全文获取类型
收费全文 | 4181篇 |
免费 | 215篇 |
国内免费 | 21篇 |
专业分类
化学 | 3198篇 |
晶体学 | 16篇 |
力学 | 80篇 |
数学 | 536篇 |
物理学 | 587篇 |
出版年
2023年 | 52篇 |
2022年 | 73篇 |
2021年 | 84篇 |
2020年 | 135篇 |
2019年 | 109篇 |
2018年 | 63篇 |
2017年 | 79篇 |
2016年 | 174篇 |
2015年 | 150篇 |
2014年 | 175篇 |
2013年 | 198篇 |
2012年 | 305篇 |
2011年 | 257篇 |
2010年 | 149篇 |
2009年 | 141篇 |
2008年 | 227篇 |
2007年 | 215篇 |
2006年 | 188篇 |
2005年 | 196篇 |
2004年 | 183篇 |
2003年 | 121篇 |
2002年 | 154篇 |
2001年 | 66篇 |
2000年 | 60篇 |
1999年 | 51篇 |
1998年 | 58篇 |
1997年 | 50篇 |
1996年 | 54篇 |
1995年 | 38篇 |
1994年 | 32篇 |
1993年 | 31篇 |
1992年 | 42篇 |
1991年 | 24篇 |
1990年 | 27篇 |
1989年 | 33篇 |
1988年 | 33篇 |
1987年 | 30篇 |
1986年 | 23篇 |
1985年 | 27篇 |
1984年 | 31篇 |
1983年 | 26篇 |
1982年 | 21篇 |
1981年 | 38篇 |
1980年 | 24篇 |
1979年 | 21篇 |
1978年 | 17篇 |
1977年 | 27篇 |
1976年 | 10篇 |
1975年 | 12篇 |
1970年 | 13篇 |
排序方式: 共有4417条查询结果,搜索用时 15 毫秒
21.
"Bare" CeO(2)(+) ions can be prepared in the gas phase by consecutive oxidation of Ce(+) with O(2) and NO(2). The ability to activate saturated and unsaturated hydrocarbons is investigated by use of Fourier-transform ion cyclotron resonance mass spectrometry. In the reactions of CeO(2)(+) with linear and branched alkanes C-H bond activation is observed almost exclusively. In contrast, both oxygen-atom transfer and C-H bond activation processes occur when thermalized CeO(2)(+) cations react with simple alkenes and aromatic compounds. C-C bond activation is not observed at all. Insight into the structural and electronic properties of neutral CeO(2) and cationic CeO(2)(+) is provided by means of quasirelativistic density-functional and ab initio pseudopotential calculations. They reveal a (2)Sigma(u)(+) ground state for CeO(2)(+) which is best described as a linear cerium dioxide with a resonating pi bond. Finally, we discuss the influence of oxo ligands on the chemistry of the cationic CeO(n)()(+) (n = 0-2) species toward hydrocarbons. 相似文献
22.
(2S,4S)-2-(tert-Butyl)-5-oxo-1,3-dioxolan-4-acetic acid ( 1 ) was dilithiated to the enolate 2 in toluene and THF with lithium diisopropylamide (LiN(i-Pr)2) under various conditions. Deuteration experiments with 2 showed that two different stable forms of the enolate could be produced, depending on whether the deprotonation of 1 had been carried out in toluene or THF. The generated (i-Pr)2NH was coordinated to the enolate and, hence, served as a strong lipophilic solvating agent. LiN(i-Pr)2 could substitute the amine ligands of the two species to a different extent. THF was partially able to displace (i-Pr)2NH that solvated the species produced in toluene, but the coordination of the amine to the one produced in THF was quantitive. Correlations with given explanations are made. 相似文献
23.
Kammerer B Kahlich R Ufer M Laufer S Gleiter CH 《Rapid communications in mass spectrometry : RCM》2004,18(4):458-464
Phenprocoumon is a commonly used oral anticoagulant of the coumarin type, and has found extensive clinical use in the treatment of thrombophlebitis, pulmonary embolism and atrial fibrillation. In the course of a clinical study to investigate the influence of genetic polymorphisms of the CYP2C9 enzyme on phenprocoumon metabolism, we developed a new enantioselective liquid chromatography/electrospray ionisation tandem mass spectrometry (LC/MS/MS) method to quantify (R)- and (S)-phenprocoumon in human plasma. HPLC separation of the enantiomers was achieved on a Chira-Grom-2 column under isocratic conditions using a water/acetonitrile/formic acid eluent. For detection and quantification a triple-quadrupole MS system was used in the selected reaction monitoring (SRM) mode. As an internal standard the structurally homologous compound warfarin was chosen. The detector response was linear with a correlation coefficient of 0.988-0.999 for (R)-phenprocoumon and 0.989-0.999 for (S)-phenprocoumon in the investigated concentration range between 62.5 and 1000 ng/mL (per enantiomer). The limit of detection (LOD) was 12.5 ng/mL. 相似文献
24.
We explore here an approach to mimic the structures and biological functions of protein loops in small synthetic molecules, by grafting the loop of interest onto an organic template comprising a bicyclic diketopiperazine, prepared by the formal coupling of (2S,4S)-4-aminoproline (Pro(NH2)) and aspartic acid (Asp). The Fmoc-protected template 4 is used to prepare cyclo(-Ala1-Asn2-Pro3-Asn4-Ala5- Ala6-Temp-) ( 5 ) and cyclo(-Ala1-Arg2-Gly3-Asp4-Temp-) ( 6 ) (where Temp = template derived from 4 ), containing the Asn-Pro-Asn-Ala (NPNA) and Arg-Gly-Asp (RGD) motifs. The conformational properties of these molecules are studied in aqueous solution by NMR and simulated-annealing methods. The NPNA motif, an immunodominant epitope on the circumsporozoite surface protein of the malaria parasite Plasmodium falciparum, is shown to adopt a stable type-I β-turn in 5 . The template in 5 adopts a preferred conformation with Pro(NH2)χ1 ≈? ?35° and the Asp moiety χ1 ≈? 70°. A different template conformation is inferred for 6 , with Pro(NH2)χ1 ≈? 0°, but the ARGD loop appears by NMR to undergo rapid conformational averaging. Solid-phase binding assays reveal that 6 displays modest antagonist activity towards both the integrin αIIbβ3 and αvβ3 receptors. 相似文献
25.
Synthesis and derivatization of daptomycin: a chemoenzymatic route to acidic lipopeptide antibiotics
Grünewald J Sieber SA Mahlert C Linne U Marahiel MA 《Journal of the American Chemical Society》2004,126(51):17025-17031
Daptomycin is a branched cyclic nonribosomally assembled acidic lipopeptide, which is the first clinically approved antibiotic of this class. Here we show that the recombinant cyclization domain of the Streptomyces coelicolor calcium-dependent antibiotic (CDA) nonribosomal peptide synthetase (NRPS) is a versatile tool for the chemoenzymatic generation of daptomycin derivatives. Linear CDA undecapeptide thioesters with single exchanges at six daptomycin-specific residues were successfully cyclized by CDA cyclase. Simultaneous incorporation of all six of these residues into the peptide backbone and elongation of the N-terminus of CDA by two residues yielded a daptomycin derivative that lacked only the beta-methyl group of l-3-methylglutamate. Bioactivity studies with several substrate analogues revealed a significant role of nonproteinogenic constituents for antibacterial potency. In accordance with acidic lipopeptides, the bioactivity of the chemoenzymatic assembled daptomycin analogue is dependent on the concentration of calcium ions. Single deletions of the four acidic residues in the peptide backbone suggest that only two aspartic acid residues are essential for antimicrobial potency. These two residues are strictly conserved among other nonribosomal acidic lipopeptides and the EF-motif of ribosomally assembled calmodulin. Based on these findings CDA cyclase is a versatile catalyst that can be used to generate novel daptomycin derivatives that are otherwise difficult to obtain by chemical modification of the parental tridecapeptide to improve further its therapeutic activity. 相似文献
26.
Ccile Pasquier Albert Gossauer Walter Keller Christoph Kratky 《Helvetica chimica acta》1987,70(8):2098-2109
(?)-(4S,16S)-8, 12-bis[de(2-carboxyethyl)]mesourobilin-IIIα hydrochloride ( 8 ) has been synthesized from the enantiomerically pure 1,4,5,10-tetrahydro-1-oxodipyrrin-9-carboxylic-acid precursor 6a whose absolute configuration was determined by X-ray diffraction analysis of the N-[(S)-1-(1-naphthyl)ethyl] carboxamide 7b . The present results prove unequivocally that an (S,S)-configurated urobilin chromophore displays a negative Cotton effect in the VIS absorption range. However, the helicity of the inherently dissymmetric chromophore remains undetermined. 相似文献
27.
The title compounds were prepared by reaction of CaSb2, SrSb2, or BaSb3 with the transition metals and antimony in sealed silica tubes. They crystallize with the cubic LaFe4P12-structure, which was refined from single-crystal X-ray data of CaFe4Sb12, SrRu4Sb12, and BaRu4Sb12 to residuals of R = 0.014, 0.016, and 0.014, respectively. The thermal parameters of the alkaline earth ions increase with decreasing ionic size. The Sb? Sb distances are greater in the iron compound than they are in the two ruthenium compounds. This is rationalized to be due to a larger portion of electrons in antibonding Sb? Sb states in the iron compound. 相似文献
28.
We have synthesized novel heterocyclic compounds from resin-bound guanidines. For this purpose, an amine immobilized on a solid support was acylated with protected amino acids. Following the deprotection, the liberated amines were guanidinylated utilizing a new member of the family of diurethane-protected triflyl guanidine reagents, N,N'-bis(allyloxycarbonyl)-N' '-triflylguanidine. The deprotected guanidines were subsequently regioselectively cyclized with beta-keto esters yielding novel compounds containing heterocyclic structures in high purities. 相似文献
29.
Klaus Rakus Sergej P. Verevkin Hans-Dieter Beckhaus Christoph Rüchardt 《欧洲无机化学杂志》1994,127(11):2225-2234
The thermolysis reactions of the tricyanomethyl compounds 10a-c were studied in solution. 2,2-Dicyano-3-methyl-3-phenylbutyronitrile ( 10a ) and 2,2-dicyano-3-methyl-3-(4-nitrophenyl)butyronitrile ( 10b ) decomposed heterolytically into carbenium ions and (CN)3C− anions, while 9-methyl-9-(tricyanomethyl)fluorene ( 10c ) underwent about 11% homolytic C-C bond cleavage into 9-methyl-9-fluorenyl- and tricyanomethyl radicals. The rates of the homolysis were determined by a radical scavenger procedure under conditions of pseudozero order kinetics. From the temperature effect on the rate constants the activation parameters were determined [ΔH ( 10c ) = 155· 2 kJ mol−1, ΔS ( 10c ) = 58· 5 J mol−1 K−1]. Standard enthalpies of formation ΔH (g) were determined for 2,2-dicyanopropionitrile ( 2 ) (422.45 kJ mol−1), 2,2-dicyanohexanenitrile ( 3 ) (349.74 kJ mol−1), 2,2-dicyano-3-phenylpropionitrile ( 4 ) (540.75 kJ mol−1), 2-butyl-2-methylhexanentrile ( 5 ) (-133.20 kJ mol−1), 2,2-dimethylpentanenitrile ( 6 ) (-45.78 kJ mol−1), and 2-methylbutyronitrile ( 7 ) (2.44 kJ mol−1) from the enthalpies of combustion and enthalpies of sublimation/vaporization. From these data and known Δ (g) values for alkanenitriles and -dinitriles, thermochemical increments for ΔH (g) were derived for alkyl groups with one, two, or three cyano groups attached. The comparison of these increments with those of alkanes reveals a strong geminal destabilization, which is interpreted by dipolar repulsions between the cyano groups. - From ΔH (g) of 10c and ΔH of its homolytic decomposition the radical stabilization enthalpy for the tricyanomethyl radical 1 RSE ( 1 ) = -18 kJ mol−1 was determined. Thus, 1 is destabilized, in comparison with the RSEs of tertiary α-cyanalkyl (23 kJ mol−1) and α,α-dicyanoalkyl (27 kJ mol−1) radicals, which were recalculated from bond homolysis measurements[4] and the new thermochemical data. This change of RSE on increasing the number of α-cyano groups is discussed as the result of the additive contributions by resonance stabilization and increasing destabilization by dipolar repulsion. The amount of the dipolar energies was estimated by molecular mechanics (MM2). 相似文献
30.
Molecular dynamics simulations show that gold clusters with about 600-3000 atoms crystallize into a Mackay icosahedron upon cooling from the liquid. A detailed surface analysis shows that the facets on the surface of the Mackay icosahedral gold clusters soften but do not premelt below the bulk melting temperature. This softening is found to be due to the increasing mobility of vertex and edge atoms with temperature, which leads to inter-layer and intra-layer diffusion, and a shrinkage of the average facet size, so that the average shape of the cluster is nearly spherical at melting. 相似文献