全文获取类型
收费全文 | 6528篇 |
免费 | 307篇 |
国内免费 | 59篇 |
专业分类
化学 | 4646篇 |
晶体学 | 24篇 |
力学 | 158篇 |
数学 | 1079篇 |
物理学 | 987篇 |
出版年
2023年 | 44篇 |
2022年 | 102篇 |
2021年 | 201篇 |
2020年 | 133篇 |
2019年 | 139篇 |
2018年 | 138篇 |
2017年 | 110篇 |
2016年 | 262篇 |
2015年 | 247篇 |
2014年 | 256篇 |
2013年 | 420篇 |
2012年 | 452篇 |
2011年 | 545篇 |
2010年 | 330篇 |
2009年 | 260篇 |
2008年 | 464篇 |
2007年 | 421篇 |
2006年 | 420篇 |
2005年 | 348篇 |
2004年 | 303篇 |
2003年 | 229篇 |
2002年 | 206篇 |
2001年 | 69篇 |
2000年 | 72篇 |
1999年 | 47篇 |
1998年 | 67篇 |
1997年 | 59篇 |
1996年 | 67篇 |
1995年 | 39篇 |
1994年 | 35篇 |
1993年 | 36篇 |
1992年 | 42篇 |
1991年 | 35篇 |
1990年 | 21篇 |
1989年 | 14篇 |
1988年 | 11篇 |
1987年 | 14篇 |
1986年 | 18篇 |
1985年 | 24篇 |
1984年 | 34篇 |
1983年 | 18篇 |
1982年 | 18篇 |
1981年 | 22篇 |
1980年 | 17篇 |
1979年 | 13篇 |
1978年 | 17篇 |
1977年 | 15篇 |
1976年 | 5篇 |
1973年 | 9篇 |
1970年 | 4篇 |
排序方式: 共有6894条查询结果,搜索用时 46 毫秒
131.
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
132.
Mezzache S Afonso C Pepe C Karoyan P Fournier F Tabet JC 《Rapid communications in mass spectrometry : RCM》2003,17(14):1626-1632
The proton affinities of proline, cis-3-methylproline and cis-3-ethylproline have been measured by the kinetic method using an ion trap instrument; the values obtained are 936, 940.5, and 943 kJ mol(-1), respectively. The experimental values are consistent with those obtained by high-level ab initio calculations (B3LYP/6-31+G*//B3LYP/6-31G* and B3P86/6-31+G*//B3LYP/6-31G*). Several conformations of neutral and protonated proline were considered, in particular the endo and exo ring structure and the position of the carboxyl group. These results show the importance of the position of the hydrogen atom of the carboxyl group in determining the most stable protonated proline structure. 相似文献
133.
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
134.
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
135.
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
136.
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
137.
Eleonora Freire Sergio Baggio Juan Carlos Muoz Ricardo Baggio 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(8):m455-m458
The structure of bis(1,10‐phenanthroline‐κ2N,N′)(thiosulfato‐κ2O:S)manganese(II) methanol solvate, [Mn(S2O3)(C12H8N2)2]·CH3OH, is made up of Mn2+ centers coordinated to two bidentate phenanthroline (phen) groups and an S,O‐chelating thiosulfate anion, forming monomeric entities. The structure of catena‐poly[[diaqua(2,9‐dimethyl‐1,10‐phenanthroline‐κ2N,N′)manganese(II)]‐μ‐thiosulfato‐κ2O:S], [Mn(S2O3)(C14H12N2)(H2O)2]n, is polymeric, consisting of Mn(dmph)(H2O)2 units (dmph is 2,9‐dimethyl‐1,10‐phenanthroline) linked by thiosulfate anions acting in an S,O‐chelating manner. 相似文献
138.
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
139.
Eleonora Freire Sergio Baggio Juan Carlos Muoz Ricardo Baggio 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(4):m221-m224
Two new nickel nitrates, diaquabis(3,4,7,8‐tetramethyl‐1,10‐phenanthroline‐κ2N,N′)nickel(II) dinitrate methanol solvate, [Ni(C16H16N2)2(H2O)2](NO3)2·CH4O, (I), and triaqua[2,4,6‐tris(2‐pyridyl)‐1,3,5‐triazine‐κ3N1,N2,N6]nickel(II) dinitrate trihydrate, [Ni(C18H12N6)(H2O)3](NO3)2·3H2O, (II), are reported. In both structures, the cation is octahedrally coordinated, to two bidentate 3,4,7,8‐tetramethyl‐1,10‐phenanthroline (tmp) and two water molecules in (I), and to one tridentate 2,4,6‐tris(2‐pyridyl)‐1,3,5‐triazine (tpt) and three water molecules in (II). Both structures are stabilized by extensive hydrogen‐bonding interactions. 相似文献
140.
Orte A Talavera EM Maçanita AL Orte JC Alvarez-Pez JM 《The journal of physical chemistry. A》2005,109(39):8705-8718
2',7'-Difluorofluorescein (Oregon Green 488, OG488) is a novel fluorescein dye derivative which presents important advantages for improving the fluorimetric applications in the biomedical and biochemical sciences. In aqueous solution it displays four prototropic forms, namely cation (C), neutral (N), monoanion (M), and dianion (D). In previous works, we found (J. Phys. Chem. A 2005, 109, 734-747, 2840-2846) that OG488 undergoes excited-state proton transfer reactions, which may affect the results from applications using this dye. We established that the excited-state proton transfer (ESPT) reactions between neutral, monoanionic, and dianionic forms of OG488 are promoted by acetate buffer, and we characterized the ground and excited species involved. We also solved the kinetics of the prototropic reactions using global compartmental analysis. In the present paper, we extend our study on the ESPT reactions of OG488 to acidic media, in which only the three prototropic species cation, neutral, and monoanion coexist. We have solved the kinetics of the three-state ESPT reaction by means of global three-compartmental analysis of a fluorescence decay surface in moderately acidic media (pH between 1.1 and 3.0), recovering the kinetic and spectral parameters of this three-state system. This system is one of the most complex solved to date, due to the strong overlap of the absorption and emission spectra of the neutral and monoanionic forms of OG488. We also found that the cation behaves as "super" photoacid, showing a very high deprotonation rate constant (1.04 x 10(11) s(-1)) and an enhanced acidity. Therefore, we also carried out experiments at very high perchloric acid concentrations, dealing with some other effects which become noteworthy at these [H(+)]. The presence of xanthylium cation quenching due to "free" water molecules, and the reduction in the amount of water clusters acting as proton acceptors, are processes which alter notably the time course of the excited-species in this high [H(+)] range. 相似文献