether L2 that has the same carbon skeleton as L does not exhibit tendency to bind lanthanide(III) ions. The X-ray crystal structure of L2 exhibit squeezed conformation of the macrocycle and spatial disposition of donor atoms that does not predispose it for coordination of lanthanide(III) ions. 相似文献
25.
Roland Boese Dieter Bläser Michael Nussbaumer Tadeusz Marek Krygowski 《Structural chemistry》1992,3(5):363-368
The crystal and molecular structure of nitrobenzene was determined at 103 K by X-ray diffraction, yieldingR=0.034 and a highly precise geometry of the molecule (esds of bond lengths 0.001 Å and bond angles 0.1°). The bond angles in the ring agree very well with additive scheme based on angular substituent parameters. X-X electron density maps support the view that the C-N bond does not exhibit-character, and, hence, the nitro group interacts with the ring mostly by inductive effects. 相似文献
26.
Sun G Kappl M Pakula T Kremer K Butt HJ 《Langmuir : the ACS journal of surfaces and colloids》2004,20(19):8030-8034
Forces across polymer melts are poorly understood despite their importance for adhesion and fabricating composite materials. Using an atomic force microscope (AFM), this interaction was measured for poly(dimethyl siloxane) (PDMS). The structure of the polymer at the surface changed during the first approximately 10 h. Afterward, short-range attractive forces were observed with short-chain PDMS (M(w) = 4200 g/mol). Using PDMS with a molecular weight (M(w) = 18 000 g/mol) above the entanglement limit, we measured a monotonically decaying repulsive force, which indicates that a quasi-immobilized layer had formed at the solid surface. Due to the small radius of curvature of the tip, forces could be measured in equilibrium. 相似文献
27.
Richard A. Bartsch David A. Babb Bronislaw P. Czech Dhimant H. Desai Eddy Chapoteau Carl R. Gebauer Wolodymyr Zazulak Anad Kumar 《Journal of inclusion phenomena and macrocyclic chemistry》1990,9(2):113-123
Four novel chromogenic benzocryptands and one side-armed chromogenic cryptand have been synthesized and their complexing abilities for alkali metal cations are described. 相似文献
28.
Peter Segl'a Martina Palicov Dusan Miklos Marian Koman Milan Melník Maria Korabik Jerzy Mrozinski Tadeusz Glowiak Markku R. Sundberg Peter Lnnecke 《无机化学与普通化学杂志》2004,630(3):470-478
Synthesis and characterization of seven new complexes [Cu(2‐MeSnic)2(CH3OH)]2 (where 2‐MeSnic is 2‐methylthionicotinate), [Cu(2‐MeSnic)2L2]2 (where L is pyridine — py, ethylnicotinate — Etnic and butylnicotinate — Bunic), [Cu(2‐MeSnic)2L2(H2O)2] (where L is py and nicotinamide — nia) and [Cu(2‐MeSnic)2(N‐Menia)2(H2O)2]·2H2O (where N‐Menia is N‐methylnicotinamide) are reported. The characterization were based on elemental analysis, infrared, electronic and EPR spectra, and magnetic susceptibility measurements over a temperature range of 1.8 — 300 K or 70 — 300 K. Three complexes of different type were studied by X‐ray analysis. The molecule of [Cu(2‐MeSnic)2(CH3OH)]2 has dimeric paddle‐wheel cage structure with a tetragonal pyramidal arrangement around CuII. The dimer results from the fact that carboxyl groups of four 2‐MeSnic anions function as bridging in a syn‐syn arrangement. On the other hand [Cu(2‐MeSnic)2(py)2]2 forms dimers with hexacoordinated CuII atoms in highly distorted coordination octahedra, each with two oxygen atoms of bridging carboxyl groups in an anti‐anti arrangement of two 2‐MeSnic anions, with two oxygen atoms of one asymmetrically chelating 2‐MeSnic anion and with two nitrogen atoms of two pyridine ligands. The temperature independent EPR spectrum for this complex exhibits an axial signal which corresponds to almost isolated S = 1/2 magnetic ions. Magnetic data for the dimer show a weak antiferromagnetic interaction between the two metal ions with J = —0.65 cm—1. The CuII atom in complex [Cu(2‐MeSnic)2(py)2(H2O)2] is hexacoordinated in an elongated centrosymmetrical tetragonal‐bipyramidal arrangement (4 + 2). Based on the molecular structure the electronic, infrared, electron paramagnetic resonance spectra and magnetic properties are discussed and stereochemistry as well as the mode of ligand coordination in new solid complexes under study have been determined. 相似文献
29.
The fragmentation patterns of the thioacyl derivatives of 2-aminothiazole and 2-aminobenzothiazole and of their ‘fixed structure’ imino and amino tautomeric forms give evidence of the predominance of the imino tautomer in the molecular ions of the trifluorothioacetyl compounds. On the other hand, the M+˙ ions of the thioacetyl and thiobenzoyl derivatives are mainly the amino tautomers. A rearrangement with elimination of RCN and formation of the 2-thiazolothione (2-benzothiazolothione) ion is characteristic for ail the compounds investigated. Additional evidence for the interpretation of the main fragmentation paths is provided by the mass spectra of the N-trideuteromethyl and N-deuterated derivatives, high-resolution mass spectrometry being used to determine the elemental composition of some selected peaks. 相似文献
30.
Oxidative damage to plasmenyl-type lipids contributes to decreased membrane barrier function, loss of membrane structure and formation of nonlamellar defects in membrane bilayers. Previous results from this laboratory have shown that membrane-soluble sensitizers (e.g. zinc phthalocyanine and bacteriochlorophyll a) mediate the photooxidation of palmitoyl plasmenylcholine (1-O-alk-1'-Z-enyl-2-palmitoyl-sn-glycero-3-phosphocholine; PPlsC) vesicles with the subsequent creation of lamellar defect structures, vesicle contents leakage and membrane-membrane fusion. Because plasmalogen lipids are significant components of sarcoplasma and myelin membranes, we sought to characterize the products of their photooxidation. This study focuses on the photooxidation of PPlsC vesicles in the presence of the water-soluble sensitizer, aluminum phthalocyanine tetrasulfonate (AlPcS4(4-)). Attack of photogenerated singlet oxygen on the 1-O-alkenyl ether linkage of PPlsC lipids was expected to generate dioxetane- and ene-type photoproducts. The products formed during continuous aerobic irradiation (28 mW/cm2, (610 nm) of PPlsC vesicles in the presence of AlPcS4(4-) were separated via reverse-phase high-performance liquid chromatography (HPLC) with electrochemical detection (ECD) or evaporative light-scattering detection (ELSD). Photooxidized dipalmitoyl-phosphatidylcholine-cholesterol vesicles (control) were used to optimize the HPLC-ECD conditions, using 7 alpha-hydroperoxy-cholesterol as standard. HPLC-ECD was found to be most sensitive for PPlsC hydroperoxides, whereas HPLC-ELSD was more sensitive for nonhydroperoxide photoproducts. The three major photoproducts formed during vesicle irradiation were isolated via preparative HPLC and then characterized by 1H-nuclear magnetic resonance and mass spectrometry. 1-Formyl-2-palmitoyl-sn-glycero-3-phosphocholine and 1-hydroxy-2-palmitoyl-sn-glycero-3-phosphocholine were identified as dioxetane cleavage products that coeluted at approximately 3 min. The second fraction (retention time [RT] = 48 min) was identified as a PPlsC allylic hydroperoxide. The third photoproduct, eluting at RT = 64 min, is tentatively identified as an oxidation product arising from allylic hydroperoxide degradation via Hock rearrangement or free radical decomposition. 相似文献