全文获取类型
收费全文 | 4682篇 |
免费 | 138篇 |
国内免费 | 22篇 |
专业分类
化学 | 3218篇 |
晶体学 | 43篇 |
力学 | 57篇 |
数学 | 865篇 |
物理学 | 659篇 |
出版年
2022年 | 31篇 |
2021年 | 45篇 |
2020年 | 56篇 |
2019年 | 67篇 |
2018年 | 36篇 |
2017年 | 39篇 |
2016年 | 99篇 |
2015年 | 68篇 |
2014年 | 82篇 |
2013年 | 186篇 |
2012年 | 180篇 |
2011年 | 199篇 |
2010年 | 99篇 |
2009年 | 134篇 |
2008年 | 242篇 |
2007年 | 196篇 |
2006年 | 187篇 |
2005年 | 221篇 |
2004年 | 171篇 |
2003年 | 179篇 |
2002年 | 179篇 |
2001年 | 86篇 |
2000年 | 99篇 |
1999年 | 64篇 |
1998年 | 88篇 |
1997年 | 81篇 |
1996年 | 99篇 |
1995年 | 80篇 |
1994年 | 95篇 |
1993年 | 76篇 |
1992年 | 61篇 |
1991年 | 64篇 |
1990年 | 58篇 |
1989年 | 49篇 |
1988年 | 64篇 |
1987年 | 52篇 |
1986年 | 40篇 |
1985年 | 53篇 |
1984年 | 75篇 |
1983年 | 44篇 |
1982年 | 58篇 |
1981年 | 73篇 |
1980年 | 62篇 |
1979年 | 47篇 |
1978年 | 38篇 |
1977年 | 45篇 |
1976年 | 39篇 |
1975年 | 34篇 |
1974年 | 31篇 |
1973年 | 33篇 |
排序方式: 共有4842条查询结果,搜索用时 31 毫秒
121.
An easy synthesis of 2-aryl-3,3-dichloroazetidines, a rather unexplored class of azaheterocycles, is described. The title compounds were easily obtained by reduction of the corresponding 4-aryl-3,3-dichloro-2-azetidinones with monochloroalane, which in turn were synthesized by a ketene-imine [2 + 2] cycloaddition. The reactivity of 3,3-dichloroazetidines with bases was investigated, yielding 2-[dimethoxy(aryl)methyl]aziridines by ring contraction when treated with sodium methoxide. Furthermore, reacting the 3,3-dichloroazetidines with sodium hydride in DMSO, followed by aqueous workup, afforded 1-alkyl-2-aroylaziridines, by hydrolysis of the intermediate 2-azetines and ring closure of the transient 3-amino-2-chloro-1-phenyl-1-propanone derivatives. Monitoring this reaction in an NMR tube, using sodium hydride in DMSO-d(6), allowed the characterization of the intermediate strained heterocyclic enamines, i.e., 2-azetines, by (1)H and (13)C NMR. 相似文献
122.
F. E. Arnold 《Journal of polymer science. Part A, Polymer chemistry》1970,8(8):2079-2089
A new fused aromatic heterocyclic tetraamine 2,3,11,12-tetraaminodiquinoxal [2,3-e,2′,3′-1] pyrene has been prepared and polymerized with 1,4,5,8-naphthalenetetracarboxylic acid and pyromellitic dianhydride. The resulting ladder polymers with inherent viscosities ranging from 0.5 to 1.0 dl/g in methanesulfonic acid exhibited thermal stabilities near 600°C in nitrogen and near 450°C in air. 相似文献
123.
The compound [PPh(4)](2)[NEt(4)][CuTe(7)] has been synthesized from the reaction of CuCl with a polytelluride solution in dimethylformamide at room temperature. The compound crystallizes with two formula units in the triclinic space group P(-)1 in a cell with dimensions a = 8.9507(18) A, b = 14.714(3) A, and c = 23.277(5) A and alpha= 86.32(3) degrees, beta= 80.17(3) degrees, and gamma= 75.63(3) degrees (T = -120 degrees C). Ab initio calculations indicate that the nonclassical [CuTe(7)](3)(-) anion is the result of joining Te(3)(2-) and [CuTe(4)](1-) fragments through donor-acceptor interactions. 相似文献
124.
Thyagarajan S Shay DT Incarvito CD Rheingold AL Theopold KH 《Journal of the American Chemical Society》2003,125(15):4440-4441
Reaction of TpR,MeCo(I) dinitrogen complexes (R = iPr, tBu) with trimethylsilyl azide yields structurally characterized compounds that imply the formation of reactive intermediates of the type TpR,MeCo=NSiMe3. These cobalt imido species apparently abstract hydrogen from the 3-substituent of the Tp-ligand, leading to the formation of amido complexes accompanied by either Co-C bond formation (R = tBu) or C-C bond formation (R = iPr). 相似文献
125.
Jee JE Eigler S Hampel F Jux N Wolak M Zahl A Stochel G van Eldik R 《Inorganic chemistry》2005,44(22):7717-7731
The polyanionic water-soluble and non-mu-oxo-dimer-forming iron porphyrin iron(III) 5(4),10(4),15(4),20(4)-tetra-tert-butyl-5(2),5(6),15(2),15(6)-tetrakis[2,2-bis(carboxylato)ethyl]-5,10,15,20-tetraphenylporphyrin, (P(8-))Fe(III) (1), was synthesized as an octasodium salt by applying well-established porphyrin and organic chemistry procedures to bromomethylated precursor porphyrins and characterized by standard techniques such as UV-vis and (1)H NMR spectroscopy. A single pK(a1) value of 9.26 was determined for the deprotonation of coordinated water in (P(8-))Fe(III)(H(2)O)(2) (1-H(2)()O) present in aqueous solution at pH <9. The porphyrin complex reversibly binds NO in aqueous solution to give the mononitrosyl adduct, (P(8-))Fe(II)(NO(+))(L), where L = H(2)O or OH(-). The kinetics of the binding and release of NO was studied as a function of pH, temperature, and pressure by stopped-flow and laser flash photolysis techniques. The diaqua-ligated form of the porphyrin complex binds and releases NO according to a dissociative interchange mechanism based on the positive values of the activation parameters DeltaS() and DeltaV() for the "on" and "off" reactions. The rate constant k(on) = 6.2 x 10(4) M(-1) s(-1) (24 degrees C), determined for NO binding to the monohydroxo-ligated (P(8-))Fe(III)(OH) (1-OH) present in solution at pH >9, is markedly lower than the corresponding value measured for 1-H(2)O at lower pH (k(on) = 8.2 x 10(5) M(-1) s(-1), 24 degrees C, pH 7). The observed decrease in the reactivity is contradictory to that expected for the diaqua- and monohydroxo-ligated forms of the iron(III) complex and is accounted for in terms of a mechanistic changeover observed for 1-H(2)O and 1-OH in their reactions with NO. The mechanistic interpretation offered is further substantiated by the results of water-exchange studies performed on the polyanionic porphyrin complex as a function of pH, temperature, and pressure. 相似文献
126.
Condensation of o-phenylenediamine with 2-, 3- and 4-nitronaphthalene-1,8-dicarboxylic anhydrides gives, in each case, an isomer mixture. Separation of the mixtures into the six isomeric mononitro-7H-benzimidazo[2,1-a]benz[de]isoquinolin-7-ones is described and the isomers characterised in respect of previously reported data on some isomers; ir and mass spectrum data are reported, the latter showing fragmentation ions which may be used in characterising dyes based on this chromophore. Other new derivatives of 7H-benzimidazo[2,1-a]benz[de]isoquinolin-7-one are also described. 相似文献
127.
A family of of open subsets of the real line is called an ω-cover of a set X iff every finite subset of X is contained in an element of . A set of reals X is a γ-set iff for every ω-cover of X there exists such that In this paper we show that assuming Martin's axiom there is a γ-set X of cardinality the continuum. 相似文献
128.
The analytical performance of a glow discharge quadrupole mass spectrometer is demonstrated using sintered tungsten as an example. The inherent problem of molecular interferences in glow discharge mass Spectrometry has been considerably reduced using neon as a complementing discharge gas. Particular attention has been paid to time dependences. Analytical figures of merit are presented. 相似文献
129.
W. Clark Still Lawrence J. Macpherson Toshiro Harada James F. Callahan Arnold L. Rheingold 《Tetrahedron》1984,40(12):2275-2281
The conformations of macrocyclic intermediates provide a useful medium through which distant chiral centers may control chemical reactions. In this paper, we show that macrocycles made by cyclization of simple acyclic starting materials with an auxiliary spacer may be used to prepare stereochemically complex acyclic products. 相似文献
130.
Comparison of the frequencies and half-widths of v1 in a variety of solvents shows that dispersion forces make a significant contribution to the effective intermolecular potential. Measurements of τv?1 and [ω2] in the CH3CN/CD3OD/CCl4 system demonstrates that long-range (hydrogen-bonding?) potentials significantly affect the relaxation rate. 相似文献