全文获取类型
收费全文 | 1014篇 |
免费 | 40篇 |
国内免费 | 4篇 |
专业分类
化学 | 779篇 |
晶体学 | 5篇 |
力学 | 21篇 |
数学 | 91篇 |
物理学 | 162篇 |
出版年
2023年 | 5篇 |
2022年 | 5篇 |
2021年 | 16篇 |
2020年 | 17篇 |
2019年 | 14篇 |
2018年 | 9篇 |
2017年 | 9篇 |
2016年 | 26篇 |
2015年 | 22篇 |
2014年 | 27篇 |
2013年 | 53篇 |
2012年 | 63篇 |
2011年 | 90篇 |
2010年 | 46篇 |
2009年 | 46篇 |
2008年 | 73篇 |
2007年 | 66篇 |
2006年 | 73篇 |
2005年 | 42篇 |
2004年 | 32篇 |
2003年 | 34篇 |
2002年 | 36篇 |
2001年 | 21篇 |
2000年 | 12篇 |
1998年 | 15篇 |
1997年 | 10篇 |
1996年 | 10篇 |
1995年 | 13篇 |
1994年 | 11篇 |
1993年 | 12篇 |
1992年 | 9篇 |
1990年 | 5篇 |
1989年 | 5篇 |
1988年 | 3篇 |
1987年 | 4篇 |
1985年 | 12篇 |
1984年 | 9篇 |
1982年 | 11篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 5篇 |
1978年 | 8篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1975年 | 8篇 |
1974年 | 5篇 |
1973年 | 4篇 |
1969年 | 4篇 |
1968年 | 3篇 |
1967年 | 3篇 |
排序方式: 共有1058条查询结果,搜索用时 31 毫秒
21.
Simulations of diffusion-limited cluster-cluster aggregation (DLCA) with no restructuring, full restructuring, and partial restructuring have been performed. The scattering patterns produced from these aggregates have been simulated using the Rayleigh-Gans-Debye approximation. Pure DLCA aggregates produced a scattering pattern with the slope of the fractal region being about -1.8. In contrast, the slope of the fractal region of the scattering pattern for fully restructured aggregates was about -2.1, indicating an increase in fractal dimension. Partial restructuring at large length scales produced an upward turn in the scattering pattern at low qr(o), while at high qr(o) the fractal section of the pure DLCA aggregate was retained. This last result was expected and is consistent with the results and postulations of several other workers. This simulation shows that the type of scattering pattern often obtained from orthokinetic or sheared aggregation can be produced by restructuring of aggregates at large length scales. Copyright 2001 Academic Press. 相似文献
22.
Novel products from attempted michael addition to 1-methyl-1,2,5,6-tetrahydropyridine-4-carbonitrile
Graeme L. Butt Leslie W. Deady Maureen F. Mackay Maruse Sadek 《Tetrahedron letters》1982,23(43):4485-4488
The reaction of some phenols with the title compound, in the presence of sodium, gives 2-(2-hydroxyaryl_piperidine derivatives. Geometrical isomers have been separated, which differ in having an equatorial (A) or axial (B) cyano group on the piperidine chair (the methyl and aryl groups are equatorial in both forms). The x-ray crystallographic structures of an example of each of A and B are reported and the proton NMR spectra are assigned. 相似文献
23.
In a combination of cobalt-mediated cationic and free radical chemistry, treatment of the allyl silane substituted β-hydroxycobaloxime 13 with pTSA led predominantly to the trans-disubstituted cyclopentane 14, which on irradiation with TEMPO produced 15. In a similar manner, the cobaloxime 18 was cyclised to 19 in the presence of pTSA which, on irradiation with a sunlamp then underwent radical mediated cyclisation to the substituted indane 20. 相似文献
24.
W. Russell Bowman Anthony J. Fletcher Peter J. Lovell Elena Hernández López Graeme B.S. Potts 《Tetrahedron》2007,63(1):191-203
Amides have been successfully used as precursors of imidoyl radicals for radical cyclisation. The amides have been converted to imidoyl selanides via reaction with phosgene to yield imidoyl chlorides followed by reaction with potassium phenylselanide. Imidoyl selanides were reacted with tributyltin hydride (Bu3SnH) as the radical mediator with triethylborane or AIBN as initiators to yield imidoyl radicals for cyclisation reactions. Imidoyl radicals have been cyclised onto alkenes to yield 2,3-substituted-indoles and -quinolines and also onto pyrroles and indoles to give bi- and tricyclic heteroarenes. 相似文献
25.
Graeme D. McAllister 《Tetrahedron》2007,63(49):12123-12130
A concise route to the anti-tumour natural product (−)-varitriol, together with its novel isomer (−)-3′-epi-varitriol, is described using a Horner-Wadsworth-Emmons (HWE)/conjugate addition/Ramberg-Bäcklund sequence as the cornerstone. The flexibility of the synthetic route has been demonstrated by the preparation of novel varitriol analogues. 相似文献
26.
Step-scan FTIR time-resolved spectroscopy (S2FTIR TRS) in acetonitrile-d3 has been used to probe the acceptor ligand in metal-to-ligand charge transfer (MLCT) excited states of amide-substituted polypyridyl complexes of RuII and in analogues appended to polystyrene. On the basis of ground-to-excited state shifts in v(C = O) of -31 cm-1 for the amide group in [RuII(bpy)2(bpyCONHEt')]2+ (bpyCONHEt' = 4'-methyl-2,2'-bipyridine-4-carboxamide-Et'; Et' = -CH2CH2BzCH2CH3) (1) and in the derivatized polystyrene abbreviated [PS-[CH2-CH2NHCObpy-RuII(bpy)2]20]40+ (3), the excited-state dipole is directed toward the amide-containing pyridyl group in the polymer side chain. Smaller shifts in v(C = O) of -17 cm-1 in [RuII(4,4'-(CONEt2)2bpy)2-(bpyCONHEt')]2+ (2) and in the derivatized polystyrene abbreviated [PS-[CH2CH2NHCObpy-RuII(4,4'-(CONEt2)2bpy)2]20]40+ (4) indicate that the excited-state dipole is directed toward one of the diamide bpy ligands. The nearly identical results for 1 and 3 and for 2 and 4 show that the molecular and electronic structures of the monomer excited states are largely retained in the polymer samples. These conclusions about dipole orientation in the polymers are potentially of importance in understanding intrastrand energy transfer dynamics. The excited-state dipole in 3 is oriented in the direction of the covalent link to the polymer backbone, and toward nearest neighbors. In 4, it is oriented away from the backbone. 相似文献
27.
Connecting two facially-protected porphyrins was expected to lead to an equal mixture of laterally-bridged doubly-protected bis-porphyrins; one in which the two porphyrin units were protected on the same face (syn) and one with the two prophyrin units protected on opposite faces (anti). Addition of a co-factor (bidentate ligand) was expected to lead predominantly to the syn-bis-porphyrin by a templated self-replication process. This concept was explored using Baldwin's capped porphyrin. Bis(capped porphyrins) were synthesised in several steps starting from zinc(II) capped porphyrin 2. Nitration of 2 followed by reduction and photo-oxidation yields a mixture of zinc(II) porphyrindiones 7 and 8 that can separated by HPLC. The condensation of 2 molar eq. of zinc(II) porphyrin-7,8-dione 8 with 1,2,4,5-benzenetetramine leads to the formation of a 1:1 mixture of syn- and anti-dizinc(II) bis(7,8-capped porphyrins), 11 and 12, respectively, that have almost identical spectroscopic properties. These two geometric isomers were distinguished by significant differences in their molecular recognition properties. Likewise the syn- and anti-dizinc(II) bis(2,3-capped porphyrins), 9 and 10, respectively, are synthesised from the related zinc(II) capped porphyrin-2,3-dione 7, and were also identified using molecular recognition studies. The molecular recognition properties of these bis(capped porphyrins) were utilised in studies of self-replicating porphyrin systems. The results show that tetraazaanthraceno-bis-porphyrins 9-12 can catalyse their own formation but self-replication was not observed. These results highlight the potential that these interesting hosts have as templates in supramolecular chemistry, synthesis and catalysis. 相似文献
28.
Boyd AS Carroll JB Cooke G Garety JF Jordan BJ Mabruk S Rosair G Rotello VM 《Chemical communications (Cambridge, England)》2005,(19):2468-2470
We report the electrochemically tuneable intramolecular hydrogen bonding interactions between a covalently linked flavin-diamidopyridine unit. 相似文献
29.
Graeme B Bolger Angela McCahill Stephen J Yarwood Michael R Steele Jim Warwicker Miles D Houslay 《BMC biochemistry》2002,3(1):24-11
Background
The cyclic AMP specific phosphodiesterase, PDE4D5 interacts with the β-propeller protein RACK1 to form a signaling scaffold complex in cells. Two-hybrid analysis of truncation and mutant constructs of the unique N-terminal region of the cAMP-specific phosphodiesterase, PDE4D5 were used to define a domain conferring interaction with the signaling scaffold protein, RACK1. 相似文献30.
S. L. Maxwell III 《Journal of Radioanalytical and Nuclear Chemistry》2006,270(3):651-655
Summary The measurement of radium isotopes in natural waters is important for oceanographic studies and for public health reasons.
Radium-226 (T1/2 = 1620 y) is one of the most toxic of the long-lived alpha-emitters present in the environment due to its long life and its
tendency to concentrate in bones, which increases the internal radiation dose of individuals. The analysis of 226Ra and 228Ra in natural waters can be tedious and time-consuming. Different sample preparation methods are often required to prepare
226Ra and 228Ra for separate analyses. A rapid method has been developed at the Savannah River Environmental Laboratory that effectively
separates both 226Ra and 228Ra (via 228Ac) for assay. This method uses MnO2 Resin from Eichrom Technologies (Darien, IL, USA) to preconcentrate 226Ra and 228Ra rapidly from water samples, along with 133Ba tracer. DGA Resinò (Eichrom) and Ln-Resinò (Eichrom) are employed in tandem to prepare 226Ra for assay by alpha-spectrometry and to determine 228Ra via the measurement of 228Ac by gas proportional counting. After preconcentration, the manganese dioxide is dissolved from the resin and passed through
stacked Ln-Resin-DGA Resin cartridges that remove uranium and thorium interferences and retain 228Ac on DGA Resin. The eluate that passed through this column is evaporated, redissolved in a lower acidity and passed through
Ln-Resin again to further remove interferences before performing a barium sulfate microprecipitation. The 228Ac is stripped from the resin, collected using cerium fluoride microprecipitation and counted by gas proportional counting.
By using vacuum box cartridge technology with rapid flow rates, sample preparation time is minimized. 相似文献