全文获取类型
收费全文 | 14544篇 |
免费 | 455篇 |
国内免费 | 322篇 |
专业分类
化学 | 9089篇 |
晶体学 | 100篇 |
力学 | 405篇 |
综合类 | 30篇 |
数学 | 1886篇 |
物理学 | 3811篇 |
出版年
2022年 | 157篇 |
2021年 | 184篇 |
2020年 | 175篇 |
2019年 | 176篇 |
2018年 | 170篇 |
2017年 | 139篇 |
2016年 | 328篇 |
2015年 | 298篇 |
2014年 | 364篇 |
2013年 | 777篇 |
2012年 | 622篇 |
2011年 | 718篇 |
2010年 | 515篇 |
2009年 | 493篇 |
2008年 | 660篇 |
2007年 | 699篇 |
2006年 | 562篇 |
2005年 | 579篇 |
2004年 | 577篇 |
2003年 | 577篇 |
2002年 | 611篇 |
2001年 | 440篇 |
2000年 | 398篇 |
1999年 | 307篇 |
1998年 | 223篇 |
1997年 | 203篇 |
1996年 | 221篇 |
1995年 | 236篇 |
1994年 | 227篇 |
1993年 | 177篇 |
1992年 | 168篇 |
1991年 | 175篇 |
1990年 | 191篇 |
1989年 | 154篇 |
1988年 | 138篇 |
1987年 | 135篇 |
1986年 | 123篇 |
1985年 | 216篇 |
1984年 | 233篇 |
1983年 | 159篇 |
1982年 | 178篇 |
1981年 | 147篇 |
1980年 | 151篇 |
1979年 | 139篇 |
1978年 | 115篇 |
1977年 | 162篇 |
1976年 | 142篇 |
1975年 | 118篇 |
1974年 | 110篇 |
1973年 | 114篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
A method for the quantification of clindamycin in animal plasma using high-performance liquid chromatography combined with electrospray ionization mass spectrometry (LC/ESI-MS/MS) is presented. Lincomycin is used as the internal standard. The sample preparation includes a simple deproteinization step with trichloroacetic acid. Chromatographic separation is achieved on an RP-18 Hypersil column using gradient elution with 0.01 M ammonium acetate and acetonitrile as mobile phase. Good linearity was observed in the range 0-10 microg ml(-1). The limit of quantification of the method is 50 ng ml(-1) and the limit of detection is 1.3 ng ml(-1). The method was shown out to be of use for pharmacokinetic studies of clindamycin formulations in dogs. 相似文献
242.
Ugo Caruso Antonella De Maria Barbara Panunzi Antonio Roviello 《Journal of polymer science. Part A, Polymer chemistry》2002,40(17):2987-2993
Two formulas of grafted polymers with metal‐containing chromophores, potentially suitable for second‐order nonlinear optics applications, are described. Two chromophores were obtained from a tridentate ligand coordinated to Cu(II) or Pd(II) ions. The organometallic chromophore fragments were grafted to poly(4‐vinylpyridine) by the pyridinic nitrogen of the host polymer. Some qualities displayed by the new metallated polymers are remarkable: (1) a high value of the first hyperpolarizability coefficient of the chromophores, (2) a high content of the grafted chromophore in the polymers (up to 60 wt %), (3) a considerable increase in the glass‐transition temperatures (up to 240 °C), (4) good thermal stability in air (ca. 280 °C), and (5) good optical transparency of the films. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2987–2993, 2002 相似文献
243.
G. J. De Jong U. A. Th. Brinkman 《Journal of Radioanalytical and Nuclear Chemistry》1977,35(1):223-232
For the system liquid anion-exchanger—Cr(III)−NCS−, an investigation has been made of the dependence of the percentage extraction of Cr(III) on parameters such as standing
time of the Cr(III)−NCS− solution, temperature, pH and type of exchanger. Quantitative extraction of e.g. 4·10−4 M Cr(III) by 0.1M Aliquat in CCl4 is easily achieved at room temperature, using 4.75M KNCS−0.05N HCl as aqueous phase. At high Cr(III) concentrations, the
complex anion present in the organic phase is Cr(NCS)
6
3−
; when working with dilute metal ion solutions, the species extracted is Cr(NCS)4 (H2O)
2
−
. Separations of mixtures containing 10−2−10−4 M Co(II), Ni(II) and Cr(III) have successfully been accomplished. 相似文献
244.
C.P. Boekel A. Jelsma J.H. Teuben H.J. De Liefde Meijer 《Journal of organometallic chemistry》1977,136(2):211-218
The thermolysis of compounds of the type Cp2VR (R = aryl) in the solid state has been studied. A distinct increase in thermal stability is observed upon substitution of the ortho-position of the aryl group. Thermal decomposition occurs with formation of RH, Cp2 V, a vanadocene homologue with the group R substituted in one of the Cp rings and, probably, a vanadocene homologue with two substituted Cp rings. It is shown that the abstraction of the hydrogen atom from the cyclopentadienyl ring, necessary for the formation of RH, is an intermolecular process, whereas the substitution of the aryl group in the Cp ring is intramolecular. A decomposition mechanism is proposed in which the group R is transferred from the vanadium atom to the C5H5 ring of the same molecule by interaction with an aryl group of another molecule. The thermal decomposition of Cp2VR is compared with that of the analogous titanium compounds. 相似文献
245.
The presence of micelles from cationic and zwitterionic surfactants increases the apparent acidity of either the keto and the enol forms of 2-phenylacetylfuran (2PAF) and 2-phenylacetylthiophene (2PAT). This effect can be attributed to the affinity of the surfactant micelles for the enolate of the two substrates. Although the equilibrium constants for keto-enol tautomerism of 2PAF and 2PAT, KT=[enol]/[ketone]=pKaKH−pKaEH, do not change much, the presence of micelles provides an efficient method for producing appreciable quantities of the enolates under mild experimental conditions and in aqueous solutions. The obtained rate-profiles for the ketonisation reactions and the consistency of the kinetic rate constants over a wide range of ‘pH’ in several overlapping buffers indicate that the pH of the aqueous pseudophase (but not that at the micellar surface) can be controlled by buffers. Moreover, the increase of the acidity and the decrease of the ‘water’ rate of ketonisation of the enols of 2PAF and 2PAT upon addition of surfactants allow the uncovery of a metal ion catalysed pathway that cannot be observed in absence of surfactants. 相似文献
246.
247.
Volonterio A Bellosta S Bravin F Bellucci MC Bruché L Colombo G Malpezzi L Mazzini S Meille SV Meli M Ramírez De Arellano C Zanda M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(18):4510-4522
Partially modified retro- (PMR) and retro-inverso (PMRI) psi[NHCH(CF(3))]Gly peptides, a conceptually new class of peptidomimetics, have been synthesized in wide structural diversity and variable length by aza-Michael reaction of enantiomerically pure alpha-amino esters and peptides with enantiomerically and geometrically pure N-4,4,4-trifluorocrotonoyl-oxazolidin-2-ones. The factors underlying the observed moderate to good diastereocontrol have been investigated. The conformations of model PMR-psi[NHCH(CF(3))]Gly tripeptides have been studied in solution by (1)H NMR spectroscopy supported by MD calculations, as well as in the solid-state by X-ray diffraction. Remarkable stability of turn-like conformations, comparable to that of parent malonyl-based retropeptides, was evidenced, as a likely consequence of two main factors: 1) severe torsional restrictions about sp(3) bonds in the [CO-CH(2)-CH(CF(3))-NH-CH(R)-CO] module, which is biased by the stereoelectronically demanding CF(3) group and the R side chain; 2) formation of nine-membered intramolecularly hydrogen-bonded rings, which have been clearly detected both in CHCl(3) solution and in some crystal structures. The former factor seems to be more important, as turn-like conformations were found in the solid-state even in the absence of intramolecular hydrogen bonding. The relative configuration of the -C*H(CF(3))NHC*H(R)- stereogenic centers has a major effect on the stability of the turn-like conformation, which seems to require a syn stereochemistry. X-ray diffraction and ab initio computational studies showed that the [-CH(CF(3))NH-] group can be seen as a sort of hybrid between a peptide bond mimic and a proteolytic transition state analogue, as it combines some of the properties of a peptidyl -CONH- group (low NH basicity, CH(CF(3))-NH-CH backbone angle close to 120 degrees, C-CF(3) bond substantially isopolar with the C=O) with some others of the tetrahedral intermediate [-C(OX)(O(-))NH-] involved in the protease-mediated hydrolysis reaction of a peptide bond (high electron density on the CF(3) group, tetrahedral backbone carbon). 相似文献
248.
J. Fernndez Rico M. Paniagua J. M. GarcíA De La Vega J. I. Fernndez-alonso P. Fantucci 《Journal of computational chemistry》1986,7(2):201-207
A direct minimization method previously presented by the authors is applied here to biconfigurational wave functions. A very moderate increasing in the time by iteration with respect to the one-determinant calculation and good convergence properties have been found. So qualitatively correct studies on singlet systems with strong biradical character can be performed with a cost similar to that required by Hartree-Fock calculations. 相似文献
249.
The reactions of 9-methyl-, 9-ethyl- and 9-propyl-anthracene with CF3CO2[2H] in C[2H]Cl3 and with FSO3[2H] in SO2ClF have been investigated. Using 4 equivalents of CF3CO2[2H] at 50° 1H-2H exchange was observed only for the 10-H and the side-chain α-hydrogens, and on using 8 and 12 equivalents at 50° also for the aromatic α-hydrogens. Treatment of the substrates with FSO3[2H] at -60° leads to the stable 9-alkyl-[10-2H]-10-anthracenium ions. On warming up to ?25° a slow 1H-2H exchange of only the 10-1H of these anthracenium ions is observed. A mechanism for the 1H-2H exchange of the aromatic and the side-chain α-hydrogens of the 9-alkylanthracenes is presented. 相似文献
250.
Umemura Y Yamagishi A Schoonheydt R Persoons A De Schryver F 《Journal of the American Chemical Society》2002,124(6):992-997
Mono- and multilayers of amphiphilic [Ru(phen)(2)(dcC12bpy)](2+) (phen = 1,10-phenanthroline, dcC12bpy = 4,4'-caboxyl-2,2'-bipyridyl didodecyl ester) hybridized with a clay mineral have been prepared by a modified Langmuir-Blodgett method, and their structures and properties have been investigated. Formation of a hybrid monolayer of the Ru(II) complex cations and the clay platelets at an air-clay suspension interface was confirmed by surface pressure-molecular area (pi-A) isotherm measurement and atomic force microscopic (AFM) observation. Multilayers were fabricated by depositing the hybrid monolayers onto glass substrates. The absorbance at 492 nm due to the Ru(II) complex cation in the multilayer increased linearly with the increase in the layer number, indicating layer-by-layer deposition of the hybrid monolayers. Because no increase in the second-harmonic generation (SHG) signal from the multilayers against the layer number was observed, the orientation of the Ru(II) complex cations in the layer would be disturbed. The hydrophilic surface of the transferred hybrid monolayer can be converted to a hydrophobic surface by dipping it in an aqueous solution of octadecylammonium chloride (ODAH(+)Cl(-)). The multilayers modified with ODAH(+) showed a quadratic relation between the SHG intensity and the layer number. This means that the Ru(II) complex cations in the multilayer are successfully oriented in a noncentrosymmetric way by the conversion of the surface property. Both a racemic mixture and an enantiomer of the Ru(II) complex cations were employed to examine the chiral effect on the film properties. The chiral contribution to the SHG signal was enhanced in the multilayer modified with ODAH(+). 相似文献