全文获取类型
收费全文 | 6231篇 |
免费 | 511篇 |
国内免费 | 31篇 |
专业分类
化学 | 4881篇 |
晶体学 | 28篇 |
力学 | 160篇 |
综合类 | 1篇 |
数学 | 1023篇 |
物理学 | 680篇 |
出版年
2022年 | 25篇 |
2021年 | 52篇 |
2020年 | 64篇 |
2019年 | 109篇 |
2018年 | 65篇 |
2017年 | 63篇 |
2016年 | 211篇 |
2015年 | 236篇 |
2014年 | 231篇 |
2013年 | 385篇 |
2012年 | 301篇 |
2011年 | 318篇 |
2010年 | 283篇 |
2009年 | 286篇 |
2008年 | 324篇 |
2007年 | 275篇 |
2006年 | 259篇 |
2005年 | 233篇 |
2004年 | 271篇 |
2003年 | 216篇 |
2002年 | 277篇 |
2001年 | 153篇 |
2000年 | 143篇 |
1999年 | 71篇 |
1998年 | 65篇 |
1997年 | 77篇 |
1996年 | 87篇 |
1995年 | 63篇 |
1994年 | 73篇 |
1993年 | 71篇 |
1992年 | 67篇 |
1991年 | 42篇 |
1990年 | 46篇 |
1989年 | 47篇 |
1988年 | 43篇 |
1987年 | 41篇 |
1986年 | 39篇 |
1985年 | 50篇 |
1984年 | 65篇 |
1983年 | 57篇 |
1982年 | 71篇 |
1981年 | 72篇 |
1980年 | 74篇 |
1979年 | 82篇 |
1978年 | 78篇 |
1977年 | 84篇 |
1976年 | 77篇 |
1975年 | 74篇 |
1974年 | 75篇 |
1973年 | 53篇 |
排序方式: 共有6773条查询结果,搜索用时 31 毫秒
161.
Néouze MA Le Bideau J Leroux F Vioux A 《Chemical communications (Cambridge, England)》2005,(8):1082-1084
The confinement of ionic liquids within a porous silica matrix was performed by a one-step non-hydrolytic sol-gel route, leading to hybrid materials (called "ionogels") featuring both the mechanical and transparency properties of silica gels and the high ionic conductivity and thermal stability of ionic liquids. 相似文献
162.
Charmantray F El Blidi L Gefflaut T Hecquet L Bolte J Lemaire M 《The Journal of organic chemistry》2004,69(26):9310-9312
Dihydroxyacetone phosphate (DHAP) was synthesized in high purity and yield in four steps starting from dihydroxyacetone dimer (DHA) (47% overall yield). DHA was converted into 2,2-dimethoxypropane-1,3-diol, which was desymmetrized by acetylation with lipase AK. The alcohol function was phosphorylated to give dibenzyl phosphate ester 4. From 4, two routes were investigated for large-scale synthesis of DHAP. First, acetate hydrolysis was performed prior to hydrogenolysis of the phosphate protective groups. The acetal hydrolysis was finally catalyzed by the phosphate group itself. Second, acetate and acetal hydrolysis were performed in one single step after hydrogenolysis. 相似文献
163.
Lee LF Adronov A Schaller RD Fréchet JM Saykally RJ 《Journal of the American Chemical Society》2003,125(2):536-540
Near-field scanning optical microscopy (NSOM) has been used to investigate the photophysical characteristics of first- to fourth-generation (G1 to G4) light-harvesting dendrimer thin films containing coumarin-343 and coumarin-2 as the core and peripheral chromophores, respectively. Thin film photoluminescence (PL) spectra exhibit a significant red shift in the lower generations (G1, G2, and G3) as compared to their respective solution PL spectra, implying the formation of excimers. Spatially resolved PL NSOM images exhibit pronounced nanoscopic domains in G1, which become more homogeneous in higher generations due to site-isolation of the core chromophore. G4 exhibits complete site-isolation for these light-harvesting dendrimer films. 相似文献
164.
The stereochemistry of the reaction of silver benzoates on chiral di-O-aroyl-1,2 iododeoxy-3-sn-glycerols has been reinvestigated. The reaction proceeds either with or without neighbouring group participation, depending on : - possible electrophilic assistance by the silver ion to the ionisation of the substrate (occuring only with dissociated silver salts), - ability of substituants R1 and R2 on the substrates to stabilize a positive charge. Rearrangements are at minimum with R1 and R2 electronreleasing groups in the substrate, silver salts un-or weakly dissociated and bearing a good nucleophile. For one case, a regio- and stereo specific reaction is observed. 相似文献
165.
Broussy S Bernardes-Génisson V Gornitzka H Bernadou J Meunier B 《Organic & biomolecular chemistry》2005,3(4):666-669
An ortho-metallation-electrophilic substitution sequence was employed as a key step to build the 4-benzoylpyridine framework. It was found that 4-benzoylpyridine-3-carboxamide and an N-pyridyl alkylated derivative both exist in a unique cyclized hemiamidal structure, not in the usually expected keto-amide open form. These structures represent fragment models of the Isoniazid-NAD adducts involved in the mechanism of action of the antituberculous drug Isoniazid. 相似文献
166.
Jean Michel Brunel 《Tetrahedron》2007,63(18):3899-3906
A new and efficient mild Pd/P(t-Bu)3 catalyst for selective reduction of various alkenes under transfer hydrogen conditions has been developed leading to the corresponding saturated derivatives in chemical yields varying from 65 to 98%. Mechanistic rationale of this reaction has been also demonstrated. 相似文献
167.
According to the subsequent treatment, azepines , , or 1,2-dihydro derivatives and are obtained in the photolysis in basic medium of β-anthracenic azides and and provide strong evidence for the intermediate formation of bicyclic aziridines. 相似文献
168.
Lucas S Ferry D Demirdjian B Suzanne J 《The journal of physical chemistry. B》2005,109(38):18103-18106
We present an experimental work devoted to study of the thermodynamical properties of solid methanol. We combine Fourier transform infrared spectroscopy (FTIR) and mass spectrometry (MS) to measure, for the first time, the vapor pressure of various methanol solid phases and determine their Clausius-Clapeyron equations. We perform our experiments between T = 130 K and the triple point temperature T(t) = 175.61 K. When methanol is condensed from its vapor below T(t), we observe three different solid phases depending on temperature. A condensation at T = 130 K forms a metastable phase with an enthalpy of sublimation deltaH(metastable-vapor) = 42.9 +/- 0.5 kJ.mol(-1). Upon heating, this phase transforms itself at T approximately 145 K to the alpha-phase that has an enthalpy of sublimation deltaH(alpha-vapor) = 46.9 +/- 0.2 kJ.mol(-1). Cooling the alpha-phase does not lead back to the metastable phase, whereas heating this alpha-phase leads to the beta-phase occurrence at T(alpha-beta) = 157.36 K. This latter one is stable until T(t) and has an enthalpy of sublimation deltaH(beta-vapor) = 44.2 +/- 0.5 kJ.mol(-1). 相似文献
169.
Rondeau D Martineau C Blanchard P Roncali J 《Journal of mass spectrometry : JMS》2002,37(10):1081-1085
Oligothienylenevinylenes/C(60) dyads n-C and triads n-C(2) are studied by electrospray mass spectrometry. A clear correlation is observed between the nature of the charged species detected by mass spectrometry, i.e. protonated molecule [M + H], (+) cation radical M(+.) and dication M(++), and the oxidation potentials of the molecules. Moreover, under defined solubility conditions, mass spectrometry provides conclusive evidences for the reversible dimerization of cation-radicals of n-C(2) compounds. 相似文献
170.
Natural Occurrence of C(15)-Epimeric Coleons C and D and its Significance to the Stereochemistry of the Formation of a Spirocyclopropanring In a previous paper some evidence was presented for a predominant occurrence of epimeric coleons ((15S)-coleon C ((15 S)- 1a ), and (15S)-coleon D ((15S)- 2a )) in Plectranthus lanuginosus [1], The proposed structures have now been proven by careful comparison of their physical data with those of the already described C(15)-epimers whose structures were deduced by X-ray analysis of a derived cis-diketone 3 [5], Examination of the epimeric purity of coleons possessing a 2-hydroxy-1-methylethyl side-chain (coleons C , D , H , I , I ′ and derivatives) isolated from seven different species (Coleus, Plectranthus and Solenostemon) has shown that these coleons always occur as a mixture of C(15)-epimers, although in most cases with one predominant epimer. The consequences for the stereochemistry of the postulated in vivo formation of the methyl-substituted spirocyclopropane substructure is discussed. 相似文献