全文获取类型
收费全文 | 43374篇 |
免费 | 9692篇 |
国内免费 | 1463篇 |
专业分类
化学 | 45960篇 |
晶体学 | 368篇 |
力学 | 742篇 |
综合类 | 1篇 |
数学 | 3969篇 |
物理学 | 3489篇 |
出版年
2022年 | 88篇 |
2021年 | 234篇 |
2020年 | 1299篇 |
2019年 | 2651篇 |
2018年 | 1076篇 |
2017年 | 711篇 |
2016年 | 3465篇 |
2015年 | 3641篇 |
2014年 | 3573篇 |
2013年 | 4437篇 |
2012年 | 3253篇 |
2011年 | 2556篇 |
2010年 | 3099篇 |
2009年 | 3039篇 |
2008年 | 2733篇 |
2007年 | 2115篇 |
2006年 | 1815篇 |
2005年 | 1941篇 |
2004年 | 1795篇 |
2003年 | 1603篇 |
2002年 | 2272篇 |
2001年 | 1463篇 |
2000年 | 1382篇 |
1999年 | 482篇 |
1998年 | 159篇 |
1997年 | 158篇 |
1996年 | 173篇 |
1995年 | 125篇 |
1994年 | 145篇 |
1993年 | 146篇 |
1992年 | 131篇 |
1991年 | 85篇 |
1990年 | 120篇 |
1989年 | 105篇 |
1988年 | 89篇 |
1987年 | 92篇 |
1986年 | 85篇 |
1985年 | 112篇 |
1984年 | 143篇 |
1983年 | 103篇 |
1982年 | 149篇 |
1981年 | 143篇 |
1980年 | 163篇 |
1979年 | 154篇 |
1978年 | 155篇 |
1977年 | 155篇 |
1976年 | 144篇 |
1975年 | 119篇 |
1974年 | 111篇 |
1973年 | 106篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The self‐assembly of NiCl2·6H2O with a diaminodiamide ligand 4,8‐diazaundecanediamide (L‐2,3,2) gave a [Ni(C9H20N4O2)(Cl)(H2O)] Cl·2H2O ( 1 ). The structure of 1 was characterized by single‐crystal X‐ray diffraction analysis. Structural data for 1 indicate that the Ni(II) is coordinated to two tertiary N atoms, two O atoms, one water and one chloride in a distorted octahedral geometry. Crystal data for 1: orthorhombic, space group P 21nb, a = 9.5796(3) Å, b = 12.3463(4) Å, c = 14.6305(5) Å, Z = 4. Through NH···Cl–Ni (H···Cl 2.42 Å, N···Cl 3.24 Å, NH···Cl 158°) and OH···Cl–Ni contacts (H···Cl 2.36 Å, O···Cl 3.08 Å, OH···Cl 143°), each cationic moiety [Ni(C9H20N4O2) (Cl)(H2O)]+ in 1 is linked to neighboring ones, producing a charged hydrogen‐bonded 1D chainlike structure. Thermogrametric analysis of compound 1 is consistent with the crystallographic observations. The electronic absorption spectrum of Ni(L‐2,3,2)2+ in aqueous solution shows four absorption bands, which are assigned to the 3A2g → 3T2g, 3T2g → 1Eg, 3T2g → 3T1g, and 3A2g → 3T1g transitions of triplet‐ground state, distorted octahedral nickel(II) complex. The cyclic volammetric measurement shows that Ni2+ is more easily reduced than Ni(L‐2,3,2)2+ in aqueous solution. 相似文献
992.
Hayati Türkmen Resul Sevinek Bekir etinkaya Santiago García‐Granda Muhittin Aygün 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(6):m277-m279
The title complex, [PdCl2(C21H26N2)(C18H15P)], shows slightly distorted square‐planar coordination around the PdII metal centre. The Pd—C bond distance between the N‐heterocyclic ligand and the metal atom is 2.028 (5) Å. The dihedral angle between the two trimethylphenyl ring planes is 36.9 (2)°. 相似文献
993.
Zhaoyang Cheng Shipei Xing Jun Guo Biao Cheng Lan‐Fang Hu Xing‐Hong Zhang Zhan Lu 《中国化学》2019,37(5):457-461
A regioselective double 1,1‐hydrosilylation of terminal aliphatic alkynes with primary silanes catalyzed by one cobalt catalyst has been developed. gem‐Bis(dihydrosilyl)alkanes containing four silicon‐hydrogen bonds are efficiently constructed in an atom‐economical manner. Tolerated substrates include simplest alkyne‐ethyne, a complicated drug derivative and various functionalized terminal aliphatic alkynes. Asymmetric approach using two catalysts is achieved with excellent enantioselectivities to access corresponding chiral products. The transformations of Si—H bonds into Si—C, Si—O, and Si—F bonds and the synthesis of enantioriched α‐hydroxysilane show synthetic utility. 相似文献
994.
Talaat Ibrahim El‐Emary 《中国化学会会志》2006,53(2):391-401
New derivatives of pyrazolo[3,4‐b]pyrazine and related heterocycles were prepared starting from 6‐amino‐3‐methyl‐1‐phenyl‐1H‐pyrazolo[3,4‐b]pyrazine‐5‐carbonitrile ( 2 ) and using the key intermediates 4 , 5 , 6 , 14 , 15 and 16 . Some of the prepared compounds were evaluated for their antifungal and antibacterial activities. 相似文献
995.
Négrier F Marceau E Che M Giraudon JM Gengembre L Löfberg A 《The journal of physical chemistry. B》2005,109(7):2836-2845
1.5 Ni wt %/Al2O3 catalysts have been prepared by incipient wetness impregnation using [Ni(diamine)x(H2O)(6-2x)]Y2 precursors (diamine = 1,2-ethanediamine (en) and trans-1,2-cyclohexanediamine (tc); x = 0, 1, and 2; Y = NO3- and Cl-), to avoid the formation, during calcination, of difficult-to-reduce nickel aluminate. N2 was chosen for thermal treatment to help reveal and take advantage of the reactions occurring between Ni2+, ligands, counterions, and support. In the case of [Ni(en)2(H2O)2]Y2 salts used as precursors, in situ UV-vis and DRIFT spectroscopies show that after treatment at 230 degrees C Ni(II) ions are grafted to alumina via two OAl bonds and that the diamine ligands still remain coordinated to grafted nickel ions but in a monodentate way, bridging the cation with the alumina surface. With Y = Cl-, the chloride counterions desorb as hydrogen chloride, and hydrogen released upon decomposition of the en ligands is able to reduce a fraction of nickel ions into metal as evidenced by XPS. In contrast, with Y = NO3-, compounds such as CO or NO are formed during thermal treatment, indicating that nitrate ions burn the en ligands. After thermal treatment at 500 degrees C, a surface phase containing Ni(II) ions forms, characterized by XPS and UV-vis spectroscopy. Temperature-programmed reduction shows that these ions can be quantitatively reduced to the metallic state at 500 degrees C, in contrast with the aluminate obtained when the preparation is carried out from [Ni(H2O)6]2+, which is reduced only partly at 950 degrees C. On the other hand, a total self-reduction of nickel complexes leading to 2-5-nm metal particles is obtained upon thermal treatment via the hydrogen released by a hydrogen-rich ligand such as tc, whatever the Y counterion. An appropriate choice of the ligand and the counterion allows then to obtain selectively Ni(II) ions or a dispersed reduced nickel phase after treatment in N2, as a result of the reactions occurring between the chemical partners present on alumina. 相似文献
996.
A series of dilute liquid crystalline solvents are used to study the effect of slight anisotropy caused by partial alignment on chemical shift and residual dipolar coupling (RDC) in small molecules. The residual dipolar couplings between protons in solutes are found to be almost independent of the local environment. It is also found that the chemical shift does not change over the concentration range observed. A linear relationship between residual dipolar coupling and liquid crystal concentration is observed at relatively low concentrations, but is severely violated at high concentrations. 相似文献
997.
A structural study by means of X‐ray and electron diffraction of intermediate oxides between Eu2Mo3O12 and Eu2W3O12 shows the existence of a new Eu2(Mo1?xWx)3O12 solid solution with the scheelite structural type. The essential feature of the x = 1/3 and x = 2/3 compounds is the presence of isolated oxygen tetrahedra, in which, according to X‐ray and electron diffraction data, W and Mo are randomly distributed. Eu atoms occupy distorted square antiprisms sites. 相似文献
998.
H. Münzel F. Michel P. P. Coetzee V. Krivan 《Journal of Radioanalytical and Nuclear Chemistry》1977,37(1):267-273
The possible application of cyclotron-produced fast neutrons to activation analysis for oxygen based on the16O(n, p)16N reaction has been investigated. Neutrons were produced by bombarding a thick beryllium target with 22 to 45 MeV deuterons.
It was found that the sensitivity increases rapidly with the energy of the deuterons. Using 45 MeV deuterons and a 10 μA beam
current a sensitivity of about 20 counts per 1 μg oxigen could be achieved, enabling the determination of less than 1 μg oxigen.
In a direct comparison it was experimentally established that the sensitivity for cyclotron-produced neutrons assuming a deuteron
beam of about 10 μA, is up to two orders of magnitude higher than that achievable for 14 MeV neutrons with a flux of about
1010 n/s. The interference of fluorine is at about the same level for both the cyclotron-produced and 14 MeV neutrons. Using cyclotron-produced
fast neutrons in the investigated energy range, sodium and magnesium can also interfere, but only to a very much lower extent. 相似文献
999.
1000.
Laurent Griffe Dr. Mary Poupot Dr. Patrice Marchand Dr. Alexandrine Maraval Dr. Cédric‐Olivier Turrin Dr. Olivier Rolland Pascal Métivier Dr. Gérard Bacquet Dr. Jean‐Jacques Fournié Dr. Anne‐Marie Caminade Dr. Rémy Poupot Dr. Jean‐Pierre Majoral Dr. 《Angewandte Chemie (International ed. in English)》2007,46(14):2334-2334