全文获取类型
收费全文 | 431篇 |
免费 | 20篇 |
专业分类
化学 | 306篇 |
晶体学 | 1篇 |
力学 | 6篇 |
数学 | 28篇 |
物理学 | 70篇 |
无线电 | 40篇 |
出版年
2023年 | 6篇 |
2022年 | 11篇 |
2021年 | 13篇 |
2020年 | 5篇 |
2019年 | 6篇 |
2018年 | 11篇 |
2017年 | 8篇 |
2016年 | 11篇 |
2015年 | 11篇 |
2014年 | 9篇 |
2013年 | 12篇 |
2012年 | 24篇 |
2011年 | 20篇 |
2010年 | 12篇 |
2009年 | 14篇 |
2008年 | 30篇 |
2007年 | 29篇 |
2006年 | 21篇 |
2005年 | 22篇 |
2004年 | 10篇 |
2003年 | 11篇 |
2002年 | 14篇 |
2001年 | 4篇 |
2000年 | 5篇 |
1999年 | 3篇 |
1996年 | 12篇 |
1995年 | 5篇 |
1994年 | 9篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 9篇 |
1990年 | 7篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1987年 | 5篇 |
1986年 | 8篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 6篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1930年 | 2篇 |
1901年 | 1篇 |
1873年 | 1篇 |
排序方式: 共有451条查询结果,搜索用时 15 毫秒
331.
Gómez-Alcántara MM Cabeza A Martínez-Lara M Aranda MA Suau R Bhuvanesh N Clearfield A 《Inorganic chemistry》2004,43(17):5283-5293
Commercial bis-(4-bromophenyl)-ether, [BrC(6)H(4)](2)-O, has been used to prepare 4-[4'-(diethoxyphosphoryl)phenoxy]phenyl-phosphonic acid diethyl ester, [(CH(3)CH(2))(2)O(3)P-C(6)H(4)](2)-O, (I) following a slight modification of the Michaelis-Arbuzov reaction. The acid hydrolysis of I gave 4-(4'-phosphonophenoxy)phenyl phosphonic acid, [H(2)O(3)P-C(6)H(4)](2)-O (II), and both compounds have been characterized by (1)H NMR and (13)C NMR. The crystal structure of II has been determined by single-crystal X-ray diffraction. II crystallizes in an orthorhombic unit cell, space group Pbcn, with a = 7.822(3) A, b = 5.821(2) A, c = 28.982(9) A, and V = 1319.7(7) A(3). The final R factor was R1 = 0.0614. The structure is layered, being held together through a hydrogen bonding network. II has been used as precursor in the syntheses of new metal (Mn, Fe, Co, Ni, Cu, and Zn) bisphosphonates. The syntheses were carried out using a fixed metal/bisphosphonic acid molar ratio of 2.1:1 and the influence of the pH in the reactions has been studied. Nine new compounds have been isolated: Mn(2)(O(3)PC(6)H(4)OC(6)H(4)PO(3)).1.5H(2)O (III), Mn(5)(OH)(2)(O(3)PC(6)H(4)OC(6)H(4)PO(3))(2).2H(2)O (IV), Fe(HO(3)PC(6)H(4)OC(6)H(4)PO(3)).0.5H(2)O (V), Co(2)(O(3)PC(6)H(4)OC(6)H(4)PO(3)).2H(2)O (VI), Ni(2)(O(3)PC(6)H(4)OC(6)H(4)PO(3)).3H(2)O (VII), Ni(2)(O(3)PC(6)H(4)OC(6)H(4)PO(3)).2H(2)O (VIII), Cu(2)(O(3)PC(6)H(4)OC(6)H(4)PO(3)) (IX), Zn(2)(O(3)PC(6)H(4)OC(6)H(4)PO(3)) (X), and Zn(HO(3)PC(6)H(4)OC(6)H(4)PO(3)H) (XI). Compound IX crystallizes in an orthorhombic unit cell, space group Pbcn, and unit cell parameters a = 8.1012(5) A, b = 5.3109(3) A, c = 29.2595(5) A, and V = 1258.8(1) A(3). Its structure has been solved by ab initio powder diffraction and refined by the Rietveld method to R(F) = 0.042. IX has a pillared layer framework with highly distorted CuO(5) groups sharing edges to give isolated dimers. XI was indexed in a monoclinic unit cell, space group P112(1), with parameters a = 9.4991(9) A, b = 5.0445(5) A, c = 29.131(2) A, gamma = 91.945(7) degrees, and V = 1395.1(3) A(3). Its structure has been refined by the Rietveld method, R(F) = 0.054, since it is isostructural with the known compound, Zn[HO(3)P(C(6)H(4))(2)PO(3)H]. All solids were also characterized by thermal analysis and IR and UV-Vis spectroscopies. 相似文献
332.
Two-dimensional laser cooling based on velocity-selective coherent population trapping is investigated theoretically for the J
g=1J
e=0 atomic transition. Wavevectors and polarizations of three laser beams are chosen to realize a coherent superposition of three degenerate ground states. For the first time in laser cooling, use is made of the electric field phases to realize coherent population trapping selective in two dimensions. Numerical solutions and analytic estimates are presented for laser cooling of helium atoms. 相似文献
333.
Dr. Diego Cortizo‐Lacalle Juan P. Mora‐Fuentes Dr. Karol Strutyński Prof. Dr. Akinori Saeki Prof. Dr. Manuel Melle‐Franco Prof. Dr. Aurelio Mateo‐Alonso 《Angewandte Chemie (International ed. in English)》2018,57(3):703-708
The properties of graphene nanoribbons are highly dependent on structural variables such as width, length, edge structure, and heteroatom doping. Therefore, atomic precision over all these variables is necessary for establishing their fundamental properties and exploring their potential applications. An iterative approach is presented that assembles a small and carefully designed molecular building block into monodisperse N‐doped graphene nanoribbons with different lengths. To showcase this approach, the synthesis and characterisation of a series of nanoribbons constituted of 10, 20 and 30 conjugated linearly‐fused rings (2.9, 5.3, and 7.7 nm in length, respectively) is presented. 相似文献
334.
An aromatic diazonium ion is considered as a reactive probe for determining the ability of a substrate-tether system to isolate adjacent, immobilized species. The immobilized probe molecule can either hydrolyze to the colorless phenol or couple to give a highly colored azo compound. The product distribution depends on whether the substrate-tether system permits interaction of adjacent species. The geometric form (cis vs. trans) of the azo compound which may form is affected by the nature of the substrate and the tether. The cis form is favored over the normally stable trans form when the substrate-tether system holds opposite ends of the doubly immobilized azo molecules together instead of stretching them apart. 相似文献
335.
Nieves Casañ Pedro Amorós Rafael Ibañez Eduardo Martínez-Tamayo Aurelio Beutrán-Porter Daniel Beltrán-Porter 《Journal of inclusion phenomena and macrocyclic chemistry》1988,6(2):193-211
Sodium-containing intercalates having as general formula Na
x
VOPOP4·(2–x)H2O (0.25x<0.50) have been obtained and characterized. Orthorhombic phases, which essentially maintain the structure of the layered oxide hydrate VOPO4·2H2O result. Intercalated sodium ions act as pillars. The presence of H3O+ ions in the parent VOPO4·2H2O and also in some reduced phases, is detected. The understanding of the structural role of the water molecules is advanced and the topotactic dehydration/rehydration processes are studied. The formation of a new metastable VOPO4·H2O phase is established. 相似文献
336.
Summary Pig brain NAD glycohydrolase immobilized on Affi-Gel 10 or nylon 6 was used for the conversion of NAD into 3-acetylpyridine
adenine dinucleotide (APAD) or 3-aminopyridine adenine dinucleotide (AAD).
A reversed-phase chromatographic system consisting of a C18 Resolve column and phosphate buffer (pH 6.2)-methanol as the mobile phase was used to monitor the production of APAD and
AAD. 相似文献
337.
Attempts were made to apply the spontaneous “biogenetic-type cyclization” of van Tamelen to the preparation of the sarpagan ring system by utilizing indolo [2,3-a]quinolizidines 10, 11, 12, and 14. The fact that the spontaneous “biogenetic-type cyclization” did not take place casts some doubt on the correctness of the earlier results. 相似文献
338.
Summary The main terpenes ofGinkgo biloba L. extracts (bilobalide, ginkgolide A, ginkgolide B and ginkgolide C) have been separated by isocratic elution on a 3 μm
C18 Spherical column using 2-propanol:water (10∶90) as eluent. 相似文献
339.
The C(12b)-C(1)-C(2)-C(3) stereochemical relationship in several racemic indolo[2,3-a]quinolizine derivatives has been determined by the application of conformational considerations to the 13C NMR spectral analysis. The proper shift assignment was confirmed by recording the spectra of selectively deuterated derivatives. The C(12b)-C(1)-C(2)-C(3) stereochemical relationship in indolo[2,3-a]quinolizines obtained by acid-induced cyclizadon of partially hydrogenated 3,5-dimethoxycarbonyl-1-[2-(3-indolyl)ethyl]pyridine derivatives is discussed. 相似文献
340.
Dr. Giacomo Saielli Dr. Franca Castiglione Dr. Michele Mauri Prof. Roberto Simonutti Prof. Andrea Mele 《Chemphyschem》2021,22(18):1880-1890
The dynamics of xenon gas, loaded in a series of 1-alkyl-3-methylimidazolium based ionic liquids, probes the formation of increasingly blurred polar/apolar nanodomains as a function of the anion type and the cation chain length. Exploiting 129Xe NMR spectroscopy techniques, like Pulse Gradient Spin Echo (PGSE) and inversion recovery (IR), the diffusion motion and relaxation times are determined for 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [CnC1im][TFSI]. A correlation between the ILs nano-structure and both xenon diffusivity and relaxation times, as well as chemical shifts, is outlined. Interestingly, comparison with previous results of the same properties in the homologous imidazolium chlorides and hexafluorophospate shows an opposite trend with the alkyl chain length. Classical molecular dynamics (MD) simulations are used to calculate the xenon and cation and anion diffusion coefficients in the same systems, including imidazolium cations with longer chains (n=4, 6, 8 … 20). An almost quantitative agreement with the experiments validates the MD simulations and, at the same time, provides the necessary structural and dynamic microscopic insights on the nano-segregation and diffusion of xenon in bistriflimide, chloride and hexafluorphosphate salts allowing to observe and rationalize the shaping effect of the cation in the nanostructure. 相似文献