全文获取类型
收费全文 | 44636篇 |
免费 | 6291篇 |
国内免费 | 9747篇 |
专业分类
化学 | 35265篇 |
晶体学 | 210篇 |
力学 | 3129篇 |
综合类 | 3篇 |
数学 | 12969篇 |
物理学 | 9098篇 |
出版年
2024年 | 17篇 |
2023年 | 1679篇 |
2022年 | 1053篇 |
2021年 | 1397篇 |
2020年 | 2315篇 |
2019年 | 1666篇 |
2018年 | 1762篇 |
2017年 | 1775篇 |
2016年 | 2755篇 |
2015年 | 2484篇 |
2014年 | 2144篇 |
2013年 | 5277篇 |
2012年 | 4026篇 |
2011年 | 3815篇 |
2010年 | 2920篇 |
2009年 | 2464篇 |
2008年 | 2767篇 |
2007年 | 3294篇 |
2006年 | 3118篇 |
2005年 | 2125篇 |
2004年 | 1722篇 |
2003年 | 1692篇 |
2002年 | 1357篇 |
2001年 | 1217篇 |
2000年 | 1066篇 |
1999年 | 1525篇 |
1998年 | 624篇 |
1997年 | 585篇 |
1996年 | 435篇 |
1995年 | 424篇 |
1994年 | 203篇 |
1993年 | 221篇 |
1992年 | 167篇 |
1991年 | 130篇 |
1990年 | 115篇 |
1989年 | 53篇 |
1988年 | 36篇 |
1987年 | 21篇 |
1986年 | 121篇 |
1985年 | 46篇 |
1984年 | 5篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1976年 | 7篇 |
1966年 | 1篇 |
1936年 | 42篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Steroidal Alkaloids from Veratrum maackii Regel with Genotoxicity on Brain‐Cell DNA in Mice 下载免费PDF全文
Yue Cong Hua‐Lan Zhu Qing‐Chun Zhang Lin Li Hai‐Yan Li Xian‐You Wang Jing‐Gong Guo 《Helvetica chimica acta》2015,98(4):539-545
Two new steroidal alkaloids, 23‐methoxycyclopamine 3‐O‐β‐D ‐glucopyranoside ( 1 ) and isoecliptalbine ( 2 ), were isolated from the root and rhizoma of Veratrum maackii Regel , together with five known compounds, i.e., verussurine ( 3 ), verabenzoamine ( 4 ), verazine ( 5 ), isoverazine ( 6 ), and verazinine ( 7 ). Their structures were established by extensive analysis of spectroscopic data, as well as by comparison with literature data. Compounds 1 – 7 could cause DNA damage in the cerebellum and cerebral cortex of mice in a dose‐dependent manner by using single‐cell gel electrophoresis (comet assay). 相似文献
992.
Wen‐Hao Chen Chang‐Ri Han Yang Hui Da‐Shuai Zhang Xin‐Ming Song Guang‐Ying Chen Xiao‐Ping Song 《Helvetica chimica acta》2015,98(5):724-730
Two new eremophilane‐type sesquiterpenoids, 1α‐hydroxyeremophila‐6,9,11‐trien‐8‐one ( 1 ), 4α‐hydroxyeremophila‐1,9‐diene‐3,8‐dione ( 2 ), and a new friedelane‐type triterpenoid, friedelane‐3α,16β‐diol ( 4 ), along with six known terpenoids, 3 and 5 – 9 , have been isolated from the stems of Drypetes congestiflora. Their structures and relative configurations were elucidated on the basis of detailed spectroscopic analyses and by comparison of their NMR data with those reported in the literature. All of the compounds, 1 – 9 , were isolated for the first time from this species. Compound 3 exhibited moderate cytotoxic activities against the A549 and B16F10 cell lines. 相似文献
993.
Ping He Sha‐Sha Tian Yan Xu He Yu Yan‐Nan Ji Hua‐Jie Zhu Jian‐Heng Li 《Helvetica chimica acta》2015,98(6):819-822
Three new phenyl ether derivatives, 3‐hydroxy‐5‐(3‐hydroxy‐5‐methylphenoxy)benzoic acid ( 1 ), 3,4‐dihydroxy‐5‐(3‐hydroxy‐5‐methylphenoxy)benzoic acid ( 2 ), 3‐[3‐hydroxy‐5‐(hydroxymethyl)phenoxy]‐5‐methylphenol ( 3 ), and three known compounds 4 – 6 were obtained from the fermentation broth of Aspergillus carneus HQ889708, which was isolated from sea water from South China Sea. The structures of compounds 1 – 3 were established on the basis of spectroscopic methods including ESI‐MS and NMR. Compounds 4 – 6 were reported before as synthesized products, herein, they are reported from nature for the first time. 相似文献
994.
C? H Arylation of olefins by triarylphosphines via C? P bond cleavage has been achieved with either Pd0 or PdII catalysts. A variety of olefins and triarylphosphines are tolerated, and we inferred that both Pd0 and PdII could function directly without pre‐oxidation or pre‐reduction. 相似文献
995.
Paecilomycines A and B,Novel Diterpenoids,Isolated from Insect‐Pathogenic Fungi Paecilomyces sp. ACCC 37762 下载免费PDF全文
Kun Zhou Xiao‐Ling Zhao Li‐Ping Han Meng‐Meng Cao Chuan Chen Bao‐Zhong Shi Du‐Qiang Luo 《Helvetica chimica acta》2015,98(5):642-649
Two new diterpenoids, named paecilomycine A ( 1 ) and paecilomycine B ( 2 ), including a novel skeleton with a five‐membered lactone ring, together with three known labdane diterpenoids, rel‐(1R,3S,4aS,5R,8aS)‐5‐[(3E)‐4‐carboxy‐3‐methylbut‐3‐en‐1‐yl]decahydro‐3‐hydroxy‐1,4a‐dimethyl‐6‐methylidenenaphthalene‐1‐carboxylic acid ( 3 ), botryosphaerin E ( 4 ), and agathic acid ( 5 ), were isolated from solid culture of the insect pathogenic fungi strain Paecilomyces sp. The structures of all compounds were established on the basis of comprehensive spectroscopic studies. The relative configurations of 1 and 2 were determined by single‐crystal X‐ray diffraction analyses. 相似文献
996.
New Cu(I) Coordination Complexes Based on Tetrathiafulvalene Substituted Triazol‐pyridine Ligand: Synthesis,Properties and Theoretical Studies 下载免费PDF全文
Two Cu(I) complexes based on the thioethyl‐bridged triazol‐pyridine ligand with tetrathiafulvalene unit (TTF‐TzPy, L ), [Cu(I)(Binap)(L)]BF4 ( 5 , Binap=2,2’‐bis(diphenylphosphino)‐1,1’‐binaphthyl) and [Cu(I)(Xantphos)(L)]BF4 ( 6 , Xantphos=9,9‐dimethyl‐4,5‐bis(diphenylphosphino)‐xanthene), have been synthesized. All new compounds are characterized by elemental analyses, 1H NMR and mass spectroscopies. The complex 5 has been determined by X‐ray structure analyses which shows that the central copper (I) ion assumes distorted tetrahedral geometry. The photophysical, computational and electrochemical properties of L and 5 ‐ 6 have been investigated. The most representative molecular orbital energy‐level diagrams and the spin‐allowed singlet? singlet electronic transitions of the three compounds have been calculated with density functional theory (DFT) and time‐dependent DFT (TD‐DFT). The luminescence bands of Cu(I) complexes 5 ‐ 6 have been assigned as mixed intraligand and metal‐to‐ligand charge transfer 3(MLCT+π→π*) transitions through analysis of the photophysical properties and DFT calculations. The electrochemical studies reveal that 5 ‐ 6 undergo reversible TTF/TTF+?/TTF2+ redox processes and one irreversible Cu+→Cu2+ oxidation process. 相似文献
997.
The standard enthalpies of formation of selected ternary half-Heusler type compositions XYZ (X = Au, Co, Fe, Ir, Ni, Pd, Pt, Rh, Ru; Y = Hf, Mn, Ti, Zr; Z = Ga, Sn) were measured using high temperature direct reaction calorimetry. The measured standard enthalpies of formation (in kJ/mole of atoms) of the half-Heusler compounds (prototype MgAgAs, Pearson symbol cF12, space group F-43m) are, IrMnSn (−29.4 ± 1.8); NiTiSn (−52.6 ± 2.4); PtHfSn (−98.8 ± 3.4); PtMnSn (−55.8 ± 2.6); PtTiSn (−93.6 ± 3.3); PtZrSn (−104.9 ± 3.8); for the B2 compound (prototype CsCl, Pearson symbol cP2, space group Pm-3m), RuMnGa (−26.9 ± 1.7); for the C1 structured (prototype CaF2, Pearson symbol cF12, space group Pm-3m) or the C1b structured compound IrMnGa (−40.9 ± 1.7). Indicative standard enthalpies of formation of the following compounds were obtained, half-Heusler compounds AuMnSn, CoTiSn, IrZrSn, NiHfSn, NiZrSn, PdHfSn, PdZrSn, RhTiSn; Heusler compound (prototype Cu2MnAl, Pearson symbol cF16, space group Fm-3m) RhMnSn; hexagonal compound (prototype BeZrSi, Pearson symbol hP6, space group P63/mmc) PtMnGa and another type of hexagonal compound (prototype RhHfSn, Pearson symbol hP18, space group P-62c) RhHfSn, IrZrsn, RhZrSn. Values were compared with ab initio calculations from AFLOW and OQMD. Lattice parameters of these compounds were determined using X-ray diffraction (XRD) analysis. Microstructures were characterized using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Selected alloys were further annealed to investigate phase transformations and phase relationships. 相似文献
998.
999.
Two New Cadmium(II) Compounds based on Distinct Second Building Subunits: Solvothermal Syntheses,Crystal Structures,and Luminescent Properties 下载免费PDF全文
Two cadmium(II) coordination polymers, namely [Cd3(bpt)2(DMA)2]n ( 1 ) and [Cd2(bpt)(btz)(DMF)]n ( 2 ) (H3bpt = biphenyl‐3,4′,5‐tricarboxylic acid, Hbtz = 1H‐benzotriazole, DMA = N,N‐dimethylacetamide; DMF = N,N‐dimethylformamide), were solvothermally synthesized and structurally characterized by single‐crystal X‐ray diffraction. Compound 1 displays a 3D framework based on trinuclear {Cd3(COO)4} subunits and can be simplified into a (4,8)‐connected topological network by viewing bpt3– ligands and trinuclear {Cd3(COO)4} units as 4‐, 8‐connected nodes, respectively. Compound 2 also displays a 3D framework but based on 1D chain subunits controlled by carboxylate groups and btz– ligands. In addition, the thermal stabilities and luminescent properties of compounds 1 and 2 were also investigated. 相似文献
1000.
Four Metal Complexes Based on Bulky Imidazole Ligands: Solvothermal Syntheses,Crystal Structures,and Fluorescence Properties 下载免费PDF全文
Xue Zhang Sai Li Yue‐Jiao He Tong Han Xiao‐Ling Wang Bang Chen Kang‐Yu Zou Zuo‐Xi Li 《无机化学与普通化学杂志》2015,641(11):1874-1880
In order to explore the influences of (de‐)protonation of the imidazole ring on the structural diversity of the resulting complexes, the imidazole‐based ligands 4, 5‐diphenylimidazole (Hdpi) and 1H‐phenanthro[9, 10‐d]imidazole (Hpi) were utilized as bulky building blocks to construct four complexes by solvothermal reactions, i.e. [Ag(Hdpi)2](NO3) · (H2O) ( 1 ), [Cu(dpi)]∞ ( 2 ), [Cu(Hpi)(NO3)]∞ ( 3 ), and [(H2pi)(NO3)] · H2O ( 4 ). In complex 1 , two Hdpi ligands adopt a monodentate pattern and coordinate with one AgI ion to form a mononuclear unit, which is further connected by hydrogen bonds into a 1D supramolecular helix. The deprotonated dpi ligand of 2 acts in bidentate mode, and bridges CuI ions to afford a 1D chain. In 3 , the NO3– ion, acts as a monodentate bridging ligand and joins CuI ions to generate a 1D chain. The Hpi ligand employs a monodentate mode to bond with CuI ions of the 1D chain. 4 is protonated and two H2pi nitrogen atoms are free of coordination. Interestingly, hydrogen bonds among the NO3– ion, the H2pi ligand, and the water molecule yield a macro ring R44(14). The resulting structural diversity reveals that the (de‐)protonation of imidazole ring directly steers the coordination number of ligand, and thus causes a significant effect on the structure, especially the dimensionality. Furthermore, the solid‐state fluorescence properties of the free ligands and compounds 1 – 4 were studied at room temperature. 相似文献