全文获取类型
收费全文 | 674篇 |
免费 | 14篇 |
国内免费 | 1篇 |
专业分类
化学 | 438篇 |
晶体学 | 7篇 |
力学 | 46篇 |
数学 | 58篇 |
物理学 | 140篇 |
出版年
2020年 | 4篇 |
2019年 | 5篇 |
2018年 | 3篇 |
2017年 | 8篇 |
2016年 | 7篇 |
2015年 | 10篇 |
2014年 | 18篇 |
2013年 | 38篇 |
2012年 | 21篇 |
2011年 | 32篇 |
2010年 | 12篇 |
2009年 | 20篇 |
2008年 | 23篇 |
2007年 | 27篇 |
2006年 | 29篇 |
2005年 | 39篇 |
2004年 | 36篇 |
2003年 | 24篇 |
2002年 | 22篇 |
2001年 | 3篇 |
2000年 | 9篇 |
1999年 | 16篇 |
1998年 | 6篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1995年 | 14篇 |
1994年 | 9篇 |
1992年 | 14篇 |
1990年 | 7篇 |
1989年 | 6篇 |
1988年 | 7篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 17篇 |
1984年 | 11篇 |
1983年 | 18篇 |
1982年 | 17篇 |
1981年 | 17篇 |
1980年 | 20篇 |
1979年 | 14篇 |
1978年 | 10篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1973年 | 5篇 |
1972年 | 8篇 |
1969年 | 8篇 |
1967年 | 2篇 |
1964年 | 4篇 |
排序方式: 共有689条查询结果,搜索用时 15 毫秒
31.
John G. Stuart Michael J. Quast Gary E. Martin Vincent M. Lynch Stanley H. Simonsen Milton L. Lee Raymond N. Castle Jerry L. Dallas Boban K. John LeRoy F. Johnson 《Journal of heterocyclic chemistry》1986,23(4):1215-1234
Syntheses of benzo[3,4]phenanthro[1,2-b]thiophene, benzo[3,4]phenanthro[2,1-b]thiophene and their 1-methyl analogs are reported as potential constituents of solvent refined coal liquids and for mutagenicity testing. The attempted synthesis of the 13-methyl analogs which gave the 11-methyl isomers is also described. Total assignments of the 1H- and 13C-nmr spectra based on long range optimized heteronuclear protoncarbon two-dimensional chemical shift correlation are reported. Carbon assignments obtained for benzo[3,4]-phenanthro[1,2-b]thiophene using this approach were confirmed with a 125 MHz 13C–13C INADEQUATE spectrum. X-Ray crystal structures were determined for benzo[3,4]phenanthro[1,2-b]thiophene and 1-methyl-benzo[3,4]phenanthro[2,1-b]thiophene. Both molecules were helically distorted from planarity. Close intramolecular contacts between the bay region H1–H13 and ClMe-H13 of 2.03 and 2.28 Å, respectively, were responsible for the distortions. There were no close intermolecular contacts of <3.5Å. both molecules refined to an R value of <0.05. 相似文献
32.
Yoshinori Tominaga Milton L. Lee Raymond N. Castle 《Journal of heterocyclic chemistry》1981,18(5):977-979
The cyclization of 2-acetonylthionaphthalene, prepared from 2-mercaptonaphthalene (1) and chloro-acetone in the presence of sodium hydroxide, with polyphosphoric acid gave 1-methylnaphtho[2,1-b]thiophene (4) in 64% overall yield from 1. By bromination with N-bromosuccinimide, 4 was converted in 40% yield into l-bromomethylnaphtho[2,1-b]thiophene (8) . Treatment of 8 with potassium cyanide in a phase-transfer medium gave l-cyanomethylnaphtho[2,1-b]thiophene (10) in good yield. Compound 10 was reduced to the corresponding aldehyde 11 and then cyclized with polyphosphoric acid to phenaleno[l,9-bc]thiophene (12) in 24% overall yield from 10. 相似文献
33.
Jain SL Bhattacharyya P Milton HL Slawin AM Crayston JA Woollins JD 《Dalton transactions (Cambridge, England : 2003)》2004,(6):862-871
Seven new pyridine dicarboxamide ligands H2L(1-7) have been synthesised from condensation reactions involving pyridine-2,6-dicarboxylic acid (H2dipic), pyridine-2,6-dicarbonyl dichloride or 2,6-diaminopyridine with heterocyclic amine or carboxylic acid precursors. Crystallographic analyses of N,N'-bis(2-pyridyl)pyridine-2,6-dicarboxamide monohydrate (H2L8 x H2O), N,N'-bis[2-(2-pyridyl)methyl]pyridine-2,6-dicarboxamide and N,N'-bis[2-(2-pyridyl)ethyl]pyridine-2,6-dicarboxamide monohydrate revealed extensive intramolecular hydrogen bonding interactions. 2,6-Bis(pyrazine-2-carboxamido)pyridine (H2L6) and 2,6-bis(pyridine-2-carboxamido)pyridine (H2L7) reacted with copper(II) acetate monohydrate to give tricopper(II) complexes [Cu3(L)2(mu2-OAc)2]. X-Ray crystallography confirmed deprotonation of the amidic nitrogen atoms and that the (L6,7)2- ligands and acetate anions hold three copper(II) ions in approximately linear fashion. H2L8. Reacted with copper(II) tetrakis(pyridine) perchlorate to give [Cu(L8)(OH2)]2 x 2H2O, in which (L8)2- was tridentate through the nitrogen atoms of the central pyridine ring and the deprotonated carboxamide groups at one copper centre, with one of the terminal pyridyl rings coordinating to the other copper atom in the dimer. The corresponding reaction using H2L7 gave [Cu3(L7)2(py)2][ClO4]2, which transformed during an attempted recrystallisation from ethanol under aerobic conditions to a tetracopper(II) complex [Cu4(L7)2(L7-O)2]. 相似文献
34.
Marvin L. Tedjamulia Hirotaka Kudo Raymond N. Castle Milton L. Lee 《Journal of heterocyclic chemistry》1984,21(2):321-325
The syntheses of naphtho[1,2–6:7,6-b′]bisbenzo[b]thiophene ( 4 ), naphtho[1,2–6:7,8-b′]bisbenzo[b]thiophene ( 5 ), naphtho[2,1-b:7,6-b′]bisbenzo[b]thiophene ( 8 ), naphtho[2,1-b:7,8-b′]bisbenzo[b]thiophene ( 9 ), naphtho-[1,2–6:5,6-b′]bisbenzo[b]thiophene ( 14 ), naphtho[1,2–6:6,5-b]bisbenzo[b]thiophene ( 17 ) and naphtho[2,1–6:-6,5-b′]bisbenzo[b]thiophene ( 23 ) are reported. 相似文献
35.
The thermodynamic and extra-thermodynamic dependencies of five types of cytochrome c in water-acetonitrile mixtures of different composition in the presence of immobilised n-octyl ligands as a function of temperature from 278 K to 338 K have been investigated. The corresponding enthalpic, entropic and heat capacity parameters, deltaHdegrees assoc, deltaS degrees assoc and delta C degrees p, have been evaluated from the observed non-linear Van't Hoff plots of these globular proteins in these heterogeneous systems. The relationships between the free energy dependencies, various molecular parameters and extra-thermodynamic dependencies (empirical correlations) of these protein-non-polar ligand interactions have also been examined. Thus, the involvement of enthalpy-entropy compensation effects has been documented for the binding of these cytochrome cs to solvated n-octyl ligands. Moreover, the results confirm that this experimental approach permits changes in molecular surface area due to the unfolding of these proteins on association with non-polar ligands as a function of temperature to be correlated with other biophysical properties. This study thus provides a general procedure whereby the corresponding free energy dependencies of globular proteins on association with solvated non-polar ligands in heterogeneous two-phase systems can be quantitatively evaluated in terms of fundamental molecular parameters. 相似文献
36.
9-Methylideneflourene (IIa) reacts rapidly with HCo(CO)4 at -67° C to give a quantitative yield of 9-methylfluorene (IIIa); k2 (13.4 ± 0.5) × 10-2 l mol-1S-1. Although the internal olefin, 9-ethylidenefluorene (IIb) reacts more slowly than IIa, it is hydrogenated about 2.5 times as fast as the terminal olefin, 1,1-diphenylethylene (I). Measurement of the rate of the reaction of IIb with DCo(CO)4 and comparison with HCo(CO)4 shows a very large inverse isotope effect kH/kD of 0.43. 相似文献
37.
Thermolysis curves for manganese(II), silver and thorium diliturates were obtained. Manganese diliturate precipitated as the octahydrate and dehydrated in two steps. Silver diliturate formed as a monohydrate from aqueous solutions. Thorium diliturate formed as a hydrate containing 20 moles of water and dehydrated in two steps.Methods for the thermogravimetric determination of manganese(II) and thorium by precipitation as the diliturates are suggested. 相似文献
38.
Eugene R. Corey Joyce Y. Corey Milton D. Glick 《Journal of organometallic chemistry》1977,129(1):17-25
The structures of 5-methyl-5-phenyl-5H-dibenzo[b,f] silepin (I) and 5-methyl- 5-phenyl-1O,11-dihydro-5H-dibenzo [b,f] silepin (II) have been determined from three-dimensional X-ray data collected by counter methods. I crystallizes in the orthorhombic space group Pnam with a 7.596(3), b 18.102(5) and c 12.190(2) Å; observed and calculated densities (Z = 4) are 1.17 and 1.18 g cm?3, respectively. II crystallizes in the monoclinic space group with a 11.115(3), b 7.920(3), c 20.765(5) Å and β 111.71(2)°; observed and calculated densities (Z = 4) are 1.17 g cm?3. Anisotropic refinement of nonhydrogen atoms, with hydrogen atoms included at fixed ideal locations, gave conventional R-factors of 4.5% (I) and 5.0% (II). Compound I exhibits the boat conformation for the tricyclic framework and is located on a crystallographically required mirror plane. Com- pound II has the expected folded boat conformation. The torsion angle about the 10,11-bond is 0.0° for I, a crystallographic symmetry requirement, and 89.9° for II. Mean SiC bond distances are 1.863 Å(I) and 1.875 Å(II). The dihedral angles between the planar benzo groups are 129.7° (I) and 137.2° (II); introduc- tion of unsaturation at the 10,11-position decreases the dihedral angle in the tri- cyclic system, i.e., the tricyclic system is more bent. 相似文献
39.
The reaction of SiBr4(g) with H2(g) in the temperature range 900–1143 K has been studied by a mass spectrometric method. Second and third law reaction enthalpies were obtained for SiBr4(g) + H2(g) = SiHBr3(g) + HBr(g), SiHBr3(g) + H2(g) = SiH2Br2(g) + HBr(g), and SiH2Br2(g) + H2(g) = SiH3Br(g) + HBr(g). From the heats of reaction, third-law ΔH£298 values of ?72.5 ± 1, ?43.2 ± 1.5 and ?15.3 ± 0.5 kcal/mole were obtained for SiHBr3(g), SiH2Br2(g), and SiH3Br(g), respectively. 相似文献
40.
Juvanc Z Markides KE Petersson P Johnson DF Bradshaw JS Lee ML 《Journal of chromatography. A》2002,982(1):119-126
A copolymeric stationary phase, consisting of a chiral selective part, i.e. (1R-trans)-N,N'-1,2-cyclohexylenebisbenzamide, and an efficient siloxane oligomeric part, was successfully applied to open tubular column GC analysis. The efficiency and the chiral selectivity of this stationary phase were studied in detail, and high capacity and efficiency at elevated GC temperatures were especially noted. Several drugs and other enantiomeric pairs were separated. The shown examples demonstrate a broad application range for this type of chiral stationary phase in GC analysis. 相似文献