全文获取类型
收费全文 | 6556篇 |
免费 | 291篇 |
国内免费 | 60篇 |
专业分类
化学 | 4677篇 |
晶体学 | 22篇 |
力学 | 155篇 |
数学 | 1082篇 |
物理学 | 971篇 |
出版年
2024年 | 7篇 |
2023年 | 45篇 |
2022年 | 149篇 |
2021年 | 205篇 |
2020年 | 135篇 |
2019年 | 143篇 |
2018年 | 134篇 |
2017年 | 111篇 |
2016年 | 254篇 |
2015年 | 244篇 |
2014年 | 255篇 |
2013年 | 424篇 |
2012年 | 449篇 |
2011年 | 542篇 |
2010年 | 330篇 |
2009年 | 261篇 |
2008年 | 457篇 |
2007年 | 417篇 |
2006年 | 418篇 |
2005年 | 349篇 |
2004年 | 298篇 |
2003年 | 224篇 |
2002年 | 205篇 |
2001年 | 65篇 |
2000年 | 69篇 |
1999年 | 44篇 |
1998年 | 68篇 |
1997年 | 56篇 |
1996年 | 62篇 |
1995年 | 36篇 |
1994年 | 34篇 |
1993年 | 37篇 |
1992年 | 39篇 |
1991年 | 32篇 |
1990年 | 21篇 |
1989年 | 15篇 |
1988年 | 10篇 |
1987年 | 13篇 |
1986年 | 18篇 |
1985年 | 24篇 |
1984年 | 31篇 |
1983年 | 18篇 |
1982年 | 21篇 |
1981年 | 22篇 |
1980年 | 17篇 |
1979年 | 14篇 |
1978年 | 23篇 |
1977年 | 21篇 |
1976年 | 8篇 |
1973年 | 9篇 |
排序方式: 共有6907条查询结果,搜索用时 15 毫秒
101.
The reaction of NaEt3BH with Nb2(-SMe2)3Cl6 results in the transfer of a hydride ion to dimethylthioether with concomitant production of methane. Further reaction with potassium di-p-tolylformamidinate, KDTolF, yields Nb2(-SMe)2(-DTolF)22-DTolF:)2.2 toluene, 1. In the latter, two thiomethoxide ions and two DTolF groups bridge the trivalent niobium atoms. Each of the other two DTolF groups chelate a metal atom to give the molecule an edge-sharing bioctahedral structure, The niobium-niobium distance of 2.655(2) A is consistent with the presence of a double bond between the metal atoms. 相似文献
102.
The physical content of the three more natural models ofGL(4) gravity is analyzed, for the case of weak fields. The first model we deal with is the linearized version of Yang's onetensor-field gravity. It is shown that this is a scalar-tensor theory, with its scalar part contained in the symmetric tensorh
, instead of appearing explicitly, externally to the symmetric tensorh
, as happens in Brans-Dicke type of scalar-tensor theories. The second and the third linearized models, which can both be derived from the fourth-order action postulated by Yang, turn out to be two-tensor decoupled systems. In both cases one of the tensors is the symmetric weak metric gravity tensor field. The second tensor appearing in these two models, representing theGL(4)-gauge field, is either a linearized symmetric affinity (in the second model) or a linearized but nonsymmetric affinity (for the third model). It is shown that in these last two cases the affinity contains a helicity-3 propagating field. The connection is also given between the fourth-order system which determines the dynamical structure (for the last two models) of the metric tensor and the third-order Yang model of gravity. Owing to the presence of helicity-3 fields we show that it is better to regard Yang's action as an action for a two-tensor system instead of trying to recover from it a pure gravity (one-tensor-field) action. Finally, it is shown what is the dynamical structure of the second and third linearized two-tensor models which can be derived from Yang's action.On leave of absence from the Universidad Simón Bolívar. 相似文献
103.
Hassan Acherki Carlos Alvarez-Ibarra Juan F. Collados Lujn María L. Quiroga-Feijo 《Tetrahedron: Asymmetry》2005,16(24):4034-4044
The base-mediated reaction of enantiomerically pure -sulfinylketimine (+)-1 with (E)-,β-disubstituted propenoate esters afforded 3,4-disubstituted-5-(p-tolylsulfinyl)-5,6-dehydropiperidin-2-ones 9-13 and 14 with high or complete diastereoselectivity. A sole diastereomer of the four possible ones, with regard to the nature of ester, was isolated, which revealed the stereocontrol of the chiral sulfinyl group in the Michael reaction and transenolization steps. In addition, the enantioselective synthesis of ethyl (+)-(3S,4aS,7aS)-1-oxo-octahydro-1H-cyclopenta[c]pyridine-3-carboxylates (+)-17 is described (five steps; 47% yield; ee 97%). The absolute configuration of stereocentres introduced in (+)-17 was assigned on the basis of 1H NMR data. 相似文献
104.
Maurício Cavicchioli Antonio Carlos Massabni Luciana Rebelo Guilherme Eduardo Ernesto Castellano Armando Paduan-Filho Ana Maria da Costa Ferreira 《Transition Metal Chemistry》2007,32(3):355-361
Structural, electrochemical and spectroscopic data of a new dinuclear copper(II) complex with (±)-2-(p-methoxyphenoxy)propionic acid are reported. The complex {tetra-μ-[(±)-2-(p-methoxyphenoxy)propionato-O,O′]-bis(aqua)dicopper(II)} crystallizes in the monoclinic system, space group P21/n with a = 14.149(1) ?, b = 7.495(1) ?, c = 19.827(1) ?, β = 90.62(1) and Z = 4. X-ray diffraction data show that the two copper(II) ions are held together through four carboxylate bridges, coordinated
as equatorial ligands in square pyramidal geometry. The coordination sphere around each copper ion is completed by two water
molecules as axial ligands. Thermogravimetric data are consistent with such results. The ligand has an “L” type shape due
to the angle formed by the β-carbon of the propionic chain and the linked p-methoxyphenoxy group. This conformation contributes to the occurrence of a peculiar structure of the complex. The complex
retains its dinuclear nature when dissolved in acetonitrile, but it decomposes into the corresponding mononuclear species
if dissolved in ethanol, according to the EPR measurements. Further, cyclic voltammograms of the complex in acetonitrile show
that the dinuclear species maintains the same structure, in agreement with the EPR data in this solvent. The voltammogram
shows two irreversible reduction waves at E
pc = −0.73 and −1.04 V vs. Ag/AgCl assigned to the Cu(II)/Cu(I) and Cu(I)/Cu° redox couples, respectively, and two successive oxidation waves at E
pa =− 0.01 and +1.41 V vs. Ag/AgCl, assigned to the Cu°/Cu(I) and Cu(I)/Cu(II) redox couples, respectively, in addition to the oxidation waves of the
carboxylate ligand. 相似文献
105.
The effect of adding linear polymers to a novel reversible electrophoretic was measured. Reversible gels are formed using the polyanionic carbohydrate polymer, gellan gum. Gellan gum forms strong stable gels in the presence of divalent cations or diamines. The gels are reversible (return to solution) by changing the ionic environment or pH. Gellan gum is an anionic polymer, and the electrophoresis gels have considerable electroosmotic flow (EOF) toward the negative electrode. We measured the EOF in gellan gum electrophoresis gels as a function of gel concentration, buffer composition, and linear polymer additive. The linear polymers used in this study were polyethylene oxide and hydroxyethyl cellulose. Both polymers reduced EOF in the gels, in a manner dependent on molecular weight. Polymers with high molecular weight were more effective at reducing EOF. The addition of polymers increased the resolution of low molecular weight DNA. Native gellan gum resolved DNA from approx 50,000 to 1000 bp. Addition of the polymers resolved DNA down to approx 50 bp, in some instances. The influence of the polymers on circular plasmid DNA was also investigated. Addition of high molecular weight polyethylene oxide reduced the electrophoretic mobility of the nicked circular form compared to the supercoiled form. 相似文献
106.
Román LU Guerra-Ramírez D Morán G Martínez I Hernández JD Cerda-García-Rojas CM Torres-Valencia JM Joseph-Nathan P 《Organic letters》2004,6(2):173-176
[structure: see text] The triterpenes 8,14-seco-oleana-8(26),13-dien-3beta-ol (1) and its acetyl derivative 2 were isolated from Stevia viscida and Stevia eupatoria, respectively. Their structures were elucidated by 2D NMR, including carbon-carbon connectivity experiments, and confirmed by X-ray diffraction analysis of ketone 3. The absolute configuration was determined by NMR analysis of the Mosher esters of 1. The biogenetic implications of the new substances are discussed. 相似文献
107.
New physically adsorbed polymer coating for reproducible separations of basic and acidic proteins by capillary electrophoresis 总被引:1,自引:0,他引:1
In this work, a new physically adsorbed coating for capillary electrophoresis (CE) is presented. The coating is based on a N,N-dimethylacrylamide-ethylpyrrolidine methacrylate (DMA-EPyM) copolymer synthesized in our laboratory. The capillary coating is simple and easy to obtain as only requires flushing the capillary with a polymer aqueous solution for 2 min. It is shown that by using these coated capillaries the electrostatic adsorption of a group of basic proteins onto the capillary wall is significantly reduced allowing their analysis by CE. Moreover, the DMA-EPyM coating provides reproducible separations of the basic proteins with RSD values for migration times lower than 0.75% for the same day (n = 5) and lower than 3.90% for three different days (n = 15). Interestingly, the electrical charge of the coated capillary wall can be modulated by varying the pH of the running buffer which makes possible the analysis of basic and acidic proteins in the same capillary. The usefulness of this coating is further demonstrated via the reproducible separation of whey (i.e. acidic) proteins from raw milk. The coating protocol should be compatible with both CE in microchips and CE-MS of different types of proteins. 相似文献
108.
Araceli Vernica Flores Carlos Alberto Prez Marco Aurlio Zezzi Arruda 《Analytica chimica acta》2005,530(2):619-305
In the present work, scandium was determined in sediment slurry samples (from three different rivers) by electrothermal atomic absorption spectrometry (ETAAS). Slurries were prepared by weighting 100 mg of dry sediment samples (≤53 μm particle sizes) and adding 6 ml of HCl:HNO3:HF (3:1:2, v/v). Accurate results were only possible due to the synergetic effect between Rh as permanent chemical modifier and acetylacetone (Acac) as complexing agent. The same platform was used for 400 heating cycles. The performance of the chemical modification was evaluated by using scanning electron microscopy (SEM), synchrotron radiation X-ray fluorescence (SRXRF) and some figures of merit (precision and detectability). The best analytical conditions were attained using 1500 and 2550 °C as pyrolysis and atomization temperatures. The scandium content in the liquid phase of the slurries ranged from 61 to 73%, thus indicating, in this study, that both liquid and solid phases play an important role in slurry analyses. An amount of 5.0–20.0 μg l−1 Sc linear range as well as LOD and LOQ of 0.19 and 0.62 μg l−1, respectively, were obtained under these conditions. The accuracy was checked by using microwave-assisted decomposition, and the results compared to those obtained with the proposed methodology (slurry analysis). By checking both sets of the results, there is no statistical difference at the 95% confidence levels. 相似文献
109.
Cheng X Kelso C Hornak V de los Santos C Grollman AP Simmerling C 《Journal of the American Chemical Society》2005,127(40):13906-13918
The process by which DNA repair enzymes recognize and selectively excise damaged bases in duplex DNA is fundamental to our mechanistic understanding of these critical biological reactions. 8-Oxoguanine (8-oxoG) is the most common form of oxidative DNA damage; unrepaired, this lesion generates a G:C-->T:A mutation. Central to the recognition and repair of DNA damage is base extrusion, a process in which the damaged base lesion or, in some cases, its partner disengages from the helix and is bound to the enzyme's active site where base excision takes place. The conformation adopted by 8-oxoG in duplex DNA is affected by the base positioned opposite this lesion; conformational changes may also take place when the damaged base binds to its cognate repair enzyme. We performed unrestrained molecular dynamics simulations for several 13-mer DNA duplexes. Oligomers containing G:C and 8oxoG:C pairs adopted Watson-Crick geometries in stable B-form duplexes; 8oxoG showed increased local and global flexibility and a reduced barrier to base extrusion. Duplexes containing the G:A mismatch showed much larger structural fluctuations and failed to adopt a well-defined structure. For the 8oxoG:A mismatch that is recognized by the DNA glycosylase MutY, the damaged nucleoside underwent spontaneous and reproducible anti-->syn transitions. The syn conformation is thermodynamically preferred. Steric hindrance and unfavorable electrostatics associated with the 8oxoG O8 atom in the anti conformation were the major driving forces for this transition. Transition events follow two qualitatively different pathways. The overall anti-->syn transition rate and relative probability of the two transition paths were dependent on local sequence context. These simulations indicate that both the dynamic and equilibrium behavior of the duplex change as a result of oxidation; these differences may provide valuable new insight into the selective action of enzymes on damaged DNA. 相似文献
110.
Merlin Rosales Janeth Navarro Ligbel Sánchez Angel González Ysaías Alvarado Raúl Rubio Carlos De La Cruz Tamara Rajmankina 《Transition Metal Chemistry》1996,21(1):11-15
Summary The complex [RuH(CO)(NCMe)2(PPh3)2]BF4 (1) is an efficient and regioselective catalyst precursor for the hydrogenation of polyaromatic nitrogen compounds such as quinoline (Q), isoquinoline (iQ), indole (ln), 5,6- and 7,8-benzoquinoline (BQ) and acridine (A) under relatively mild reaction conditions (125 °C, 4 atm H2). The order of individual initial rates was: A > Q > 5,6-BQ > 7,8-BQ > ln > iQ, reflecting both steric and electronic effects. For the regioselective homogeneous hydrogenation of A to 9,10-dihydroacridine (DHA) catalysed by complex (1), a kinetic study was carried out; the experimentally determined rate law was r = k
1 [Ru] [H2]. These findings are consistent with a mechanism involving the hydrogenation of [RuH(CO)(A)(NCMe)(PPh3)2]BF4 to yield DHA and the unsaturated species [RuH(CO)(NCMe)(PPh3)2]BF4 in the rate-determining step. 相似文献