全文获取类型
收费全文 | 2153篇 |
免费 | 40篇 |
国内免费 | 6篇 |
专业分类
化学 | 1492篇 |
晶体学 | 18篇 |
力学 | 34篇 |
数学 | 237篇 |
物理学 | 418篇 |
出版年
2022年 | 12篇 |
2021年 | 29篇 |
2020年 | 30篇 |
2019年 | 30篇 |
2018年 | 23篇 |
2017年 | 12篇 |
2016年 | 49篇 |
2015年 | 37篇 |
2014年 | 25篇 |
2013年 | 126篇 |
2012年 | 92篇 |
2011年 | 141篇 |
2010年 | 60篇 |
2009年 | 60篇 |
2008年 | 123篇 |
2007年 | 99篇 |
2006年 | 132篇 |
2005年 | 90篇 |
2004年 | 96篇 |
2003年 | 87篇 |
2002年 | 70篇 |
2001年 | 35篇 |
2000年 | 17篇 |
1998年 | 22篇 |
1997年 | 15篇 |
1996年 | 23篇 |
1995年 | 21篇 |
1994年 | 28篇 |
1993年 | 19篇 |
1992年 | 27篇 |
1991年 | 17篇 |
1990年 | 17篇 |
1989年 | 13篇 |
1988年 | 21篇 |
1987年 | 23篇 |
1986年 | 18篇 |
1985年 | 54篇 |
1984年 | 33篇 |
1983年 | 29篇 |
1982年 | 42篇 |
1981年 | 29篇 |
1980年 | 35篇 |
1979年 | 33篇 |
1978年 | 22篇 |
1977年 | 17篇 |
1976年 | 22篇 |
1975年 | 14篇 |
1974年 | 22篇 |
1973年 | 16篇 |
1972年 | 11篇 |
排序方式: 共有2199条查询结果,搜索用时 468 毫秒
51.
Ralph C. Dougherty Derek Horton Kerstin D. Philips Joseph D. Wander 《Journal of mass spectrometry : JMS》1973,7(7):805-816
2-Acetamido-1,3,4,6-tetra-O-acetyl-2-deoxy-α-D-glucopyranose (I), and its analogs specifically mono (trideuterioacetylated) at O-1 (III), at N-2 (II), at O-4 (IV) and at O-6 (V), have been examined by high-resolution mass spectrometry. From the elemental compositions of the fragment ions, the mass-number shifts resulting from deuterium incorporation and analysis of metastable transitions, it has been possible to specify in detail the fragmentation pathways undergone by this molecule. The principal degradations of I proceed by initial rapid decomposition of the molecular ion (whose intensity is insignificant) by three routes: (i) by loss of the C-1 acetoxyl group as a radical to give the glycosyl cation (a), (ii) by loss of the 1-acetyl group as a radical to give an acyclic ion m/e 346 (b) and (iii) by loss of a C-6 fragment and acetic acid derived from the 3-acetate group to give m/e 241 (c). 相似文献
52.
Critical factors for high-performance physically adsorbed (dynamic) polymeric wall coatings for capillary electrophoresis of DNA 总被引:2,自引:0,他引:2
Doherty EA Berglund KD Buchholz BA Kourkine IV Przybycien TM Tilton RD Barron AE 《Electrophoresis》2002,23(16):2766-2776
Physically adsorbed (dynamic) polymeric wall coatings for microchannel electrophoresis have distinct advantages over covalently linked coatings. In order to determine the critical factors that control the formation of dynamic wall coatings, we have created a set of model polymers and copolymers based on N,N-dimethylacrylamide (DMA) and N,N-diethylacrylamide (DEA), and studied their adsorption behavior from aqueous solution as well as their performance for microchannel electrophoresis of DNA. This study is revealing in terms of the polymer properties that help create an "ideal" wall coating. Our measurements indicate that the chemical nature of the coating polymer strongly impacts its electroosmotic flow (EOF) suppression capabilities. Additionally, we find that a critical polymer chain length is required for polymers of this type to perform effectively as microchannel wall coatings. The effective mobilities of double-stranded (dsDNA) fragments within dynamically coated capillaries were determined in order to correlate polymer hydrophobicity with separation performance. Even for dsDNA, which is not expected to be a strongly adsorbing analyte, wall coating hydrophobicity has a deleterious influence on separation performance. 相似文献
53.
Thomas P.E.Auf Der Heyde Craig S. Green Derek E. Needham David A. Thornton Gareth M. Watkins 《Journal of Molecular Structure》1981
The infrared spectra of the complexes [M(pyO)(H2O)Cl2] (M = Mn, Fe, Co, Ni, Cu; pyO = pyridine N-oxide) have been determined. Assignments of ν M-Opy, νM-OH2 and ν M-Cl are made by observing the effects of deuterating the coordinated pyO and H2O and replacing chloride by bromide in the Mn(II) complex. Assignments of metal—ligand modes in the mixed ligand complexes [M(pyO)(dmso)X2] (dmso = dimethylsulphoxide) are made by comparison with the spectra of (ML2X2] (L = pyO, dmso) and by observing the effects of deuteration of pyO and dmso. Structural aspects of the spectra are discussed. 相似文献
54.
Chen SH Su AC Chang CS Chen HL Ho DL Tsao CS Peng KY Chen SA 《Langmuir : the ACS journal of surfaces and colloids》2004,20(20):8909-8915
Morphological effects in luminescence properties of a representative semiconducting polymer, poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV), has recently attracted much attention. Previous studies indicated that short-term heat treatment of solution-cast MEH-PPV films may result in the formation of mesomorphic order that is responsible for the "red" emission around 640 nm, in contrast to the single-chromophore "yellow" emission near 590 nm from the disordered matrix. On the basis of microscopic and spectroscopic evidence for films cast from freshly prepared and aged solutions, here we show that prolonged storage of MEHyellowPPV solutions at room temperature or lower may result in retardation of the thermally induced mesophase formation in the subsequently cast films. According to small-angle neutron scattering and differential scanning calorimetric observations over aged MEH-PPV/toluene solutions, we propose that the suppressed transformation into mesomorphic order is due to further development of nanocrystalline aggregates that serve as physical cross-links among MEH-PPV chains in the solution state upon long-term storage. These solvent-induced nanocrystalline aggregates, however, do not exhibit new spectroscopic features beyond the suppression of "red" emission at 640 nm from the mesomorphic phase. 相似文献
55.
Jain SL Bhattacharyya P Milton HL Slawin AM Crayston JA Woollins JD 《Dalton transactions (Cambridge, England : 2003)》2004,(6):862-871
Seven new pyridine dicarboxamide ligands H2L(1-7) have been synthesised from condensation reactions involving pyridine-2,6-dicarboxylic acid (H2dipic), pyridine-2,6-dicarbonyl dichloride or 2,6-diaminopyridine with heterocyclic amine or carboxylic acid precursors. Crystallographic analyses of N,N'-bis(2-pyridyl)pyridine-2,6-dicarboxamide monohydrate (H2L8 x H2O), N,N'-bis[2-(2-pyridyl)methyl]pyridine-2,6-dicarboxamide and N,N'-bis[2-(2-pyridyl)ethyl]pyridine-2,6-dicarboxamide monohydrate revealed extensive intramolecular hydrogen bonding interactions. 2,6-Bis(pyrazine-2-carboxamido)pyridine (H2L6) and 2,6-bis(pyridine-2-carboxamido)pyridine (H2L7) reacted with copper(II) acetate monohydrate to give tricopper(II) complexes [Cu3(L)2(mu2-OAc)2]. X-Ray crystallography confirmed deprotonation of the amidic nitrogen atoms and that the (L6,7)2- ligands and acetate anions hold three copper(II) ions in approximately linear fashion. H2L8. Reacted with copper(II) tetrakis(pyridine) perchlorate to give [Cu(L8)(OH2)]2 x 2H2O, in which (L8)2- was tridentate through the nitrogen atoms of the central pyridine ring and the deprotonated carboxamide groups at one copper centre, with one of the terminal pyridyl rings coordinating to the other copper atom in the dimer. The corresponding reaction using H2L7 gave [Cu3(L7)2(py)2][ClO4]2, which transformed during an attempted recrystallisation from ethanol under aerobic conditions to a tetracopper(II) complex [Cu4(L7)2(L7-O)2]. 相似文献
56.
The crystal and molecular structures of three sydnone derivatives are reported. The compound 3-cyclohexylsydnone crystallizes in space group C2/c of the monoclinic system with sixteen molecules in a cell of dimensions a = 19.326 (3), b = 9.471 (2), c = 20.005 (4)Å, β = 106.85(1)°. The structure has been refined to a final value of 0.0581 for the conventional R-factor based on 2222 independent observed intensities. Form I of 3-(3-pyridyl)sydnone crystallizes in space group P2/n of the monoclinic system with eight molecules in a cell of dimensions a = 7.317(2), b = 9.283 (2), c = 20.891 (6) Å, β = 99.61(2)°. The structure has been refined to a final value of 0.0514 for the conventional R-factor based on 1208 independent observed intensities. Form II of 3-(3-pyridyl)sydnone crystallizes in space group P21/c of the monoclinic system with eight molecules in a cell of dimensions a=9.073 (2), b = 22.267 (5). c = 7.494(2)Å, β = 112.15 (2)°. The structure has been refined to a final value of 0.0462 for the conventional R-factor based on 1330 independent observed intensities. Each of the three structures contains two crystallographically independent molecules in the cell. In the case of 3-cyclohexylsydnone, one of the independent molecules exhibits disorder around the exocyclic bond at N(3). A comparison of bond lengths indicates that the (electron donating) cyclohexyl group brings about enhanced electron density in the N(3)-C(4) bond, and possibly in the N(3)-N(2) bond. All three structures studied here exhibit intermolecular hydrogen bonding involving C(4)-H(4)…O(6) interactions. Although there are no stacking interactions in the cyclohexyl derivative, there is evidence for such interactions in the 3-pyridyl derivatives. 相似文献
57.
The ligands [Ph2P(O)NP(E)Ph2]− (E=S I; E=Se II) can readily be complexed to a range of palladium(II) starting materials affording new six-membered Pd–O–P–N–P–E palladacycles. Hence ligand substitution reaction of the chloride complexes [PdCl2(bipy)] (bipy=2,2′-bipyridine), [{Pd(μ-Cl)(L–L)}2] (HL–L=C9H13N or C12H13N), [{Pd(μ-Cl)Cl(PMe2Ph)}2] or [PdCl2(PR3)2] [PR3=PPh3; 2PR3=Ph2PCH2CH2PPh2or cis-Ph2PCH=CHPPh2] with either I (or II) in thf or CH3OH gave [Pd{Ph2P(O)NP(E)Ph2-O,E}(bipy)]PF6, [Pd{Ph2P(O)NP(E)Ph2-O,E}(L–L)], [Pd{Ph2P(O)NP(E)Ph2-O,E}Cl(PMe2Ph)] or [Pd{Ph2P(O)NP(E)Ph2-O,E} (PR3)2]PF6 in good yields. All compounds described have been characterised by a combination of multinuclear NMR [31 P{1 H} and 1 H] and IR spectroscopy and microanalysis. The molecular structures of five complexes containing the selenium ligand II have been determined by single-crystal X-ray crystallography. Three different ring conformations were observed, a pseudo-butterfly, hinge and in the case of all three PR3 complexes, pseudo-boat conformations. Within the Pd–O–P–N–P–Se rings there is evidence for π-electron delocalisation. 相似文献
58.
The reactivity of W(NPh)(o-(Me3SiN)2C6H4)(py)2 and W(NPh)(o-(Me3SiN)2C6H4)(pic)2 (py=pyridine; pic=4-picoline) with unsaturated substrates has been investigated. Treatment of W(NPh)(o-(Me3SiN)2C6H4)(py)2 with diphenylacetylene or 2,3-dimethyl-1,3-butadiene generates W(NPh)(o-(Me3SiN)2C6H4)(eta2-PhCCPh) and W(NPh)(o-(Me3SiN)2C6H4)(eta4-CH2=C(Me)C(Me)=CH2), respectively, while the addition of ethylene to W(NPh)(o-(Me3SiN)2C6H4)(py)2 generates the known metallacycle W(NPh)(o-(Me3SiN)2C6H4)(CH2CH2CH2CH2). The addition of 2 equiv of acetone to W(NPh)(o-(Me3SiN)2C6H4)(pic)2 provides the azaoxymetallacycle W(NPh)(o-(Me3SiN)2C6H4)(OCH(Me)2)(OC(Me)2-o-C5H3N-p-Me), the result of acetone insertion into the ortho C-H bond of picoline. Similarily, the addition of 2 equiv of RC(O)H [R=Ph, tBu] to W(NPh)(o-(Me3SiN)2C6H4)(py)2 generates W(NPh)(o-(Me3SiN)2C6H4)(OCH2R)(OCHR-o-C5H4N) [R=Ph, tBu,]. In contrast, reaction between W(NPh)(o-(Me3SiN)2C6H4)(py)2 and 2-pyridine carboxaldehyde yields the diolate W(NPh)(o-(Me3SiN)2C6H4)(OCH(C5H4N)CH(C5H4N)O). The synthesis of W(NPh)(o-(Me3SiN)2C6H4)(PMe3)(py)(eta2-OC(H)C6H4-p-Me), formed by the addition of p-tolualdehyde to a mixture of W(NPh)(o-(Me3SiN)2C6H4)(py)2 and PMe3, suggests that an eta2-aldehyde intermediate is involved in the formation of the azaoxymetallacycle, while the isolation of W(NPh)(o-(Me3SiN)2C6H4)(Cl)(OC(Me)(CMe3)-o-C5H4N), formed by the reaction of pinacolone with W(NPh)(o-(Me3SiN)2C6H4)(py)2, in the presence of adventitious CH2Cl2, suggests that the reaction proceeds via the hydride W(NPh)(o-(Me3SiN)2C6H4)(H)(OC(Me)(CMe3)-o-C5H4N). 相似文献
59.
Aragoni MC Arca M Devillanova FA Isaia F Lippolis V Mancini A Pala L Slawin AM Woollins JD 《Inorganic chemistry》2005,44(26):9610-9612
We report the first examples of metal dithiolenes belonging to the class [M(R-dmet)(2)] [R-dmet = formally monoreduced N-substituted thiazolidine-2,4,5-trithione; R = Et, M = Ni (1), Pd (2), Pt (3)]. A comparative spectroscopic, electrochemical, and density functional theory theoretical investigation indicates that [M(R-dmet)(2)] complexes show features intermediate between those of the dithiolenes belonging to the previously reported classes [M(R,R'-timdt)(2)] and [M(dmit)(2)] (R,R'-timdt = formally monoreduced N,N'-disubstituted imidazolidine-2,4,5-trithione; dmit = 2-thioxo-1,3-dithiole-4,5-dithiolato). UV-vis-near-IR spectroscopy and cyclic voltammetry/differential pulsed voltammetry measurements performed on 1 and 3 proved that the new dithiolenes are stable as neutral, monoanionic, and bianionic species and feature a near-IR electrochromic absorption falling at about 1000 and 1250 nm for neutral and monoanionic species, respectively. 相似文献
60.