全文获取类型
收费全文 | 2147篇 |
免费 | 67篇 |
国内免费 | 4篇 |
专业分类
化学 | 1647篇 |
晶体学 | 4篇 |
力学 | 19篇 |
数学 | 326篇 |
物理学 | 222篇 |
出版年
2023年 | 29篇 |
2022年 | 15篇 |
2021年 | 23篇 |
2020年 | 61篇 |
2019年 | 50篇 |
2018年 | 27篇 |
2017年 | 17篇 |
2016年 | 40篇 |
2015年 | 28篇 |
2014年 | 39篇 |
2013年 | 66篇 |
2012年 | 68篇 |
2011年 | 95篇 |
2010年 | 39篇 |
2009年 | 44篇 |
2008年 | 69篇 |
2007年 | 62篇 |
2006年 | 71篇 |
2005年 | 68篇 |
2004年 | 55篇 |
2003年 | 43篇 |
2002年 | 38篇 |
2001年 | 17篇 |
2000年 | 21篇 |
1999年 | 15篇 |
1998年 | 18篇 |
1996年 | 14篇 |
1995年 | 20篇 |
1994年 | 20篇 |
1991年 | 21篇 |
1990年 | 15篇 |
1987年 | 15篇 |
1986年 | 18篇 |
1985年 | 27篇 |
1984年 | 29篇 |
1983年 | 25篇 |
1982年 | 38篇 |
1981年 | 22篇 |
1980年 | 24篇 |
1979年 | 42篇 |
1978年 | 25篇 |
1977年 | 34篇 |
1976年 | 26篇 |
1975年 | 24篇 |
1974年 | 19篇 |
1973年 | 14篇 |
1967年 | 19篇 |
1959年 | 14篇 |
1956年 | 20篇 |
1934年 | 23篇 |
排序方式: 共有2218条查询结果,搜索用时 15 毫秒
41.
The synthesis of aza-2 pyrrolo[1,2-a]indole derivatives was achieved by a 1,3-dipolar cycloaddition reaction of dimethylacetylenedicarboxylate with a mesoionic ion. 相似文献
42.
Max Robba Daniel Maume Jean-Charles Lancelot 《Journal of heterocyclic chemistry》1979,16(6):1193-1196
The 1,2-dihy dro-1,2,4-triazino[4,5-a] indole-1-thiones 2 and 3 obtained by sulphurization of the 1,2-dihydro-1,2,4-triazino-[4,5-a] indol-1-ones 1 and 4 with phosphorus pentasulphide are intermediate products in the synthesis of 1,2,4-triazino[4,5-a]-indolc 10 and 4-methyl-1,2,4 omdp;e 19. The thiolactam-thiolactim tautomerism was studied using the ir and nmr spectra and allowed N. and S.substitutions. The methylation with dimethyl sulphate afforded mixtures of N. and S. methyl derivatives which were separated by chromatographic analysis and identified by |21|0H nmr and unequivocal synthesis. 相似文献
43.
Max Robba Jean-Charles Lancelot Daniel Maume Alain Rabaron 《Journal of heterocyclic chemistry》1978,15(7):1159-1163
1,2,4-Triazino[4,5-b]indazol-1(2H)one and its derivatives were prepared by transposition of 3-[2-(-1,3,4-oxadiazolyl)]indazole or by ring closure of indazole ethoxymethylidenehydrazides. The synthesis of 1,2,3,4-tetrahydro-l,2,4-triazino[4,5-b]indazole-1,4-dione was achieved by cyclising the N-carbethoxyhydrazide of indazole-3-carboxylic acid and the synthesis of 1,2,4-triazino-[4,5-b]indazol-4(3H)one was made by cyclising the N-carbethoxy-hydrazone of indazole-3-carboxaldehyde. The Oxydation of 1,2,4-triazino[4,5-b]indazole-l(2H)thione gave 1,2,4-triazino-[4,5-b]indazoles. Nmr spectral data are reported. 相似文献
44.
Single crystals of dihydrodibenzo[b, i]-1, 4, 8, 11-tetraazacyclotetradecinenickel iodide show metallic conductivity between room temperature and approx. 110 K. The crystals are orthorhombic, space group Ibam with four formula units per cell of the following dimensions: a = 20.245, b = 13.416, c = 6.418 Å. The analogous palladium complex is isomorphous, the cell constants being a = 20.452, b = 13.430 and c = 6.499 Å. A number of other partially oxidized metal tetraaza [14]annulenes also show electrical conductivity. 相似文献
45.
Berthold Fischer Helmut W. Schmalle Markus R. Baumgartner Max Viscontini 《Helvetica chimica acta》1997,80(1):103-110
The first crystal structure of a molybdenum complex 9 with a hydrogenated pterin and a sulfur ligand contributes to the discussion about the active center of molybdenum and tungsten enzymes containing a molybdopterin cofactor. Complex 9 was synthesized through a redox reaction of [MoVIO2 (LN-S2)] ( 8 ; LN-S2 = pyridine-2, 6-bis(methanethiolato)) with 5, 6, 7, 8-tetrahydropterin ( 7 ). 2 HCl (H4Ptr.2 HCl). The complex crystallizes, with a non-coordinating Cl-atom acting as a counterion, in the monoclinic space group C2/c (No. 15) with cell dimensions a = 22.900(5), b = 10.716(2), c = 17.551(4) Å, β = 120.36(3)°, and Z = 8. We interpret 9 as [MoIVO(LN-S2)(H+-q-H2Ptr)]Cl (q = quinonoid; H2Ptr = dihydropterin), i.e., a MoIV monooxo center coordinated by a pyridine-2, 6-bis(methanethiolato) ligand and a protonated dihydropterin. The spectroscopic properties of this new complex are comparable to those of other crystalline molybdenum complexes of hydrogenated pterins without additional S-coordination. The slightly H2O-soluble complex 9 reacts with the natural enzyme substrate DMSO very slowly, possibly due to the lack of easily dissociable ligands at the metal center. 相似文献
46.
Yamien Effi Jean-Charles Lancelot Sylvain Rault Max Robba 《Journal of heterocyclic chemistry》1986,23(1):17-23
The reactivity of pyrrolo[1,2-a]thieno[2,3-e]pyrazine towards quaternisation, bromination, nitration, Friedel-Crafts, Vilsmeier-Haack and Reissert reactions was studied. Assignment of structures to the product is based upon ir, nmr and mass spectral studies. 相似文献
47.
Patrick Dallemagne Michel Boulouard Sylvain Rault Max Robba 《Journal of heterocyclic chemistry》1993,30(3):799-802
Synthesis of thieno[2′,3′:5,4]cyclopenta[3,2-d]oxazole and thiazole derivatives are achieved by insertion of carbon dioxide and disulfide into 4-amino-5-chloro-5,6-dihydro-4H-cyclopenta[b]thiophen-6-one. 相似文献
48.
The configurational isomers of astaxanthin (3,3′-dihydroxy-β,β-carotene-4,4′-dione) from the flesh of salmon (Salmo salar and Oncorhynchus) caught at different places in Europe and Canada were isolated and analyzed as (?)-camphanic acid diesters by means of HPLC. The biological variation in the composition of the configurational isomers in seven fish was surprisingly similar: 78 to 85% of (3S, 3′S)-astaxanthin, 12 to 17% (3R, 3′R)-astaxanthin and 2 to 6% meso-astaxanthin. 相似文献
49.
Katharina Schiedt Max Vecchi Ernst Glinz Trond Storebakken 《Helvetica chimica acta》1988,71(4):887-896
Diets supplemented with astaxanthin and canthaxanthin, respectively, and a control diet without carotenoid additions, were fed to 1½-year-old Atlantic salmon (Salmo salar, L.) for one year. The integuments were investigated as to their quantitative and qualitative carotenoid composition. Astaxanthin and canthaxanthin deposited in the skin amounted to 20 and 14% of the total carotenoids only. Seventy % must be considered as metabolites of astaxanthin and canthaxanthin and 10% as basic xanthophylls also present in the control groups. Astaxanthin apparently underwent the following metabolic pathway: astaxanthin→idoxanthin→adonixanthin→zeaxanthin→zeaxanthin 5,6-epoxides. Reduction of the 4′-carbonyl group was stereospecific leading to the (4′R)-idoxanthin. Canthaxanthin was obviously converted to β,β-carotene via 4′-hydroxyechinenone, echinenone, and 4-hydroxy-β,β-carotene. 相似文献
50.
Max H. Feinberg Christian J. Ducauze 《Fresenius' Journal of Analytical Chemistry》1991,339(9):604-607
Summary Chemometrics have been described as effective tools for exploring chemical data, and many software packages are now available on micro-computers. This work evaluate their suitability for environmental analytical chemistry. Guidelines for multivariate method selection are proposed. They are based on the type of variables and the goal of the study. Two examples are proposed to illustrate these methods and their efficiency. Firstly it is shown that a global assessment of Rhine basin mercuric pollution in the Alsace region is possible with Multiple Correspondence Factor Analysis. Several goals are simultaneously reached: a mapping of pollution, a detection of pollution changes during the study period and an evaluation of bio-accumulation as a function of fish species. Secondly the modeling of industrial soil pollution by heavy metals is studied by Multiple Linear Regression. It demonstrates that chemometrics provide us with necessary tools for environmental analytical chemistry but also for toxicological studies or ecosystem modeling. 相似文献