全文获取类型
收费全文 | 12920篇 |
免费 | 292篇 |
国内免费 | 87篇 |
专业分类
化学 | 9073篇 |
晶体学 | 123篇 |
力学 | 281篇 |
数学 | 1650篇 |
物理学 | 2172篇 |
出版年
2023年 | 67篇 |
2022年 | 97篇 |
2021年 | 128篇 |
2020年 | 167篇 |
2019年 | 156篇 |
2018年 | 126篇 |
2017年 | 115篇 |
2016年 | 232篇 |
2015年 | 241篇 |
2014年 | 296篇 |
2013年 | 552篇 |
2012年 | 716篇 |
2011年 | 923篇 |
2010年 | 459篇 |
2009年 | 406篇 |
2008年 | 830篇 |
2007年 | 851篇 |
2006年 | 873篇 |
2005年 | 887篇 |
2004年 | 768篇 |
2003年 | 629篇 |
2002年 | 589篇 |
2001年 | 181篇 |
2000年 | 167篇 |
1999年 | 145篇 |
1998年 | 158篇 |
1997年 | 177篇 |
1996年 | 212篇 |
1995年 | 139篇 |
1994年 | 145篇 |
1993年 | 137篇 |
1992年 | 112篇 |
1991年 | 97篇 |
1990年 | 72篇 |
1989年 | 78篇 |
1988年 | 75篇 |
1987年 | 79篇 |
1986年 | 76篇 |
1985年 | 106篇 |
1984年 | 111篇 |
1983年 | 78篇 |
1982年 | 78篇 |
1981年 | 102篇 |
1980年 | 95篇 |
1979年 | 73篇 |
1978年 | 61篇 |
1977年 | 61篇 |
1976年 | 44篇 |
1975年 | 60篇 |
1973年 | 43篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Regioselective ortho lithiation of 2-, 3-, and 4-halopyridines is achieved with lithium diisopropylamide (?78°, tetrahydrofuran) to afford, upon quenching with electrophilic reagents, 2,3- and 3,4-disubstituted pyridines in good to excellent yield. 相似文献
92.
Lázló Markó 《Journal of organometallic chemistry》1981,213(1):271-275
Two new mixed iron cobalt carbonyl sulphide clusters HFe2(CO)9S and Fe2Co(CO)8(NO)S have been prepared and characterized. The hydride complex undergoes acidic dissociation in polar solvents and the resulting anion was isolated as (Et4N)[Fe2Co(CO)9S]. An efficient high pressure synthesis has been found for H2Fe3(CO)9S. 相似文献
93.
Photocycloadditlon of (+)-isoplperltenone and cyclobutene-1-carboxylic acid gives an adduct which upon reduction with NaCNBH3 followed by thermolysis yields an isomer of isoaristolactone in an overall yield of 26%. 相似文献
94.
Butler MS 《Natural product reports》2005,22(2):162-195
Natural product and natural product-derived compounds that are being evaluated in clinical trials or in registration (current 31 December 2004) have been reviewed. Natural product derived drugs launched in the United States of America, Europe and Japan since 1998 and new natural product templates discovered since 1990 are discussed. 相似文献
95.
The synthesis and chemical properties of a series of amino acid derivatives containing the thiolsulphonate functionality are described. 相似文献
96.
Michael E. Jung John A. Lowe Mark A. Lyster Manabu Node Rudolf W. Pfluger Richard W. Brown 《Tetrahedron》1984,40(22):4751-4766
Several mono- and bicyclic 6-methoxy-2-pyrones having substituents at C4 and C5 can be prepared regiospecifically by either of two routes: (1) regiospecific construction of a glutaconic half-ester followed by dehydrative cyclization, and (2)regiospecific Friedel-Crafts acylation of 6-methoxy-2-pyrones at C5. These pyrones undergo clean and regiospecific Diels-Alder cycloadditions with various unsymmetrical dienophiles, e.g., quinones, unsaturated esters, etc, with subsequent loss of carbon dioxide. In this manner several polycyclic aromatic natural products have been prepared such as chrysophanol, helminthosporin, pachybasin, 2-acetylemodin and the purported structure for orientalone. The utility of this approach for the synthesis of the anthracyclines is demonstrated by its use in the preparation of various tetracyclic intermediates for anthracycline synthesis. 相似文献
97.
Evert J. Ditzel Xavier L. R. Fontaine Norman N. Greenwood John D. Kennedy Mark Thornton-Pett 《无机化学与普通化学杂志》1992,616(10):79-85
The action of SMe2 on the ten-vertex nido-ruthenaborane [6-(η6-C6Me6)RuB9Hl3] ( 1 ) provides a high-yield route to the unsubstituted isocloso-ruthenaborane [1-(η6-C6Me6)RuB9H9] (2). The benzene analogue [1-(η6-C6Me6)RuB9H9] is prepared similarly. By contrast, reaction of (1) with PhNH2 gives a variety of B-phenylamino isocloso derivatives, including orange crystals of [1-(η6-C6Me6)-2-(PhNH)-isocloso-1-RuB 9 H8] ( 3 ), red-orange [1-(η6-C6Me6)-2,3-(PhNH)2-isocloso-1-RuB9H7] ( 4 ) and dark-red [1-(η6-C6Me6)-5,6,7-(PhNH)3-isocloso-1-RuB9H6] ( 5 ). Detailed 1H and 11B nmr properties of these various compounds are described. The structure of ( 3 ) has been established by a single-crystal X-ray diffraction study of the solvate [1-(η6-C6Me6)-2-(PhNH)-isocloso-1-RuB9H8] · 1/2 CH2Cl2; the crystals were monoclinic, space group C2/c, with a = 1895.1(3), b = 1556.6(3), c = 1716.4(3) pm, β = 104.37(1)° and z = 8. 相似文献
98.
Micellar electrokinetic chromatography (MEKC) of highly hydrophobic compounds is generally difficult using sodium dodecyl sulfate micellar solutions. The polymeric surfactant, polysodium undecyl sulfate (poly-SUS) has been used to separate moderately to highly hydrophobic polychlorinated biphenyl (PCB) congeners by MEKC in the absence of cyclodextrins. Parameters such as concentration of acetonitrile (ACN), polymeric surfactant concentration, and the effect of pH were examined. Optimum MEKC conditions to get baseline resolution of nine PCBs was 7.5 mM borate in 40% (v/v) ACN fraction buffered at pH 9.2 using 0.5% (w/v) poly-SUS. The applied voltage was 30 kV and the temperature was maintained at 25 degrees C. Elution order for each PCB congener was found to be dependent on the degree of chlorination and hydrophobic character. 相似文献
99.
Hughes RP Meyer MA Tawa MD Ward AJ Williamson A Rheingold AL Zakharov LN 《Inorganic chemistry》2004,43(2):747-756
Reaction of Pd(TMEDA)(CH(3))(2) [TMEDA = tetramethylethylenediamine] with fluoroalkyl iodides R(F)I affords a series of square planar Pd(II) complexes Pd(TMEDA)(CH(3))(R(F)) [R(F) = CF(2)CF(3) (9), CFHCF(3) (10), CH(2)CF(3) (11)], presumably by oxidative addition followed by reductive elimination of CH(3)I. The solid-state structures of each compound have been determined by single crystal X-ray diffraction studies, allowing the effect of increasing alpha-fluorination on the structural trans-influence of alkyl ligands to be examined. In these compounds there is no significant difference observed in the trans-influence of the three fluorinated alkyl ligands toward the trans-N atom, although a significant cis-influence on the neighboring methyl ligand is apparent. Oxidative addition of the same series of fluoroalkyl ligands to the corresponding Pt(TMEDA)(CH(3))(2) affords octahedral Pt(IV) complexes trans-Pt(TMEDA)(CH(3))(2)(R(F))I [R(F) = CF(2)CF(3) (12), CFHCF(3) (13), CH(2)CF(3) (14)] as the kinetic products. In each case, subsequent isomerization to the corresponding all cis-isomers is observed; in the case of 13, the stereocenter at the alpha-carbon results in two diastereomeric cis-isomers, which are formed at different rates. The molecular structures of 13 and its more stable all cis-isomer 16b have been crystallographically determined. Kinetic studies of the trans-cis isomerization reactions show the mechanism to involve a polar transition state, presumably involving iodide dissociation, followed by rearrangement of the cation, and iodide recombination. High dielectric solvents increase the rate, but solvent coordinating ability has no effect. Dissolved salts (LiI, LiOTf) show normal accelerative salt effects, with no inhibition in the case of added iodide, consistent with the formation of an intimate ion pair intermediate. The kinetic parameters show that the trans-effects of fluoroalkyl ligands in these compounds follow the order expected from the relative sigma-donor properties of the ligands, with CF(2)CF(3) < CFHCF(3) < CH(2)CF(3). 相似文献
100.
We report the gas-phase preparation of negatively charged glycine as well as the Gly(H(2)O)(1,2) (-) complexes by entrainment of the neutral precursor into an ionized supersonic expansion tuned to optimize the (H(2)O)(n) (-)Ar(m) clusters. The photoelectron spectrum of Gly(-) displays the signature of a dipole-bound species, with sufficient vibrational fine structure to characterize the core neutral as a higher energy, non-zwitterionic isomer of the amino acid. 相似文献