全文获取类型
收费全文 | 6396篇 |
免费 | 511篇 |
国内免费 | 32篇 |
专业分类
化学 | 4993篇 |
晶体学 | 32篇 |
力学 | 159篇 |
综合类 | 1篇 |
数学 | 1043篇 |
物理学 | 711篇 |
出版年
2022年 | 38篇 |
2021年 | 54篇 |
2020年 | 66篇 |
2019年 | 114篇 |
2018年 | 67篇 |
2017年 | 63篇 |
2016年 | 212篇 |
2015年 | 238篇 |
2014年 | 233篇 |
2013年 | 394篇 |
2012年 | 306篇 |
2011年 | 328篇 |
2010年 | 292篇 |
2009年 | 289篇 |
2008年 | 330篇 |
2007年 | 282篇 |
2006年 | 265篇 |
2005年 | 236篇 |
2004年 | 274篇 |
2003年 | 228篇 |
2002年 | 282篇 |
2001年 | 154篇 |
2000年 | 145篇 |
1999年 | 72篇 |
1998年 | 67篇 |
1997年 | 80篇 |
1996年 | 90篇 |
1995年 | 64篇 |
1994年 | 73篇 |
1993年 | 72篇 |
1992年 | 69篇 |
1991年 | 42篇 |
1990年 | 48篇 |
1989年 | 49篇 |
1988年 | 42篇 |
1987年 | 41篇 |
1986年 | 39篇 |
1985年 | 49篇 |
1984年 | 70篇 |
1983年 | 57篇 |
1982年 | 72篇 |
1981年 | 75篇 |
1980年 | 76篇 |
1979年 | 83篇 |
1978年 | 79篇 |
1977年 | 87篇 |
1976年 | 77篇 |
1975年 | 76篇 |
1974年 | 78篇 |
1973年 | 54篇 |
排序方式: 共有6939条查询结果,搜索用时 15 毫秒
131.
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
132.
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
133.
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
134.
Jacqueline Ficini Jean dAngelo Arnaud Eman Anne Marie Touzin 《Tetrahedron letters》1976,17(9):683-686
135.
136.
Enrico Borgarello Jean Desilvestro Michael Grtzel Ezio Pelizzetti 《Helvetica chimica acta》1983,66(6):1827-1834
Conduction band electrons produced by band gap excitation of TiO2-particles reduce efficiently thiosulfate to sulfide and sulfite. \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm 2e}_{{\rm cb}}^ - ({\rm TiO}_{\rm 2}) + {\rm S}_{\rm 2} {\rm O}_3^{2 - } \longrightarrow {\rm S}^{2 - } + {\rm SO}_3^{2 - } $\end{document} This reaction is confirmed by electrochemical investigations with polycrystalline TiO2-electrodes. The valence band process in alkaline TiO2-dispersions involves oxidation of S2O to tetrathionate which quantitatively dismutates into sulfite and thiosulfate, the net reaction being: \documentclass{article}\pagestyle{empty}\begin{document}$ 2{\rm h}^{\rm + } ({\rm TiO}_{\rm 2}) + 0.5{\rm S}_{\rm 2} {\rm O}_{\rm 3}^{{\rm 2} - } + 1.5{\rm H}_{\rm 2} {\rm O} \longrightarrow {\rm SO}_3^{2 - } + 3{\rm H}^{\rm + } $\end{document} This photodriven disproportionation of thiosulfate into sulfide and sulfite: \documentclass{article}\pagestyle{empty}\begin{document}$ 1.5{\rm H}_{\rm 2} {\rm O } + 1.5{\rm S}_{\rm 2} {\rm O}_{\rm 3}^{{\rm 2} - } \mathop \to \limits^{h\nu} 2{\rm SO}_3^{2 - } + {\rm S}^{{\rm 2} - } + 3{\rm H}^{\rm + } $\end{document} should be of great interest for systems that photochemically split hydrogen sulfide into hydrogen and sulfur. 相似文献
137.
Turbiez M Frère P Allain M Videlot C Ackermann J Roncali J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(12):3742-3752
Hybrid oligothiophenes based on a various combinations of thiophene and 3,4-ethylenedioxythiophene (EDOT) groups have been synthesized. UV/Vis absorption spectra show that the number and relative positions of the EDOT groups considerably affect the width of the HOMO-LUMO gap and the rigidity of the conjugated system. Analysis of the crystallographic structure of two hybrid quaterthiophenes confirms that insertion of two adjacent EDOT units in the middle of the molecule leads to a self-rigidification of the conjugated systems by intramolecular SO interactions. Cyclic voltammetry data shows that the first oxidation potential of the oligomers decreases with increasing chain length and increasing number of EDOT groups for a given chain length. Electrochemical studies and theoretical calculations show that the positions of the EDOT units in the conjugated chain control the potential difference (DeltaE(p)) between the first and second oxidation steps. Moving the EDOT groups from the outer to the inner positions of the conjugated system increases DeltaE(p). Theoretical calculations confirm that this phenomenon reflects an increase of the intramolecular coulombic repulsion between positive charges in the dication. A thin-film field-effect transistor was fabricated by vacuum sublimation of a pentamer with alternating thiophene-EDOT structure, and the hole mobility was determined. 相似文献
138.
When α,β-unsaturated γ-dimethoxymethyl cyclohexenones are excited to the S2(π,π*) state, certain unimolecular reactions can be observed to compete with S2 → S1 internal conversion. These reactions do not occur from the S1(n,π*) or the lowest T(π,π* and n,π*) states. They comprise the radical elimination of the formylacetal substituent (cf. 8 , 9 → 32 + 33 ), γ → α formylacetal migration (cf. 6 → 27 , 8 → 30 , 9 → 34 , 12 → 37 ), and a cyclization process involving the transfer of a methoxyl hydrogen to the α carbon and ring closure at the β position (cf. 6 → 28 , 8 → 31 , 12 → 38 , 20 → 40 + 41 ). The quantum yield of the ring closure 20a → 40a + 41a is 0.016 at ≤ 0.05M concentration. It is independent of the excitation wavelength within the π→π* absorption band (238–254 nm), but Φ ( 40a + 41a ) decreases at higher concentrations. According to the experimental data the reactive species of these specifically π→π*-induced transformations is placed energetically higher than the S1(n,π*) state, and it is either identical with the thermally equilibrated S2(n,π*) state, or reached via this latter state. The linear dienone 14 undergoes a similar π→π*-induced cyclization (→ 42 ) whereas the benzohomologue 26 proved unreactive, and the dienone 22 at both n → π and π→π* excitation only gives rise to rearrangements generally characteristic of cross-conjugated cyclohexadienones. 相似文献
139.
Geometry optimization of the cis and the trans isomers of several octahedral dioxo complexes of d(2) electronic configuration are performed using the gradient-corrected density functional theory (B3LYP and, for some key structures, BP86). With only monodentate sigma donor ligands (ReO(2)(NH(3))(4)(+), 7), the usual energy order is found (i.e., the trans isomer is the most stable). Complexes with a chelating bidentate ligand, OsO(2)(OCH(2)CH(2)O)(NH(3))(2) (10) and ReO(2)(HN=CHCH=NH)(NH(3))(2)(+) (11), are used as models for the experimental complexes 5 and 2 in which the arrangement of the O=M=O unit is trans and cis, respectively. Our calculations actually show an inversion of the relative energy of the two isomers in going from 10 to 11: while the trans isomer is found to be the most stable in 10, the unusual cis diamagnetic isomer is favored by about 29 kcal mol(-)(1) in 11. This result is traced to the geometric and electronic properties of the bidentate ligand, in particular an acute bite angle and good pi acceptor character. In complex 14 with a bipyridine chelating ligand (weaker pi acceptor than diaza-1,4-butadiene in 11), this energy difference is, however, reduced to 7.5 kcal mol(-)(1) (partial geometry optimization). 相似文献
140.
Roncali J 《Chemical Society reviews》2005,34(6):483-495
This tutorial review covers recent contributions in the area of linear pi-conjugated systems bound to fullerenes in view of their application as active materials in photovoltaic devices. The first part discusses the concepts of double-cable polymer and molecular hetero-junction and presents several examples of chemically or electrochemically synthesized C60-derivatized conjugated polymers. The second and main part of the article concerns the various classes of C60-derivatized pi-conjugated oligomers designed in view of their utilization in single-component photovoltaic devices. Thus, C60-containing pi-conjugated systems such as oligoarylenevinylenes, oligoaryleneethynylenes and oligothiophenes are discussed on the basis of the relationships between molecular structure, photophysical properties and performances of the derived photovoltaic devices. A brief last section presents some recent examples of surface-attached molecular hetero-junctions based on self-assembled monolayers and discusses possible routes for future research. 相似文献