全文获取类型
收费全文 | 1732篇 |
免费 | 108篇 |
国内免费 | 401篇 |
专业分类
化学 | 2126篇 |
晶体学 | 29篇 |
力学 | 2篇 |
综合类 | 12篇 |
物理学 | 72篇 |
出版年
2024年 | 2篇 |
2023年 | 13篇 |
2022年 | 22篇 |
2021年 | 26篇 |
2020年 | 54篇 |
2019年 | 52篇 |
2018年 | 54篇 |
2017年 | 49篇 |
2016年 | 68篇 |
2015年 | 71篇 |
2014年 | 87篇 |
2013年 | 223篇 |
2012年 | 115篇 |
2011年 | 91篇 |
2010年 | 69篇 |
2009年 | 90篇 |
2008年 | 106篇 |
2007年 | 99篇 |
2006年 | 105篇 |
2005年 | 120篇 |
2004年 | 122篇 |
2003年 | 113篇 |
2002年 | 65篇 |
2001年 | 55篇 |
2000年 | 35篇 |
1999年 | 31篇 |
1998年 | 32篇 |
1997年 | 24篇 |
1996年 | 27篇 |
1995年 | 33篇 |
1994年 | 28篇 |
1993年 | 29篇 |
1992年 | 35篇 |
1991年 | 17篇 |
1990年 | 7篇 |
1989年 | 8篇 |
1988年 | 16篇 |
1987年 | 6篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 6篇 |
1980年 | 2篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1974年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有2241条查询结果,搜索用时 31 毫秒
31.
Reiko Saito Yoichi Saito Hideyasu Kamoshita Yoichi Tokubuchi 《Journal of polymer science. Part A, Polymer chemistry》2012,50(16):3444-3451
Highly heterotactic poly(4‐vinyl pyridine)s (P4VPs) with the fraction of mr content (fmr) > 0.81 were synthesized by free radical polymerization of 4‐vinyl pyridine (4VP) with randomly methylated β‐cyclodextrin (β‐RMCD) in acidic aqueous media of HNO3 and CF3COOH at 40 °C. The heterotacticity of P4VP strongly depended on the neutralization of 4VP. The complete neutralization of 4VP with HNO3 or CF3COOH increased the heterotacticity of P4VP, whereas atactic P4VP was obtained in water. The partial decomposition of β‐RMCD by HCl reduced the heterotacticity of P4VP (fmr ≈ 0.74). The structures of inclusion complexed monomers were determined by Job's plot, 2D NMR with nuclear Overhauser enhancement spectroscopy analyses, and simulation by MM2. The 1:2 complex with [β‐RMCD]:[4VP] with meso placement of 4VPs in β‐RMCD was formed when 4VP was completely neutralized with acid, whereas the 1:1 complex was formed in water. The mechanism of heterospecific control by using β‐RMCD was proposed. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012 相似文献
32.
33.
34.
35.
One-pot conversion of amino coumarin to corresponding coumarinyl isothiocyanate using carbon disulfide, iodine, and pyridine is described. In this reaction, pyridine acts as a solvent as well as a catalyst to give coumarinyl isothiocyanate. Excellent yield, short reaction time, and mild reaction conditions make this method a useful synthetic procedure for one-pot preparation of coumarinyl isothiocyanate. 相似文献
36.
《Mendeleev Communications》2021,31(5):624-627
Heterometallic Coii-Lii compounds with N-heterocyclic carbene 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), tri phenylphosphine (Ph3P) and pyridine (py), [Co2Li2(Piv)6(IMes)2] (Piv is the anion of pivalic acid), [Co2Li2(Piv)6(Ph3P)2] and [Co2Li2(Fur)6(py)2] (Fur is the anion of 2-furoic acid), respectively, have been prepared and structurally characterized by X-ray crystallography. Easy-plane magnetic anisotropy in Coii complexes with pseudo-tetrahedral cores CoO3X (X = C, P and N) was revealed by measuring the magnetic properties together with quantum-chemical calculations using the SA-CASSCF/NEVPT2 approach. The field-induced slow magnetic relaxation of the complexes was mainly attributed to the Raman and direct processes. 相似文献
37.
Anton Dmitrienko Prof. Melanie Pilkington Prof. Georgii I. Nikonov 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(18):5730-5736
The AlI compound NacNacAl ( 1 , NacNac = [ArNC(Me)CHC(Me)NAr]−, Ar = 2,6-iPr2C6H3) serves as a template for the chemoselective coupling between carbonyls (benzophenone, fenchone, isophorone, p-tolyl benzoate, N,N-dimethylbenzamide, (1-phenylethylidene)aniline) and pyridine. With the CH-acidic ketone (1R)-(+) camphor, the reaction affords a hydrido alkoxide compound of Al, formed as the result of enolization, whereas an enolizable imine, (1-phenylethylidene)aniline, and the bulky ketone isophorone, still chemoselectively couple with pyridine. In contrast, reaction with the ester p-tolyl benzoate results in cleavage of the ester bond together with replacement of the alkoxy group by a hydrogen atom of the pyridine moiety. This study demonstrates that for carbonyl substrates featuring phenyl substituents, the reaction proceeds via intermediate formation of η2(C,X)-coordinated (X = O, N) carbonyl adducts, whereas the reaction of 1 with (R)-(−)-fenchone in the absence of pyridine leads to CH activation in the pendant isopropyl group of the Ar substituent of the NacNac ligand. 相似文献
38.
Shuhei Akahori Prof. Dr. Takahiro Sasamori Prof. Dr. Hiroshi Shinokubo Prof. Dr. Yoshihiro Miyake 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(18):5675-5682
Self-assembly of π-conjugated molecules in solution generally occurs owing to either an enthalpic or an entropic gain; however, designing π-conjugated systems that simultaneously exhibit enthalpically and entropically favorable self-assembly behavior is challenging. Herein, the self-assembly behavior of tetraazatetrathia[8]circulenes is disclosed, which is driven by both enthalpy and entropy. Single-crystal X-ray diffraction analysis demonstrated that molecules of these tetraazatetrathia[8]circulenes form face-to-face stacked dimers with a 1D columnar structure owing to the circularly arranged dipole moments. Importantly, concentration- and temperature-dependent 1H NMR spectra revealed that the formation of self-assemblies of tetraazatetrathia[8]circulenes in chloroform and methanol is favored by both enthalpic and entropic factors. The unique association behavior is due to the presence of sp2-hybridized nitrogen atoms, which weakly coordinate to the hydrogen atoms of these solvents and reduce the π-electron density of the circulene cores. 相似文献
39.
Structure–reactivity studies of chloro and isothiocyanato diorganotin (IV) complexes based on multidentate N‐donor ligands: Synthesis,spectral characterization and crystal structures 下载免费PDF全文
The reaction of [SnMe2Cl2] with the bidentate ligand 4,7‐phenanthroline (4,7‐phen) resulted in the formation of [SnMe2Cl2 (4,7‐phen)]n ( 1a ) which is probably a polymeric chain in solution. On the other hand, the reaction of [SnEt2Cl2] with 4,7‐phen afforded the complex [Sn2Et4Cl4 (κ1‐N‐4,7‐phen)2(μ‐κ2‐N,N‐4,7‐phen)] ( 1b ) which dissociates in dimethylsulfoxide solution. The reaction of [SnR2Cl2] (R = Me, Et) with 2,2′‐biquinoline (biq) yielded the complexes [SnMe2Cl2 (κ2‐N,N‐biq)] ( 2a ) and [SnEt2Cl2 (κ1‐N‐biq)2] ( 2b ) in the solid state. Moreover, the reaction of [SnR2Cl2] (R = Me, Et) with the tridentate ligand 4′‐(2‐furyl)‐2,2′:6′,2″‐terpyridine (ftpy) resulted in the formation of ionic penta‐ and hexa‐coordinated tin complexes [SnMe2Cl (ftpy)][SnMe2Cl3] ( 3a ) and [SnEt2Cl (ftpy)]Cl ( 3b ). The reaction of [SnMe2 (NCS)2] with ftpy afforded the hepta‐coordinated complex [SnMe2 (NCS)2(ftpy)] ( 4a ). The products were fully characterized using elemental analysis, and infrared, UV–visible, multinuclear (1H, 13C, 119Sn) NMR, DEPT‐135°, HH‐COSY and HSQC NMR spectroscopies. The crystal structure of complex 3a reveals that it contains the simultaneous presence of penta‐ and hexa‐coordinated tin (IV) atoms. Notably, the crystal structure of complex 4a shows that tin (IV) is hepta‐coordinated in a pentagonal bipyramidal geometry SnC2N5 by three nitrogen atoms of ftpy, two nitrogen atoms of NCS? and two Me groups with trans‐[SnMe2] configuration. These data indicate the influence of halide or pseudo‐halide group on the coordination number and geometry of tin. Hirshfeld surface analysis and two‐dimensional fingerprint plots were calculated for 3a and 4a which show the π–π interaction between molecules in the solid is relatively weak. 相似文献
40.
The title compound 2-(((6-chloropyridin-3-yl)methyl)thio)-5-(pyridin-4-yl)-1,3,4-thiadiazole 5(C26H18Cl2N8S4) was synthesized, and its structure was confirmed by 1H NMR, MS and elemental analyses and X-ray diffraction. It crystallizes in the triclinic system, space group P1 with a = 9.452(4), b = 12.335(4), c = 13.017(5) A, α = 90.624(5), β = 110.541(5), γ =104.879(4)°, Dc = 1.561 g/cm3, Z = 2, V = 1364.9(9) A3, F(000) = 656, the final R = 0.0300 and w R = 0.0635 for 4206 observed reflections with I 2σ(I). The preliminary biological test showed that the title compound has activities against Stemphylium lycopersici(Enjoji) Yamamoto, Fusarium oxysporum. sp. cucumebrium, and Botrytis cinerea with inhibitory activities to be 9.82%, 44.44% and 20.00%, respectively. 相似文献