全文获取类型
收费全文 | 2750篇 |
免费 | 57篇 |
国内免费 | 39篇 |
专业分类
化学 | 1738篇 |
晶体学 | 15篇 |
力学 | 74篇 |
数学 | 339篇 |
物理学 | 680篇 |
出版年
2021年 | 26篇 |
2020年 | 24篇 |
2019年 | 19篇 |
2018年 | 16篇 |
2016年 | 34篇 |
2015年 | 30篇 |
2014年 | 40篇 |
2013年 | 106篇 |
2012年 | 100篇 |
2011年 | 130篇 |
2010年 | 80篇 |
2009年 | 55篇 |
2008年 | 120篇 |
2007年 | 148篇 |
2006年 | 123篇 |
2005年 | 111篇 |
2004年 | 111篇 |
2003年 | 103篇 |
2002年 | 105篇 |
2001年 | 79篇 |
2000年 | 73篇 |
1999年 | 57篇 |
1998年 | 41篇 |
1997年 | 43篇 |
1996年 | 50篇 |
1995年 | 39篇 |
1994年 | 50篇 |
1993年 | 47篇 |
1992年 | 50篇 |
1991年 | 29篇 |
1990年 | 42篇 |
1989年 | 41篇 |
1988年 | 26篇 |
1987年 | 26篇 |
1986年 | 25篇 |
1985年 | 50篇 |
1984年 | 37篇 |
1983年 | 25篇 |
1982年 | 39篇 |
1981年 | 30篇 |
1980年 | 41篇 |
1979年 | 35篇 |
1978年 | 46篇 |
1977年 | 37篇 |
1976年 | 33篇 |
1975年 | 31篇 |
1974年 | 19篇 |
1973年 | 39篇 |
1972年 | 15篇 |
1971年 | 15篇 |
排序方式: 共有2846条查询结果,搜索用时 31 毫秒
71.
Ron S. Dickson Gary S. Evans Gary D. Fallon 《Journal of organometallic chemistry》1982,236(2):C49-C52
The complexes (η-C5Me5)2Rh2(μ-CO) {μ-η2,η2-C(O)CRCR} are obtained from reactions between (η-C5Me5)2Rh2(CO)2 and the alkynes RCCR (R CF3, CO2Me, or Ph) at 25°C. The molecular geometry of the complex with R CF3 has been established by X-ray diffraction; the bridging 'ene-one' unit adopts a μ-η2,η2 conformation. Other complexes isolated from these reactions include (η-C5Me5)Rh(C6R6) (R CF3, CO2Me), (η-C5Me)2Rh2(C4R4) (R CO2Me) and (η-C5Me5)2Rh2(CO2C2R2) (R Ph). The reaction between (η-C5Me5)2Rh2(CO)2 and C6F5CCC6F5 gives (η-C5Me5)2Rh2(CO)2(C6F5C2C6F5). Mononuclear complexes such as (η-C5Me5)Co(C4R4CO) are the major products isolated from reactions between (η-C5Me5)2CO2(CO)2 and alkynes at 25°C. 相似文献
72.
Phenylalanine, tyrosine and dihydroxyphenylalanine (DOPA) were identified unambiguously and quantitatively determined in single ventral thoracic nerve cords from the locust, Schistocerca gregaria, by gas chromatography-negative-ion chemical ionisation mass spectrometry. Deuterium-labelled analogues of each compound were added to a single ventral thoracic nerve cord in hydrochloric acid; the tissue was homogenised and the suspension centrifuged. The remaining hydrochloric acid was eliminated azeotropically by repeated additions of acetonitrile followed by evaporation under a stream of nitrogen and the resultant residue derivatised by reaction with hexafluoroisopropanol and pentafluoropropionic anhydride. Under negative-ion chemical ionisation conditions, the hexafluoroisopropanol-pentafluoropropionyl derivatives produced characteristic ions which were sufficiently abundant to be suitable for selected-ion monitoring. This method is highly specific and gave a limit of detection below the nanogram level. The amounts of phenylalanine, tyrosine and DOPA in a single ventral thoracic nerve cord were, respectively, 194 +/- 81, 347 +/- 88 and 11 +/- 11 ng per tissue. 相似文献
73.
C. Evans D. Weir J. C. Scaiano A. Mac Eachern J. T. Arnason P. Morand B. Hollebone L. C. Leitch B. J. R. Philogegne 《Photochemistry and photobiology》1986,44(4):441-451
The photochemistry of α-terthienyl (αT) and its mono- and dodo derivatives has been examined using nanosecond laser Hash photolysis techniques. The triplet states of these intermediates have been characterized, and show strong triplet-triplet absorptions with maxima in the 450 to 490 nm region. The triplet lifetimes are normally reduced by efficient triplet-triplet annihilation and self-quenching both of which approach diffusion control. Triplet lifetimes in methanol obtained by extrapolation to zero laser dose and zero concentration are 30, 12.5 and 9.4 μs for αT and its mono- and dodo derivatives, respectively; the effect of iodo substitution on the lifetimes is attributed to heavy atom effects. The triplet states are efficiently quenched by oxygen and the electron acceptor methyl viologen, while amines tire very poor triplet quenchers. The iodo derivatives are photolabile. undergoing C-I bond cleavage from the singlet state, a process that was studied in benzene solvent, where the complex between iodine atoms and benzene can be readily characterized. Modification of αT by replacement of the central thiophene ring by an aromatic ring (i.e. DTB) causes drastic changes in the triplet and singlet state kinetic and spectroscopic characteristics. 相似文献
74.
Simple yet accurate expressions for the electrical double-layer interaction force and energy between the particles that hold for a wide range of surface potential is required in the modeling, simulation, and optimization of many processes employed in industry. In this paper, simple approximate expressions for the interaction are obtained based on the asymptotic results and the numerical solution to the Poisson-Boltzmann equation for identical parallel plates with constant surface potential up to 180/z(i) mV at 25 degrees C, and the Derjaguin approximation. Within the moderate surface potential range, the semianalytical expressions agree well with the exact numerical results and are convenient to use for the purpose of process modeling and simulation. Copyright 2000 Academic Press. 相似文献
75.
The differences between the far-infrared absorption of liquid (+)3-methylcyclohexanone and the racemic mixture are interpreted as supporting evidence for the direct observation of rotation-translation coupling. 相似文献
76.
Wet oxidation of BPL porous carbon by H2O2 and HNO3 changes the pore structure and also increases the concentration of surface oxides. KOH uptake is increased by oxidation and is seen to be dependent on KOH concentration and to a lesser extent on inert electrolyte concentration. Enthalpy of immersion measurements have allowed the estimation of the enthalpy of hydration of the surface oxides and the enthalpy of neutralization of the easily accessible acid oxides. 相似文献
77.
Catherine A. Evans 《Tetrahedron》2005,61(26):6309-6314
Amine-catalyzed coupling reactions of allenoate esters and α,β-unsaturated carbonyls lead to a diverse range of α,α′-disubstituted allenoates. With appropriately substituted monomers, intermolecular reactions can lead to pyrimidone products. Alternatively, with amine substituted allenoates, a 7-endo-dig cyclization can be carried out such that a divergent pathway is observed that leads to azepine scaffolds. 相似文献
78.
Dinitrogen can be reduced to the planar M2(mu-eta2:eta2-N2) structure without employing cyclopentadienyl or complicated polydentate ligands using the recently discovered divalent oxidation states of Tm(II), Dy(II), and Nd(II). Complexes of these ions with common monodentate amide and aryloxide ligands can effect N2 reduction. THF solutions of LnI2 (Ln = Tm, Dy) in the presence of 2 equiv of NaN(SiMe3)2 reduce dinitrogen to form {[(Me3Si)2N]2(THF)Ln}2(mu-eta2:eta2-N2) complexes that have planar Ln2N2 units and 1.264(7) and 1.305(6) A NN bonds consistent with (N2)2- moieties. With the stronger reductant Nd(II), aryloxides are sufficient ancillary ligands: the NdI2/2KOC6H3tBu2-2,6 (KOAr) system forms [(ArO)2(THF)2Nd]2(mu-eta2:eta2-N2), which has a 1.242(7) A NN bond. 相似文献
79.
N-[2-(Dimethylamino)ethyl]acridine-4-carboxamide is a new experimental antitumour agent which has excellent in vivo activity against the Lewis lung tumour in mice. A reversed-phase high-performance liquid chromatographic method is described for the measurement of this agent in plasma. The internal standard was N-[2-(diethylamino)ethyl]acridine-4-carboxamide. The compounds of interest were extracted from plasma (0.2 ml) with acetonitrile and further purified on C18 solid-phase extraction Bond Elut columns. After elution with acetonitrile-ammonium acetate buffer and evaporation, the final separation was carried out on a C18 muBondapak column with fluorimetric detection. Over the plasma concentration range 100-5000 nM, the intra- and inter-assay coefficients of variation were less than 4.1 and 7.7%, respectively. The accuracy of the method varied from 97 to 105% of the theoretical values. The lowest concentration which could be measured with acceptable accuracy (+/- 10%) and precision (coefficient of variation less than 10%) was 10 nM. The method was sufficiently sensitive to allow pharmacokinetic analyses of 30 mumol/kg doses for more than six half-lives (t1/2) in rabbits (t1/2 = 4) and mice (t1/2 = 1.3 h). 相似文献
80.
Diffusion coefficients of tetradecyltrimethylammonium bromide in aqueous solutions have been determined at 25, 95 and 135 °C using the Taylor dispersion technique. The diffusion coefficient exhibits a minimum at a surfactant concentration above the critical micelle concentration (CMC). The results are interpreted in terms of electrostatic coupling and rapid exchange between micelle, surfactant monomer and counterions. 相似文献