全文获取类型
收费全文 | 11846篇 |
免费 | 755篇 |
国内免费 | 687篇 |
专业分类
化学 | 12440篇 |
晶体学 | 117篇 |
力学 | 6篇 |
综合类 | 25篇 |
数学 | 152篇 |
物理学 | 548篇 |
出版年
2023年 | 92篇 |
2022年 | 144篇 |
2021年 | 233篇 |
2020年 | 421篇 |
2019年 | 337篇 |
2018年 | 278篇 |
2017年 | 271篇 |
2016年 | 350篇 |
2015年 | 247篇 |
2014年 | 325篇 |
2013年 | 743篇 |
2012年 | 1317篇 |
2011年 | 489篇 |
2010年 | 425篇 |
2009年 | 609篇 |
2008年 | 685篇 |
2007年 | 838篇 |
2006年 | 636篇 |
2005年 | 595篇 |
2004年 | 566篇 |
2003年 | 473篇 |
2002年 | 340篇 |
2001年 | 292篇 |
2000年 | 234篇 |
1999年 | 237篇 |
1998年 | 190篇 |
1997年 | 272篇 |
1996年 | 289篇 |
1995年 | 304篇 |
1994年 | 207篇 |
1993年 | 181篇 |
1992年 | 150篇 |
1991年 | 101篇 |
1990年 | 62篇 |
1989年 | 47篇 |
1988年 | 51篇 |
1987年 | 41篇 |
1986年 | 37篇 |
1985年 | 29篇 |
1984年 | 31篇 |
1983年 | 12篇 |
1982年 | 26篇 |
1981年 | 14篇 |
1980年 | 17篇 |
1979年 | 12篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1975年 | 4篇 |
1974年 | 3篇 |
1973年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
In this paper we analyze some classes of abstract simplicial complexes relying on algebraic models arising from module theory. To this regard, we consider a left-module on a unitary ring and find models of abstract complexes and related set operators having specific regularity properties, which are strictly interrelated to the algebraic properties of both the module and the ring.Next, taking inspiration from the aforementioned models, we carry out our analysis from modules to arbitrary sets. In such a more general perspective, we start with an abstract simplicial complex and an associated set operator. Endowing such a set operator with the corresponding properties obtained in our module instances, we investigate in detail and prove several properties of three subclasses of abstract complexes.More specifically, we provide uniformity conditions in relation to the cardinality of the maximal members of such classes. By means of the notion of OSS-bijection, we prove a correspondence theorem between a subclass of closure operators and one of the aforementioned families of abstract complexes, which is similar to the classic correspondence theorem between closure operators and Moore systems. Next, we show an extension property of a binary relation induced by set systems when they belong to one of the above families.Finally, we provide a representation result in terms of pairings between sets for one of the three classes of abstract simplicial complexes studied in this work. 相似文献
2.
《Arabian Journal of Chemistry》2022,15(3):103640
Complexes of transition metals [Co(II), Cd(II) and Mo(0)] with a new enaminone (PA) 3-chloro-4-((4-methoxyphenyl)amino)pent-3-en-2-one were synthesized and afterwards characterized by 1H NMR, 13C NMR, FAB-MS, UV–Vis, ICP-OES, TGA and FTIR. The spectroscopic and conductance data suggested that the ligand (PA) is attached to the metal ions in bidentate, neutral form through the nitrogen atom of amino group and the oxygen of carbonyl group. Metal complexes displayed octahedral geometries. In vitro urease inhibition and cytotoxic activities of all the compounds were evaluated. Results revealed that Co(II) complex (PA-Co) was even more significant than the reference drug thiourea. Analysis of the cytotoxicity indicated that, the Co(II) complex has more cytotoxic effect than enaminone ligand and other complexes when assessed on the human cancer cell lines MCF-7.Molecular docking simulation was also performed to find out the putative binding mode within the target protein. 相似文献
3.
《Arabian Journal of Chemistry》2022,15(2):103628
In this paper, a novel Zn(II) and Co(II) Schiff base complexes were synthesized by template method via refluxing 2,3-Naphthalenedicarboxaldehyde, Metal(II) chloride (Metal = Zn or Co), and L-phenylalanine. ZnO and Co3O4 nanoparticles were synthesized by thermal decomposition of Zn(II) and Co(II) complexes, respectively. The products were characterized using different instruments such as CHN, Conductivity, FT-IR, XRD, HR-TEM, and UV–Vis spectrophotometer. The experimental results of elemental analysis for Zn(II) and Co(II) complexes, agree with the calculated results, indicating that the Zn(II) and Co(II) complexes have 1:1 ligand/metal ratios. The molar conductance of the Zn(II) and Co(II) complexes, is less than 5 Ω?1cm?1mol?1, confirming the non-electrolytic nature of the synthesized complexes. The average crystallite diameter of the ZnO and Co3O4 samples is 39.64 and 30.38 nm, respectively. The optical energy gap of the ZnO and Co3O4 samples are 2.75 and 3.25 eV, respectively. Methylene blue dye was utilized to examine the photocatalytic properties of the synthesized nanoparticles using UV irradiations in the absence and presence of hydrogen peroxide. The % degradation of the methylene blue dye in the presence of hydrogen peroxide using ZnO and Co3O4 samples after 40 min is 94.55 and 98.98, respectively. Six pathogenic microbes were utilized to examine the antimicrobial properties of the synthesized Schiff base complexes and their nanoparticles: Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Streptococcus species, Aspergillus species, and Candida species. Zn(II) and Co(II) complexes display inhibition towards all the studied microbes. Besides, ZnO and Co3O4 nanoparticles exhibit less inhibition towards Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Streptococcus species. Moreover, ZnO and Co3O4 nanoparticles have no activity towards Aspergillus and Candida species. 相似文献
4.
Sergej Stipurin Prof. Dr. Thomas Strassner 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(70):e202202227
A series of phosphorescent bimetallic platinum(II) complexes is presented, which were synthesized by the combination of bidentate cyclometalated N-heterocyclic carbene ligands and different bridging diphenylformamidinates. The complexes were characterized by standard techniques and additionally two solid-state structures could be obtained. Photoluminescence measurements revealed the strong emissive behavior of the compounds with quantum yields of up to 90 % and emission lifetimes of approx. 2 μs. The effect of the substitution pattern in the bridging ligands on the structural and photophysical properties of the complexes was examined in detail and rationalized by density functional theory calculations (PBE0/6-311G*). 相似文献
5.
6.
Organotin complexes of Schiff bases (derived from the condensation of hydrazides with salicylaldehyde derivatives) were prepared and their characterization was done using several spectroscopic techniques like FTIR, NMR (1H, 13C, and 119Sn) and mass spectrometry. The spectroscopic data of the ligands and their corresponding complexes revealed that the Schiff bases chelated to the tin metal in a tridentate manner through –ONO atoms (oxygen atom of the hydroxyl group of the salicylaldehydic derivatives, the nitrogen atom of azomethine group, and the oxygen atom of enolic group present in the carboxylic acid hydrazides). Around tin atom pentacoordinated geometry was exhibited. The synthesized ligands and their complexes have been assessed for their biological potency (antibacterial, antifungal and antioxidant using Ciprofloxacin, Fluconazole and Ascorbic acid as reference compounds) and few of the compounds showed optimistic activity. The ligands having electron withdrawing group attached showed greater antimicrobial activity as compared to the other ligands. The complexes showed the better activity than the ligands. The general trend followed by the complexes was diphenyl ?> ?dibutyl ?> ?dimethyl substituted complexes. Compound 11 was the most active against microbes. The antioxidant activity increased with electron donating group. The phenyl substituted complexes showed better activity as compared to the dibutyl and dimethyl substituted complexes. Compound 20 was the best antioxidant. 相似文献
7.
Matthew M. Wootten Dr. Sofja Tshepelevitsh Prof. Ivo Leito Prof. Jonathan Clayden 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(59):e202202247
A molecular system comprising a cationic zinc complex and an amino acid-derived ambident ligand having phosphate and carboxylate binding sites undergoes a series of rearrangements in which the metal cation migrates autonomously from one site to another. The location of the metal is identified by the circular dichroism spectrum of a ligated bis(2-quinolylmethyl)-(2-pyridylmethyl)amine (BQPA) chromophore, which takes a characteristic shape at each binding site. Migration is fuelled by the decomposition of trichloroacetic acid to CO2 and CHCl3, which progressively neutralises the acidity of the system as a function of time, revealing in sequence binding sites of increasing basicity. The migration rate responds to control by variation of the temperature, water content and triethylamine concentration, while an excess of fuel controls the duration of an induction period before the migration event. 相似文献
8.
《Mendeleev Communications》2022,32(4):482-484
Rhodium(III) complexes catalyze the insertion of carbenoids generated from diazo compounds into E?H bonds (E = B, Si, N, but not C), although less efficiently than classical rhodium(II) carboxylates, despite formally higher oxidation state of the metal. 相似文献
9.
Dr. Arijit Singha Hazari Shubhadeep Chandra Prof. Sanjib Kar Prof. Dr. Biprajit Sarkar 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(24):e202104550
Metal complexes of multi-porphyrins and multi-corroles are unique systems that display a host of extremely interesting properties. Availability of free meso and β positions allow formation of different types of directly linked bis-porphyrins giving rise to intriguing optical and electronic properties. While the fields of metalloporphyrin and corroles monomer have seen exponential growth in the last decades, the chemistry of metal complexes of bis-porphyrins and bis-corroles remain rather underexplored. Therefore, the impact of covalent linkages on the optical, electronic, (spectro)electrochemical, magnetic and electrocatalytic activities of metal complexes of bis-porphyrins and -corroles has been summarized in this review article. This article shows that despite the (still) somewhat difficult synthetic access to these molecules, their extremely exciting properties do make a strong case for pursuing research on these classes of compounds. 相似文献
10.
《Mendeleev Communications》2022,32(6):777-779
The reactions of aryllithium reagents o-LiC6H4CH2NR2 with (MeO)2CO afford two new tris(aryl)carbinols bearing pendant-NR2 donor groups in the side chain [o-R NCH C H ] COH [R = Me, R + R = (CH) ]. These alcohols feature helical chirality due to differently inclined aromatic fragments and are presented in a crystalline cell as two M and P enantiomers. Carbinol (R = Me) readily reacts with (Me3SiCH2)3Sc(THF)2 to give a scandium bis(alkyl) complex [(o-C6H4CH2NMe2)3CO]Sc(CH2SiMe3)2 featuring rigid binding of the alkoxy anion through a κ1-O, κ2-N chelating coordination mode 相似文献