全文获取类型
收费全文 | 7764篇 |
免费 | 172篇 |
国内免费 | 24篇 |
专业分类
化学 | 5855篇 |
晶体学 | 45篇 |
力学 | 70篇 |
数学 | 1115篇 |
物理学 | 875篇 |
出版年
2020年 | 77篇 |
2019年 | 76篇 |
2016年 | 151篇 |
2015年 | 146篇 |
2014年 | 118篇 |
2013年 | 262篇 |
2012年 | 249篇 |
2011年 | 326篇 |
2010年 | 187篇 |
2009年 | 196篇 |
2008年 | 295篇 |
2007年 | 302篇 |
2006年 | 276篇 |
2005年 | 310篇 |
2004年 | 289篇 |
2003年 | 234篇 |
2002年 | 202篇 |
2001年 | 124篇 |
2000年 | 104篇 |
1999年 | 110篇 |
1998年 | 111篇 |
1997年 | 131篇 |
1996年 | 129篇 |
1995年 | 124篇 |
1994年 | 113篇 |
1993年 | 123篇 |
1992年 | 111篇 |
1991年 | 112篇 |
1990年 | 85篇 |
1989年 | 110篇 |
1988年 | 119篇 |
1987年 | 107篇 |
1986年 | 113篇 |
1985年 | 138篇 |
1984年 | 107篇 |
1983年 | 109篇 |
1982年 | 112篇 |
1981年 | 100篇 |
1980年 | 107篇 |
1979年 | 96篇 |
1978年 | 88篇 |
1977年 | 85篇 |
1976年 | 82篇 |
1974年 | 95篇 |
1973年 | 86篇 |
1971年 | 75篇 |
1961年 | 141篇 |
1960年 | 195篇 |
1959年 | 102篇 |
1958年 | 115篇 |
排序方式: 共有7960条查询结果,搜索用时 31 毫秒
151.
By means of metal-atom ligand-vapor cocondensation (CC) hexa(eta1-arsenine)molybdenum (11) and hexa(eta1-arsenine)tungsten (12) have been prepared; in the molybdenum case, the sandwich complex bis(eta6-arsenine)molybdenum (10) has been isolated as the primary product. The structure of 12 follows from a single-crystal X-ray diffraction study. Based on the results of CC synthesis, chromium binds to arsenine in the eta6-mode exclusively, molybdenum features both options eta1 and eta6, and for tungsten only eta1 coordination is observed. 相似文献
152.
Tang T Qu J Müllen K Webber SE 《Langmuir : the ACS journal of surfaces and colloids》2006,22(18):7610-7616
Recently the synthesis of water-soluble and fluorescent perylene diimides has been reported (Müllen, K.; et al. Angew. Chem., Int. Ed. 2004, 43, 1528; Chem.-Eur. J. 2004, 10, 5297). We have characterized the photophysics of two of these compounds (anionic n-PDI, CAS Reg. No. 694438-88-5. and cationic p-PDI, CAS Reg. No. 817207-4-7) in pure water, dimethyl sulfoxide (DMSO), and aqueous NaCl. These studies, supported by molecular dynamics simulations, have led to the conclusion that these compounds form weakly interacting aggregated species in pure water. n-PDI and p-PDI have been incorporated in polyelectrolyte films of poly(styrene sulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDAC) following the layer-by-layer (LBL) methodology. The optical density and fluorescence intensity of the PDI-LBL films grew linearly with the number of layers, and the PDI was not extracted by subsequent polyelectrolyte deposition. The PDI fluorescence quantum yield was substantially diminished in these films, which we interpret as a self-quenching effect, enhanced by inter- and intralayer energy transfer. Energy-transfer studies to the incorporated cationic dye Brilliant Green (BG) has demonstrated that the BG resides in the same PSS-rich region as p-PDI and is largely excluded from the region that contains n-PDI (PDAC-rich). 相似文献
153.
Reinforced self-assembly of hexa-peri-hexabenzocoronenes by hydrogen bonds: from microscopic aggregates to macroscopic fluorescent organogels 总被引:1,自引:0,他引:1
Dou X Pisula W Wu J Bodwell GJ Müllen K 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(1):240-249
Hexa-peri-hexabenzocoronene derivatives (HBCs) that have hydrogen-bonding functionalities (either amido or ureido groups) adjacent to the aromatic cores have been synthesized to study the effects of intracolumnar hydrogen bonds on the self-assembly behavior of HBCs. The hydrogen bonds effectively increased the aggregation tendency of these compounds in solution. In the bulk state, the typical columnar supramolecular arrangement of HBCs was either stabilized substantially (1 a, 1 b, 2 a, and 2 b), or suppressed by dominant hydrogen-bonding interactions (3). For some of the compounds (1 a, 2 a, and 2 b), the supramolecular arrangement adopted in the liquid-crystalline state was even retained after annealing, presumably owing to the reinforcement of the pi-stacking interactions by the hydrogen bonds. Additionally, the combined effect of the hydrogen bonds and pi-stacking of the aromatic moieties led to the formation of fluorescent organogels, whereby some derivatives were further investigated as novel low molecular-mass organic gelators (LMOGs). 相似文献
154.
Suri JT Mitsumori S Albertshofer K Tanaka F Barbas CF 《The Journal of organic chemistry》2006,71(10):3822-3828
Dihydroxyacetone variants have been explored as donors in organocatalytic aldol reactions with various aldehyde and ketone acceptors. The protected form of dihydroxyacetone that was chosen for in-depth study was 2,2-dimethyl-1,3-dioxan-5-one, 1. Among the catalysts surveyed here, proline proved to be superior in terms of yield and stereoselectivities in the construction of various carbohydrate scaffolds. In a fashion analogous to aldolase enzymes, the de novo preparation of L-ribulose, L-lyxose, D-ribose, D-tagatose, 1-amino-1-deoxy-D-lyxitol, and other carbohydrates was accomplished via the use of 1 and proline. In reactions using 2,2-dimethyl-1,3-dioxan-5-one 1 as a donor, (S)-proline can be used as a functional mimic of tagatose aldolase, whereas (R)-proline can be regarded as an organocatalytic mimic of fuculose aldolase. 相似文献
155.
Naka Seidel Klaus Jacob Piero Zanello Marco Fontani 《Journal of organometallic chemistry》2001,620(1-2)
The reaction of {C,N-[Fe(η5-C5H5)(η5-C5H3(CH2NMe2)-2)]}Li, (FcN)Li, with zinc chloride affords the diorganozinc complex (FcN)2Zn (1). In solution, 1 appears as a mixture of rac and meso diastereomers, whereas in the solid state it crystallizes solely as a rac diastereomer. The ratio of rac/meso diastereomers in solution is solvent-, temperature- and concentration-dependent, consistent with an intermolecular exchange between diastereomers. An intramolecular dynamic phenomenon involving dissociation and recoordination of Zn---N bonds was also observed. The reaction of 1 with zinc chloride yields the monoorganozinc compound (FcN)ZnCl (2) as a slightly soluble yellow microcrystalline powder. 相似文献
156.
Two routes for the introduction of highly fluorescent peryleneimide chromophores into the scaffolding of polyphenylene dendrimers via iterative Diels-Alder cycloadditions are presented. The key intermediates for the divergent dendrimer buildup were two cyclopentadienone branching units carrying two peryleneimides and two masked terminal alkynes. The difference between the two reagents is the mode of incorporation of the chromophores. In the first case, the chromophores were attached to the alpha-position of the tetraphenylcyclopentadienones. In the second case, peryleneimides are used as a "spacer" in the beta-position of the cyclopentadienones giving rise to dendrimers with extended molecular diameters (up to 12 nm) and 24 chromophores within their scaffold. Absorption and emission characteristics of the new multichromophoric nanoparticles were investigated and compared to those of the parent dyes. Additionally, an asymmetrically substituted first-generation dendrimer with six perylene diimide chromophores and one ester functionality is reported. The ester serves as a potential anchor group, and this nanoemitter paves the way to a multichromophoric fluorescence label. All dendrimers have good solubility in common organic solvents, high fluorescence quantum yields, and defined distances between the chromophores, making them attractive candidates for single-molecule spectroscopy. 相似文献
157.
Renate Borowski Dieter Rehder Klaus von Deuten 《Journal of organometallic chemistry》1981,220(1):45-54
Photo-reaction between the ligands Ph2ECH2EPh2 (E = P: dppm, E = As: dpam, E = Sb: dpsm), L, and the vanadium complexes η5-C5H5V(CO)4 and [Et4N][V(CO)6] yields monosubstituted mononuclear (dpsm) and dinuclear, ligand-bridged complexes (dpam, dpsm). With dppm, the final products are disubstituted chelate complexes, but monosubstituted mono- and dinuclear species are formed as intermediates.The shielding of the 51V nucleus decreases in the series dpsm > dppm > dpam and {M(CO)n} > {M(CO)n?1} L > {M(CO)n?1}2μ-L > {M(CO)n?2}dppm ({M(CO)n}[V(CO)6]?, η5-C5H5V(CO)4). The half-widths of the NMR signals are greater for dinuclear than for mononuclear complexes.The crystal and molecular structures of η5-C5H5V(CO)3As2Ph4 have been determined. The compound crystallizes in the space group P21/c with a = 1347.8, b = 1020.0, c = 2085.2 pm and β = 82.3°. Due to steric crowding, the 51V shielding is low composed to that of {η5-C5H5V(CO)3}2μ-dpam. 相似文献
158.
159.
160.
Biesemans M Martins JC Jurkschat K Pieper N Seemeyer S Willem R 《Magnetic resonance in chemistry : MRC》2004,42(9):776-780
It is demonstrated that long-range nJ(1H,117Sn) coupling constants down to 0.3 Hz, can be accurately quantified from non-linear fitting of the sine modulation of the associated 1H-117Sn correlation cross-peak intensities sampled as a function of the heteronuclear antiphase coherence preparation time in the 1H-117Sn J-HMQC pulse sequence. The contribution of additional, undesired modulations is illustrated and assessed using the product operator formalism, and is traced back to contributions that arise from miss-setting of the wandering 180 degrees pulse angle in the constant time period. The power of the method and its use in the characterization of weak intramolecular donor-acceptor interactions are illustrated by the determination of long-range nJ(1H,117Sn) coupling constants of bis[3-(dimethylamino)propyl]tin derivatives, [Me2N(CH2)3]2SnR2 (', R = Me; 3, R = Ph; 4, R = t-Bu). By comparing these with the values found for the corresponding bis(4-methylpentyl)tin derivatives, [Me2CH(CH2)3]2SnR2 (2', R = Me; 3', R = Ph), which lack such interactions, the use of long-range coupling constants to detect intramolecular donor-acceptor interactions is evaluated. It is concluded that nJ(1H,117Sn) couplings up to six bonds through an organic carbon chain can be quantified, whether donor-acceptor interactions are present or not. Furthermore, evidence is presented that, when two scalar coupling pathways co-exist, the pathway involving an intramolecular donor-acceptor interaction can have opposite sign, thus decreasing the overall coupling constant to a value smaller than that actually measured in the absence of a donor-acceptor interaction, where only one coupling pathway is active. There is nevertheless clear numerical value discrimination in the series of compounds investigated between long-range couplings in derivatives without weak intramolecular donor-acceptor interactions and those where such interactions can exist. 相似文献