全文获取类型
收费全文 | 6603篇 |
免费 | 315篇 |
国内免费 | 24篇 |
专业分类
化学 | 5369篇 |
晶体学 | 32篇 |
力学 | 81篇 |
数学 | 657篇 |
物理学 | 803篇 |
出版年
2024年 | 2篇 |
2023年 | 58篇 |
2022年 | 157篇 |
2021年 | 291篇 |
2020年 | 201篇 |
2019年 | 177篇 |
2018年 | 137篇 |
2017年 | 133篇 |
2016年 | 260篇 |
2015年 | 256篇 |
2014年 | 278篇 |
2013年 | 429篇 |
2012年 | 509篇 |
2011年 | 554篇 |
2010年 | 377篇 |
2009年 | 340篇 |
2008年 | 452篇 |
2007年 | 443篇 |
2006年 | 365篇 |
2005年 | 338篇 |
2004年 | 259篇 |
2003年 | 200篇 |
2002年 | 196篇 |
2001年 | 64篇 |
2000年 | 40篇 |
1999年 | 59篇 |
1998年 | 39篇 |
1997年 | 44篇 |
1996年 | 38篇 |
1995年 | 37篇 |
1994年 | 18篇 |
1993年 | 20篇 |
1992年 | 17篇 |
1991年 | 17篇 |
1990年 | 17篇 |
1989年 | 12篇 |
1988年 | 6篇 |
1987年 | 13篇 |
1986年 | 10篇 |
1985年 | 15篇 |
1984年 | 17篇 |
1983年 | 10篇 |
1982年 | 9篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1978年 | 7篇 |
1977年 | 2篇 |
1975年 | 3篇 |
1965年 | 2篇 |
1955年 | 1篇 |
排序方式: 共有6942条查询结果,搜索用时 31 毫秒
51.
Sannomiya M Rodrigues CM Coelho RG dos Santos LC Hiruma-Lima CA Souza Brito AR Vilegas W 《Journal of chromatography. A》2004,1035(1):47-51
The methanolic extract of the leaves of the medicinal plant Byrsonima crassa (Malpighiaceae) contain flavonoids with antioxidant activity. They were separated in a preparative scale using high-speed counter-current chromatography. The optimum solvent system used was composed of a mixture of ethyl acetate-n-propanol-water (140:8:80 (v/v/v)) and led to a successful separation between monoglucosilated flavonoids (quercetin-3-O-alpha-L-arabinoside, quercetin-3-O-beta-D-galactoside) and the biflavonoid amentoflavone in only 3.5 h. The purities of quercetin-3-O-alpha-L-arabinoside (95 mg), quercetin-3-O-beta-D-galactoside (16 mg) and the biflavonoid amentoflavone (114 mg) were all isolated at purity over 95%. Identification was performed by 1H NMR, 13C NMR and UV analyses. 相似文献
52.
Silva L Coutinho A Fedorov A Prieto M 《Journal of photochemistry and photobiology. B, Biology》2003,72(1-3):17-26
Nystatin is a polyene antibiotic frequently applied in the treatment of topical fungal infections. In this work, a 7-nitrobenz-2-oxa-1,3-diazole (NBD) hexanoyl amide derivative of nystatin was synthesized and its detailed photophysical characterization is presented. The average conformation of the labelled antibiotic in tetrahydrofuran, ethanol and methanol was determined by intramolecular (tetraene to NBD) fluorescence resonance energy transfer measurements. At variance with the literature [Can. J. Chem. 63 (1985) 77-85], it was concluded that there is no need to invoke a solvent-dependent conformational equilibrium between extended and closed conformers of the antibiotic, because the mean tetraene-to-NBD separating distance was found to remain constant (approximately 18 A) in all the solvents studied. In addition, the large solvent dependence of the fluorescence anisotropy observed for the non-derivatized nystatin, was rationalized on the basis of the prolate ellipsoidal geometry of the molecule. It was concluded that the rod shaped and amphipathic antibiotic remains monomeric in different solvents within the concentration range studied (2-20 microM). 相似文献
53.
Pérez-Juste I Graña AM Carballeira L Mosquera RA 《The Journal of chemical physics》2004,121(21):10447-10455
Linear sulfur-carbon chains C(n)S (n=1-6) of astronomical interest were examined by means of several theoretical methods. The three smallest compounds of the series were chosen to evaluate the performance of several computational models, including Hartree-Fock theory, density functional theory with the Becke's three parameter exchange functional and the correlation functional of Lee, Yang, and Parr (B3LYP), and electron-correlated methods (second-order Moller-Plesset perturbation method (MP2), configuration interaction method including single and double excitations (CISD), and quadratic configuration interaction method including single and double excitations (QCISD) in combination with a large variety of basis sets. The systematic comparison between the experiment and theory indicates that the B3LYP/6-311G** method can be considered suitable for the study of the electronic structures of the C(n)S compounds. The electronic ground states of the C(n)S molecules alternate between 1Sigma and 3Sigma for odd and even values of n, respectively. The B3LYP/6-311G** wave functions for these electronic ground states were analyzed by means of the atoms in molecules (AIM) and natural bond orbital (NBO) methods. Both approaches suggest that the electronic structures for the singlet and triplet compounds must be considered separately. According to the NBO method, singlet compounds can be properly represented by acetylenic structures with alternating single and triple bonds (S[triple bond]C-C[triple bond]C...). However, triplet compounds are better described by means of double bond-double bond cumulenic structures (S=C=C=C=C...) as a consequence of the average between different alpha and beta electronic densities. AIM delocalization indexes and NBO interactions between localized orbitals also indicate that these structures are strongly pi delocalized. Finally, the different singlet and triplet structures proposed provide a consistent explanation for the geometries, dipole moments, and spin-density values of the C(n)S compounds studied. 相似文献
54.
Four new fluorescent macrocyclic ligands derived from biphenyl are described. The new compounds have been used in liquid-liquid extraction experiments and the influence of pH has been studied in those ligands containing carboxylic groups. The results obtained for the latter ligands have been compared with those observed in the presence of an external acid. 相似文献
55.
Ana A. C. Pacheco Arnaldo F. da Silva Filho Kristoffer Kortsen Magnus W. D. Hanson-Heine Vincenzo Taresco Jonathan D. Hirst Muriel Lansalot Franck D'Agosto Steven M. Howdle 《Chemical science》2021,12(3):1016
Reversible addition–fragmentation chain transfer (RAFT) dispersion polymerisation of methyl methacrylate (MMA) is performed in supercritical carbon dioxide (scCO2) with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) present as chain transfer agent (CTA) and surprisingly shows good control over PMMA molecular weight. Kinetic studies of the polymerisation in scCO2 also confirm these data. By contrast, only poor control of MMA polymerisation is obtained in toluene solution, as would be expected for this CTA which is better suited for acrylates. In this regard, we select a range of CTAs and use them to determine the parameters that must be considered for good control in dispersion polymerisation in scCO2. A thorough investigation of the nucleation stage during the dispersion polymerisation reveals an unexpected “in situ two-stage” mechanism that strongly determines how the CTA works. Finally, using a novel computational solvation model, we identify a correlation between polymerisation control and degree of solubility of the CTAs. All of this ultimately gives rise to a simple, elegant and counterintuitive guideline to select the best CTA for RAFT dispersion polymerisation in scCO2.RAFT dispersion polymerisation of methyl methacrylate is performed in scCO2 with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) present as chain transfer agent (CTA) and surprisingly shows good control over PMMA molecular weight. 相似文献
56.
Basurto S García S Neo AG Torroba T Marcos CF Miguel D Barberá J Ros MB de la Fuente MR 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(18):5362-5376
Several new liquid-crystalline indene and pseudoazulene systems are reported. These molecules give rise to either columnar hexagonal mesophases and/or columnar plastic phases. The unique nature of these compounds stems from their non-classical discotic structure. Although the molecules have rigid aromatic cores, they lack terminal tails and instead the polarizable atoms (S, halogens) or polar groups (CN, CO) act as unusual soft parts. On the basis of many structurally related materials, we conclude that for this type of compound molecular stacking in the solid state is a prerequisite for the appearance of a columnar mesophase, although other intermolecular interactions within the layers are also important in establishing liquid-crystalline order. The behavior reported for these mesomorphic molecules opens up new possibilities in the search for related molecular interactions that might be useful for the construction of supramolecular architectures with particular properties. 相似文献
57.
[reaction: see text] A simple and efficient stereoselective synthesis of aziridine-2-phosphonate 3, and -phosphine oxide 5 by diastereoselective addition of Grignard reagents to 2H-azirine phosphonate 1 and -phosphine oxide 4 is reported. Similarly, the addition of heterocyclic amines and benzenethiol to aziridines 1 and 4 yielded functionalized aziridines 10, 11, and 18. These aziridines are used as intermediates for the regioselective synthesis of beta-aminophosphine oxides 6 and beta-aminophosphonates 7, as well as alpha- aminophosphonates 8. Phenylsulfenyl-substituted alpha-aminophosphorus derivatives 15 and 19 are obtained directly from benzenethiol and 2H-azirine phosphonates 1 and -phosphine oxides 4. 相似文献
58.
M. Jesús Rodríguez M. Isabel Fernndez Ana M. Gonzlez‐Noya Marcelino Maneiro Rosa Pedrido Miguel Vzquez Bruno Donnadieu Manuel R. Bermejo 《无机化学与普通化学杂志》2005,631(11):2161-2166
M(HL)(H2O)n complexes have been obtained by the electrochemical reaction of Fe, Co, Ni, Cu, Zn and Cd anodes with the potentially pentadentate and trianionic asymmetrical Schiff base 3‐aza‐N‐{2‐[1‐aza‐2‐(5‐nitro‐2‐hydroxylphenyl)‐vinyl]phenyl}‐4‐(5‐nitro‐2‐hydroxyphenyl)but‐3‐enamide (H3L), containing a hard amido donor atom. The complexes have been characterized by elemental analysis, mass spectrometry, IR and 1H NMR spectroscopies, magnetic measurements and molar conductivities. Co(HL)(H2O) ( 2 ) has been found to rearrange in DMF solution into a crystallographically solved octahedral complex, CoL1(H2O)2 ( 7 ) [where H2L1 is the symmetrical Schiff base ligand N,N′‐(1,2‐phenylene)‐bis(5‐nitro‐3‐hydroxysalicylidenimine)]. A hydrolysis mechanism is discussed to explain this rearrangement. 相似文献
59.
Uranium(VI and IV) and thorium(IV) give cathodic indentations in supporting electrolytes prepared from 0.1M perchloric acid, 0.5 M ammonium thiocyanate and 5·10-3M cupferron (solution A) or from 0.1 M succinic-succinate buffer pH 4, 0.1 M sodium chloride, 10-3M cupferron and 0.05% gelatine (solution B). The uranium indentation on the dE/dt = f(E) curve (Q=0.75 and 0.73) permits its detection at the 3·10-7M level. The thorium indentation (Q=0.78) permits its detection at the 4·10-7M level in solution B. Methods for the elimination of interfering ions for the uranium determination are described. In the determination of thorium, Ga(III), Fe(III), Ti(IV) and U(VI) interfere. 相似文献
60.
Matilde Fondo Ana M. García Deibe Noelia Ocampo Manuel R. Bermejo Jesús Sanmartín 《无机化学与普通化学杂志》2005,631(11):2041-2045
The heptadentate Schiff base H3L reacts with cobalt(II) acetate in methanol to form the discrete dinuclear complex Co2L(OAc)2(OMe)(H2O)2 ( 1 ·2H2O). The reaction of 1 ·2H2O with NMe4OH·5H2O in methanol gives rise to displacement of the acetate by methanolate groups, yielding Co2L(OMe)3(H2O) ( 2 ·1H2O). Recrystallizations of the Schiff base, 1 ·2H2O and 2 ·H2O in different solvents, produce single crystals of H3L, 1 ·2.5H2O and 2 ·2MeOH, respectively. The crystal structures of 1 ·2.5H2O and 2 ·2MeOH show the cobalt atoms double bridged by and endogenous phenol oxygen atom and an exogenous methanolate oxygen donor, giving rise to Co2O2 cores with Co···Co distances of ca. 2.87 Å. 相似文献