全文获取类型
收费全文 | 424篇 |
免费 | 15篇 |
专业分类
化学 | 292篇 |
晶体学 | 1篇 |
力学 | 7篇 |
数学 | 59篇 |
物理学 | 80篇 |
出版年
2022年 | 4篇 |
2021年 | 6篇 |
2020年 | 8篇 |
2019年 | 3篇 |
2018年 | 8篇 |
2017年 | 7篇 |
2016年 | 8篇 |
2015年 | 7篇 |
2014年 | 6篇 |
2013年 | 15篇 |
2012年 | 15篇 |
2011年 | 17篇 |
2010年 | 11篇 |
2009年 | 13篇 |
2008年 | 19篇 |
2007年 | 25篇 |
2006年 | 22篇 |
2005年 | 11篇 |
2004年 | 20篇 |
2003年 | 12篇 |
2002年 | 22篇 |
2001年 | 6篇 |
2000年 | 8篇 |
1999年 | 6篇 |
1998年 | 8篇 |
1997年 | 6篇 |
1996年 | 14篇 |
1995年 | 6篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1992年 | 6篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1986年 | 3篇 |
1985年 | 8篇 |
1984年 | 7篇 |
1983年 | 3篇 |
1982年 | 7篇 |
1981年 | 9篇 |
1980年 | 11篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1974年 | 3篇 |
1970年 | 3篇 |
1969年 | 2篇 |
1967年 | 2篇 |
1966年 | 9篇 |
1962年 | 2篇 |
排序方式: 共有439条查询结果,搜索用时 15 毫秒
101.
102.
The rate of detrapping of atomic hydrogen from several octasilsesquioxanes is the same for dissolved and solid samples and is independent of the presence of other species such as free radicals or oxygen; varying the cage substituents leads to only minor differences in the activation parameters. Hydrogen atoms are found to be more strongly stabilized in homosubstituted octasilsesquioxanes compared with singly Ge-substituted cages. A kinetic isotope effect observed for the detrapping of H and D from MeT(8) is ascribed to the difference in the zero-point energies of the trapped atoms. There is a secondary H/D isotope effect in the temperature dependence of the (29)Si-superhyperfine splitting constants in the range 228-353 K. Cage relaxation has a substantial effect on the detrapping barrier but little influence on the intracage potential. Calculations using a rigid cage approximation give satisfactory agreement with zero-point parameters extracted from experimental data. Different model chemistries yield qualitatively different pictures of the dependence of the hyperfine coupling constant of the trapped H atom upon the detrapping coordinate. Within an isotropic approximation of the vibrational displacements, the B3LYP data give fairly close agreement with the experimental temperature dependence, subject to a shift of the absolute value related to known weaknesses of the method. For the Si(7)Ge cage, it is found that the transition state in which the H atom passes through a Ge-containing face is strongly favored, accounting for the larger detrapping rate parameters observed experimentally for this species. 相似文献
103.
Novel integrated paired emitter-detector diode (PEDD) as a miniaturized photometric detector in HPLC
O'Toole M Lau KT Shazmann B Shepherd R Nesterenko PN Paull B Diamond D 《The Analyst》2006,131(8):938-943
A novel low power, low cost, highly sensitive, miniaturized light emitting diode (LED) based flow detector has been used as optical detector for the detection of sample components in high performance liquid chromatography (HPLC). This colorimetric detector employs two LEDs, one operating in normal mode as a light source and the other is reverse biased to work as a light detector. Instead of measuring the photocurrent directly, a simple timer circuit is used to measure the time taken for the photocurrent generated by the emitter LED (lambda(max) 500 nm) to discharge the detector LED (lambda(max) 621 nm) from 5 V (logic 1) to 1.7 V (logic 0) to give digital output directly without using an A/D converter. Employing a post-column reagent method, a Nucleosil 100-7 column (functionalized with iminodiacetic acid (IDA) groups) was used to separate a mixture of transition metal complexes, manganese(II) and cobalt(II) in 4-(2-pyridylazo)-resorcinol (PAR). All optical measurements were taken by using both the in-built HPLC variable wavelength detector and the proposed paired-emitter-detector-diode (PEDD) optical detector configured in-line for data comparison. The concentration range investigated using the PEDD was found to give a linear response to the Mn(II) and Co(II) PAR complexes. The effects of flow rate and emitter LED light source intensity were investigated. Under optimised conditions the PEDD detector offered a linear range of 0.9-100 microM and LOD of 0.09 microM for Mn-PAR complex. A linear range of 0.2-100 microM and LOD of 0.09 microM for Co-PAR complex was achieved. 相似文献
104.
Nick C. Johnstone 《Journal of organometallic chemistry》2010,695(2):170-11381
Aluminium complexes bearing the N,N-chelating ligand 1,4-bis(2-hydroxy-3,5-di-tert-butyl)piperazine (1) have been synthesised. Both monometallic and bimetallic aluminium methyl complexes (2 and 3, respectively) were prepared by treatment of 1 with the appropriate amount of AlMe3. Complex 2 can be converted to 3 by addition of excess AlMe3. Bimetallic aluminium-ethyl complex 4 was also prepared. Treatment of 1 with AlEt2Cl afforded the monometallic chloride complex 5. Treatment of this latter complex with potassium alkoxides (KOR, R = Me, Et, iPr, tBu) or AgOTf afforded the corresponding aluminium alkoxide complexes (6, R = Et; 7, R = Me; 8, R = iPr; 9, R = tBu; 10, R = OTf) in good yields. Aluminium ethoxide complex 6 was also synthesised by treatment of 1 with AlEt2OEt. All of these complexes were tested as potential catalysts in the ring-opening polymerisation of rac-lactide and caprolactone with limited success. 相似文献
105.
Poineau F Johnstone EV Weck PF Kim E Forster PM Scott BL Sattelberger AP Czerwinski KR 《Journal of the American Chemical Society》2010,132(45):15864-15865
Technetium trichloride has been synthesized by reaction of Tc(2)(O(2)CCH(3))(4)Cl(2) with HCl(g) at 300 °C. The mechanism of formation mimics the one described earlier in the literature for rhenium. Tc(2)(O(2)CCH(3))(2)Cl(4) [P1?; a = 6.0303(12) ?, b = 6.5098(13) ?, c = 8.3072(16) ?, α = 112.082(2)°, β = 96.667(3)°, γ = 108.792(3)°; Tc-Tc = 2.150(1) ?] is formed as an intermediate in the reaction at 100 °C. Technetium trichloride is formed above 250 °C and is isostructural with its rhenium homologue. The structure consists of Tc(3)Cl(9) clusters [R3?m; a = b = 10.1035(19) ?, c = 20.120(8) ?], and the Tc-Tc separation is 2.444(1) ?. Calculations on TcX(3) (X = Cl, Br) have confirmed the stability of TcCl(3) and suggest the existence of a polymorph of TcBr(3) with the ReBr(3) structure. 相似文献
106.
107.
108.
Comely AC Gibson SE Hales NJ Johnstone C Stevenazzi A 《Organic & biomolecular chemistry》2003,1(11):1959-1968
Carbonylcobalt(0) species have been used as linkers between alkynes and a polymer support for the first time. The alkynes may be loaded indirectly onto a phosphine functionalised polymer via their hexacarbonyldicobalt(0) complex, or directly onto a cobalt coated polymer. The alkynes have been released either as alkynes, thus providing a traceless method of immobilising alkynes, or by reaction with an alkene to generate a cyclopentenone via the Pauson-Khand reaction. The cobalt coated polymers produced during this study were shown to catalyse the Pauson-Khand reaction. 相似文献
109.
Hydrogen isotope studies have shown that heterogeneous catalytic transfer hydrogenolysis of C-O bonds in tetrazolyl ethers of phenols (1) in the liquid phase proceeds via direct transfer of hydrogen from an active hydrogen donor centre to the ether on the catalyst surface and not through transfer of hydrogen atoms from the catalyst surface. 相似文献
110.
A new approach to functionalized spiropiperidines through tandem RCM and nitrogen-directed reactions
Edwards AS Wybrow RA Johnstone C Adams H Harrity JP 《Chemical communications (Cambridge, England)》2002,(14):1542-1543
The synthesis of a functionalized spiropiperidine via a tandem ring closing metathesis strategy is described, furthermore, the regio- and stereoselective functionalization of this compound has been achieved through a novel nitrogen-directed epoxidation reaction. 相似文献