全文获取类型
收费全文 | 2328篇 |
免费 | 33篇 |
国内免费 | 13篇 |
专业分类
化学 | 1409篇 |
晶体学 | 28篇 |
力学 | 83篇 |
数学 | 419篇 |
物理学 | 435篇 |
出版年
2020年 | 18篇 |
2019年 | 17篇 |
2018年 | 18篇 |
2017年 | 21篇 |
2016年 | 32篇 |
2015年 | 28篇 |
2014年 | 24篇 |
2013年 | 110篇 |
2012年 | 85篇 |
2011年 | 93篇 |
2010年 | 56篇 |
2009年 | 64篇 |
2008年 | 100篇 |
2007年 | 109篇 |
2006年 | 100篇 |
2005年 | 103篇 |
2004年 | 108篇 |
2003年 | 78篇 |
2002年 | 74篇 |
2001年 | 48篇 |
2000年 | 45篇 |
1999年 | 55篇 |
1998年 | 28篇 |
1997年 | 32篇 |
1996年 | 32篇 |
1995年 | 23篇 |
1994年 | 25篇 |
1993年 | 22篇 |
1992年 | 32篇 |
1991年 | 20篇 |
1990年 | 25篇 |
1989年 | 36篇 |
1988年 | 14篇 |
1987年 | 24篇 |
1986年 | 29篇 |
1985年 | 44篇 |
1984年 | 50篇 |
1983年 | 36篇 |
1982年 | 38篇 |
1981年 | 58篇 |
1980年 | 37篇 |
1979年 | 43篇 |
1978年 | 38篇 |
1977年 | 44篇 |
1976年 | 25篇 |
1975年 | 33篇 |
1974年 | 35篇 |
1973年 | 37篇 |
1972年 | 26篇 |
1970年 | 18篇 |
排序方式: 共有2374条查询结果,搜索用时 16 毫秒
31.
S. N. Dmitrieva N. I. Sidorenko A. I. Vedernikov L. G. Kuz’mina J. A. K. Howard T. M. Buslaeva S. P. Gromov 《Russian Chemical Bulletin》2007,56(5):993-1002
The condensation reaction of 1,2-bis(2-haloethoxy)-4-nitrobenzenes with acyclic α,ω-(oxa)alkanedithiols in the presence of
alkali metal carbonates produced a series of nitrobenzodithiacrown ethers with macrocycles of different size. The structures
of three ethers were established by X-ray diffraction. A new method was developed for the synthesis of nitrobenzomonothia-15-crown-5
ether. Nitro derivatives of benzodithiacrown ethers were tested as reagents for extraction of palladium(II), platinum(IV), and rhodium(III) from hydrochloric acid solutions. Extraction of PdII salts was found to be highly selective compared to that of PtIV and RhIII salts. Benzodithia-15-crown-5 ether is the most efficient extractant for palladium(II). Reduction of nitrobenzothiacrown ethers with hydrazine hydrate in the presence of a platinum catalyst afforded their amino
derivatives.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 958–966, May, 2007. 相似文献
32.
Laser-induced luminescence in quasi-one-dimensional Ba(Pt, Ni)(CN)4·nH2O crystals has been measured from 5 to 300 K. A well-defined luminescence peak is observed, corresponding to an excited electronic state of platinum and nickel. Energy transfer from this Pt—Ni excited state to Pt(CN)42? clusters is found to be very much dependent on the temperature. 相似文献
33.
Bruce M. Howard William Fenical Edward V. Arnold Jon Clardy 《Tetrahedron letters》1979,20(31):2841-2844
The structure of a unique bromine-containing ketal, obtusin (), has been determined by spectral and X-ray crystallographic techniques. Obtusin is a natural component of the Mediteranean red alga . 相似文献
34.
Howard C. Haas Monis J. Manning Martin H. Mach 《Journal of polymer science. Part A, Polymer chemistry》1970,8(7):1725-1730
Differential thermal analysis has been used to study the fusion of aqueous thermally reversible gels of gelatin and polyacrylylglycinamide (PAG). In the case of gelatin gels, endotherms close to the melting point are readily observed and these are sometimes preceeded by a small exothermic heat of gel reorganization. Calculations are presented to show that breaking of the gelatin gel network requires only a small fraction of the observed endothermic heat of fusion and that most of the heat is required for melting larger crystallites within gelatin aggregates and for perhaps a helix → coil transition. Failure to observe endotherms by DTA over the known temperature range of fusion of PAG gels is consistent with prior measurements and conclusions. The noncrystallinity of PAG gels and soluble aggregates together with a heat of crosslinking of only ?5 to ?10 kcal/mole of crosslinks places the heat of fusion of PAG gels outside the lower limits of DTA sensitivity. 相似文献
35.
FrankH. Allen Raju Mondal NigelA. Pitchford JudithA.K. Howard 《Helvetica chimica acta》2003,86(4):1129-1139
Symmetry‐modified principal‐component analysis has been used to visualise the geometrical distortions of three‐coordinate metal centres observed in crystal structures retrieved from the Cambridge Structural Database. It is shown that compounds of Cu, Ag, Hg, Zn, and Au dominate the dataset, and exhibit distortions away from the trigonal planar archetype towards a) T‐shaped and b) Y‐shaped geometries. A small number of compounds, principally of Cu and Ag, also show distortions towards trigonal pyramidal geometries. The interconversions from Y‐shaped geometries, through the trigonal planar form to T‐shaped geometries are clearly mapped by the PC analysis. For HgII complexes, it is possible to interpret the transition from T‐shaped geometries to the trigonal planar form in terms of a reaction pathway for ligand addition to linear L1? HgII? L2 species. 相似文献
36.
Boyd DR Sharma ND King AW Shepherd SD Allen CC Holt RA Luckarift HR Dalton H 《Organic & biomolecular chemistry》2004,2(4):554-561
Direct and indirect evidence, of unexpected stereoselective reductase-catalysed deoxygenations of sulfoxides, was found. The deoxygenations proceeded simultaneously, with the expected dioxygenase-catalysed asymmetric sulfoxidation of sulfides, during some biotransformations with the aerobic bacterium Pseudomonas putida UV4. Stereoselective reductase-catalysed asymmetric deoxygenation of racemic alkylaryl, dialkyl and phenolic sulfoxides was observed, without evidence of the reverse sulfoxidation reaction, using anaerobic bacterial strains. A purified dimethyl sulfoxide reductase, obtained from the intact cells of the anaerobic bacterium Citrobacter braakii DMSO 11, yielded, from the corresponding racemates, enantiopure alkylaryl sulfoxide and thiosulfinate samples. 相似文献
37.
Tsai P Ichikawa K Mailer C Pou S Halpern HJ Robinson BH Nielsen R Rosen GM 《The Journal of organic chemistry》2003,68(20):7811-7817
Apparent rate constants, at acidic pH and neutral pH for the reaction of a family of ester-containing 5-carboxyl-5-methyl-1-pyrroline N-oxides with superoxide (O2*-) were estimated, using ferricytochrome c as a competitive inhibitor. It was of interest to note that the rate constants were similar among the different nitrones and not that significantly different from that found for 5-(diethoxyphosphoryl)-5-dimethyl-1-pyrroline N-oxide. At acidic pH, the rate constant for spin trapping O2*- was 3-fold greater than that at physiological pH. Subsequent experiments determined the half-life of aminoxyls, derived from the reaction of these nitrones with O2*-. The EPR spectra were modeled by using a global analysis method. The results clearly demonstrated that EPR spectra of all the aminoxyls were inconsistent with a model that included a single gamma-hydrogen splitting. A better interpretation modeled them as two diastereomers with identical nitrogen splittings and slightly different beta-hydrogen splittings. Detailed line width analyses slightly favored an equal line width-unequal population ratio for the two diastereomers. 相似文献
38.
We present the first recording of the high-resolution spectrum of an induced chiral dimer. Three conformers of the induced chiral dimers of ethanol have been observed using a pulsed molecular-beam Fourier transform microwave spectrometer. The rotational constants of the normal isotopomers of the three species have been determined to be (a) A=5113.826(5), B=1329.7214(4), and C=1257.5151(3) MHz, (b) A=5086.459(5), B=1316.6508(4), and C=1243.6329(4) MHz, and (c) A=4851.608(5), B=1369.7558(6), and C=1243.4184(4) MHz. The observed species have been assigned to calculated structures via Kraitchman double substitution analyses and ab initio calculations. The Kraitchman analyses and the fitted centrifugal distortion parameters suggest that the deuterium bond is significantly stronger than the hydrogen bond in the dimers of ethanol. 相似文献
39.
Kinetic control of the stereoselectivity of protonation of enolates and other strongly delocalized anionic species is involved in a large number of organic reactions. Protonation occurring from the less hindered side of the, e.g., enolic system affords the less stable of two diastereomers. However, one apparent discrepancy has been in the synthesis of prostaglandins. The present research deals with the source of this behavior. A curious effect of the substituent at the enolic alpha carbon was uncovered. In certain instances an alpha substituent is forced to twist into a conformation blocking the proton donor from its side, thus reversing the stereochemistry of protonation. In the course of this research, a number of five-ring enols of varying structure were investigated. Finally, the ketonization reaction course has been studied theoretically. 相似文献
40.
Acyclovir was synthesized in five steps from 7-formamido-5-methylthiofurazano[3,4-d]pyrimidine 2 . Alkylation of 2 with 2-(benzoyloxy)ethoxymethyl chloride, followed by reductive cleavage of the furazan ring gave 9-[[2-(benzoyloxy)ethoxy]methyl]-2-(methylthio)adenine 5 . Hydrolysis of the 6-amino group of 5 , followed by amination of 7 with ammonia gave 9-[(2-hydroxyethoxy)methyl]guanine ( 1 , acyclovir). 相似文献