全文获取类型
收费全文 | 11896篇 |
免费 | 888篇 |
国内免费 | 1628篇 |
专业分类
化学 | 5635篇 |
晶体学 | 62篇 |
力学 | 336篇 |
综合类 | 168篇 |
数学 | 6663篇 |
物理学 | 1548篇 |
出版年
2024年 | 4篇 |
2023年 | 148篇 |
2022年 | 181篇 |
2021年 | 320篇 |
2020年 | 352篇 |
2019年 | 408篇 |
2018年 | 347篇 |
2017年 | 392篇 |
2016年 | 338篇 |
2015年 | 273篇 |
2014年 | 463篇 |
2013年 | 1231篇 |
2012年 | 439篇 |
2011年 | 564篇 |
2010年 | 489篇 |
2009年 | 660篇 |
2008年 | 708篇 |
2007年 | 907篇 |
2006年 | 733篇 |
2005年 | 678篇 |
2004年 | 601篇 |
2003年 | 651篇 |
2002年 | 508篇 |
2001年 | 417篇 |
2000年 | 348篇 |
1999年 | 341篇 |
1998年 | 327篇 |
1997年 | 297篇 |
1996年 | 220篇 |
1995年 | 173篇 |
1994年 | 168篇 |
1993年 | 106篇 |
1992年 | 104篇 |
1991年 | 79篇 |
1990年 | 64篇 |
1989年 | 48篇 |
1988年 | 49篇 |
1987年 | 48篇 |
1986年 | 30篇 |
1985年 | 47篇 |
1984年 | 26篇 |
1983年 | 17篇 |
1982年 | 22篇 |
1981年 | 20篇 |
1980年 | 10篇 |
1979年 | 11篇 |
1978年 | 11篇 |
1977年 | 12篇 |
1976年 | 4篇 |
1936年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Dr. Xiaoming Jie Dr. Qiu Sun Dr. Constantin G. Daniliuc Robert Knitsch Prof. Dr. Michael Ryan Hansen Prof. Dr. Hellmut Eckert Dr. Gerald Kehr Prof. Dr. Gerhard Erker 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(6):1269-1273
The active six-membered cyclo-FLP 6 undergoes a rapid P/B addition reaction to carbon dioxide. At elevated temperature, the resulting heterobicyclo[2.2.2]octane derived product 7 undergoes ring opening and equilibrates with the cyclotetramer (7)4 . In the large macrocyclic structure, four monomeric six-membered cyclo-FLP units are connected by four CO2 molecules to form the supramolecular ring system. The P/B cyclo-FLP 6 undergoes a variety of additional cycloaddition reactions. 相似文献
42.
Martín Calvelo Alejandro Lamas Dr. Arcadio Guerra Dr. Manuel Amorín Dr. Rebeca Garcia-Fandino Prof. Dr. Juan R. Granja 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(26):5846-5858
Cyclic peptides with disc-shaped structures have emerged as potent building blocks for the preparation of new biomaterials in fields ranging from biological to material science. In this work, we analyze in depth the self-assembling properties of a new type of cyclic peptides based on the alternation of α-residues and cyclic δ-amino acids (α,δ-CPs). To examine the preferred stacking properties adopted by cyclic peptides bearing this type of amino acids, we carried out a synergistic in vitro/in silico approximation by using simple dimeric models and then extended to nanotubes. Although these new cyclic peptides (α,δ-CPs) can interact either in a parallel or antiparallel fashion, our results confirm that although the parallel β-sheet is more stable, it can be switched to the antiparallel stacking by choosing residues that can establish favorable cross-strand interactions. Moreover, the subsequent comparison by using the same methodology but applied to α,γ-CPs models, up to the moment assumed as antiparallel-like d,l -α-CPs, led to unforeseen conclusions that put into question preliminary conjectures about these systems. Surprisingly, they tend to adopt a parallel β-sheet directed by the skeleton interactions. These results imply a change of paradigm with respect to cyclic peptide designs that should be considered for dimers and nanotubes. 相似文献
43.
Daniela Cappello Dr. Ryan R. Maar Prof. Dr. Viktor N. Staroverov Prof. Dr. Joe B. Gilroy 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(24):5522-5529
The creation of dimeric boron difluoride complexes of chelating N-donor ligands is a proven strategy for the enhancement of the optoelectronic properties of fluorescent dyes. We report dimers based on the boron difluoride hydrazone (BODIHY) framework, which offer unique and sometimes unexpected substituent-dependent absorption, emission, and electrochemical properties. BODIHY dimers have low-energy absorption bands (λmax=421 to 479 nm, ϵ=17 200 to 39 900 m −1 cm−1) that are red-shifted relative to monomeric analogues. THF solutions of these dimers exhibit aggregation-induced emission upon addition of water, with emission enhancement factors ranging from 5 to 18. Thin films of BODIHY dimers are weakly emissive as a result of the inner-filter effect, attributed to intermolecular π-type interactions. BODIHY dimers are redox-active and display two one-electron oxidation and two one-electron reduction waves that strongly depend on the N-aryl substituents. These properties are rationalized using density-functional theory calculations and X-ray crystallography experiments. 相似文献
44.
Mark R. Bortolus Dr. Hélène P. A. Mercier Prof. Gary J. Schrobilgen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(41):8935-8950
The reactions of the fluoride-ion donor, XeF6, with the fluoride-ion acceptors, M′OF4 (M′=Cr, Mo, W), yield [XeF5]+ and [Xe2F11]+ salts of [M′OF5]− and [M2O2F9]− (M=Mo, W). Xenon hexafluoride and MOF4 react in anhydrous hydrogen fluoride (aHF) to give equilibrium mixtures of [Xe2F11]+, [XeF5]+, [(HF)nF]−, [MOF5]−, and [M2O2F9]− from which the title salts were crystallized. The [XeF5][CrOF5] and [Xe2F11][CrOF5] salts could not be formed from mixtures of CrOF4 and XeF6 in aHF at low temperature (LT) owing to the low fluoride-ion affinity of CrOF4, but yielded [XeF5][HF2]⋅CrOF4 instead. In contrast, MoOF4 and WOF4 are sufficiently Lewis acidic to abstract F− ion from [(HF)nF]− in aHF to give the [MOF5]− and [M2O2F9]− salts of [XeF5]+ and [Xe2F11]+. To circumvent [(HF)nF]− formation, [Xe2F11][CrOF5] was synthesized at LT in CF2ClCF2Cl solvent. The salts were characterized by LT Raman spectroscopy and LT single-crystal X-ray diffraction, which provided the first X-ray crystal structure of the [CrOF5]− anion and high-precision geometric parameters for [MOF5]− and [M2O2F9]−. Hydrolysis of [Xe2F11][WOF5] by water contaminant in HF solvent yielded [XeF5][WOF5]⋅XeOF4. Quantum-chemical calculations were carried out for M′OF4, [M′OF5]−, [M′2O2F9]−, {[Xe2F11][CrOF5]}2, [Xe2F11][MOF5], and {[XeF5][M2O2F9]}2 to obtain their gas-phase geometries and vibrational frequencies to aid in their vibrational mode assignments and to assess chemical bonding. 相似文献
45.
Dr. Mateusz Janeta Prof. Tadeusz Lis Prof. Sławomir Szafert 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(60):13686-13697
In the present research, the synthesis, spectroscopic characterization, and structural investigations of a unique ZnII complex of imine-functionalized polyhedral oligomeric silsesquioxane (POSS) is designed, and hereby described, as a catalyst for the synthesis of cyclic carbonates from epoxides and CO2. The uncommon features of the designed catalytic system is the elimination of the need for a high pressure of CO2 and the significant shortening of reaction times commonly associated with such difficult transformations like that of styrene oxide to styrene carbonate. Our studies have shown that imine-POSS is able to chelate metal ions like ZnII to form a unique coordination complex. The silsesquioxane core and the hindrance of the side arms (their steric effect) influence the construction process of the homoleptic Zn4@POSS-1 complex. The compound was characterized in solution by NMR (1H, 13C, 29Si), ESI-MS, UV/Vis spectroscopy and in the solid state by thermogravimetric/differential thermal analysis (TG-DTA), elemental analysis, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), cross-polarization magic angle spinning (CP MAS) NMR (13C, 29Si) spectroscopy, and X-ray crystallography. 相似文献
46.
Sara Bertuzzi Dr. Ana Gimeno Reyes Núñez-Franco Dr. Ganeko Bernardo-Seisdedos Sandra Delgado Dr. Gonzalo Jiménez-Osés Dr. Oscar Millet Prof. Dr. Jesús Jiménez-Barbero Dr. Ana Ardá 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(67):15643-15653
The interaction of human galectin-1 with a variety of oligosaccharides, from di-(N-acetyllactosamine) to tetra-saccharides (blood B type-II antigen) has been scrutinized by using a combined approach of different NMR experiments, molecular dynamics (MD) simulations, and isothermal titration calorimetry. Ligand- and receptor-based NMR experiments assisted by computational methods allowed proposing three-dimensional structures for the different complexes, which explained the lack of enthalpy gain when increasing the chemical complexity of the glycan. Interestingly, and independently of the glycan ligand, the entropy term does not oppose the binding event, a rather unusual feature for protein-sugar interactions. CLEANEX-PM and relaxation dispersion experiments revealed that sugar binding affected residues far from the binding site and described significant changes in the dynamics of the protein. In particular, motions in the microsecond-millisecond timescale in residues at the protein dimer interface were identified in the presence of high affinity ligands. The dynamic process was further explored by extensive MD simulations, which provided additional support for the existence of allostery in glycan recognition by human galectin-1. 相似文献
47.
Dr. Yongliang Zhang Botao Wu Dr. Mingdong Zhong Prof. Dr. Wen-Xiong Zhang Prof. Dr. Zhenfeng Xi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(69):16472-16479
Transition-metal alkylidenes have exhibited wide applications in organometallic chemistry and synthetic organic chemistry, however, cyclic Schrock-carbene-like bis-alkylidenes of group 4 metals with a four-electron donor from an alkylidene have not been reported. Herein, the synthesis and characterization of five-membered cyclic bis-alkylidenes of titanium ( 4 a , b ) and zirconium ( 5 a , b ) are reported, as the first well-defined group 4 metallacyclopentatrienes, by two-electron reduction of their corresponding titana- and zirconacyclopentadienes. DFT analyses of 4 a show a four-electron donor (σ-donation and π-donation) from an alkylidene carbon to the metal center. The reaction of 4 a with N,N′-diisopropylcarbodiimide (DIC) leads to the [2+2]-cycloaddition product 6 . Compound 4 a reacted with CO, affording the oxycyclopentadienyl titanium complex 7 . These reactivities demonstrate the multiple metal–carbon bond character. The reactions of 4 a or 5 a with cyclooctatetraene (COT) or azobenzene afforded sandwich titanium complex 8 or diphenylhydrazine-coordinated zirconacyclopentadiene 9 , respectively, which exhibit two-electron reductive ability. 相似文献
48.
49.
50.
New pyridinium-functionalized metalloporphyrins MEtPpBr4 (M = Zn2+, Co2+, Ni2+, Cu2+; EtPp = 5, 10, 15, 20-tetra(4-(3-(N-ethyl-4-pyridyl)pyrazolyl)phenyl)porphyrin) were synthesized as bifunctional catalysts for the cycloaddition reactions of epoxides and CO2. The effects of catalyst loading, CO2 pressure, reaction temperature and time on catalytic activity were investigated. ZnEtPpBr4 ( 1 ) and CoEtPpBr4 ( 2 ) exhibited efficient activities in the cycloaddition reactions of various epoxides with CO2 as at 120 °C under 2 MPa of CO2 pressure without solvent. Most of corresponding cyclic carbonates could be obtained in almost quantitative yields and > 99.9% selectivity with molar ratio of epoxide/catalyst 2222 after 8 hr of reaction. 相似文献