全文获取类型
收费全文 | 1661篇 |
免费 | 73篇 |
国内免费 | 17篇 |
专业分类
化学 | 1182篇 |
晶体学 | 17篇 |
力学 | 96篇 |
数学 | 188篇 |
物理学 | 268篇 |
出版年
2023年 | 24篇 |
2022年 | 79篇 |
2021年 | 81篇 |
2020年 | 55篇 |
2019年 | 84篇 |
2018年 | 74篇 |
2017年 | 55篇 |
2016年 | 93篇 |
2015年 | 58篇 |
2014年 | 91篇 |
2013年 | 156篇 |
2012年 | 113篇 |
2011年 | 105篇 |
2010年 | 73篇 |
2009年 | 73篇 |
2008年 | 74篇 |
2007年 | 58篇 |
2006年 | 48篇 |
2005年 | 36篇 |
2004年 | 25篇 |
2003年 | 28篇 |
2002年 | 28篇 |
2001年 | 11篇 |
2000年 | 15篇 |
1999年 | 7篇 |
1998年 | 17篇 |
1997年 | 12篇 |
1996年 | 8篇 |
1995年 | 6篇 |
1994年 | 13篇 |
1993年 | 8篇 |
1992年 | 5篇 |
1991年 | 6篇 |
1990年 | 10篇 |
1989年 | 13篇 |
1988年 | 12篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 10篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 11篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1968年 | 2篇 |
1967年 | 3篇 |
排序方式: 共有1751条查询结果,搜索用时 13 毫秒
21.
Aloui M Chambers DJ Cumpstey I Fairbanks AJ Redgrave AJ Seward CM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(11):2608-2621
The technique of intramolecular aglycon delivery (IAD), whereby a glycosyl acceptor is temporarily appended to a hydroxyl group of a glycosyl donor is an attractive method that can allow the synthesis of 1,2-cis glycosides in an entirely stereoselective fashion. 2-O-Allyl protected thioglycoside donors are excellent substrates for IAD, and may be glycosylated stereoselectively through a three-step reaction sequence. This sequence consists of quantitative yielding allyl bond isomerisation, to produce vinyl ethers that can then undergo N-iodosuccinimide mediated tethering of the desired glycosyl acceptor, and subsequent intramolecular glycosylation, to yield either alpha-glucosides or beta-mannosides accordingly. Although attempted one-pot tethering and glycosylation is hampered by competitive intermolecular reaction with excess glycosyl acceptor, this problem can be simply overcome by the use of excess glycosyl donor. Allyl mediated IAD is a widely applicable practical alternative to other IAD approaches for the synthesis of beta-mannosides, that is equally applicable for alpha-gluco linkages. It is advantageous in terms of both simplicity of application and yield, and in addition has no requirement for cyclic 4,6-protection of the glycosyl donor. 相似文献
22.
Mohamed R. Mahmoud Hussein S. Elkashef Maher M. Ahmed 《Monatshefte für Chemie / Chemical Monthly》1980,111(5):1213-1224
The electronic absorption spectra of some 2-styryl-4-phenyl-thiazole ethiodides are studied in organic solvents of different polarities. The shorter wavelength band appearing in the visible region is assigned to an intramolecular charge transfer (CT)-transition originating from the phenyl moiety to the positively charged hetero ring, while the longer wavelength one is due to an intermolecular CT-transition from the iodide ion to the 2-styryl-4-phenyl-thiazolinium cation. These assignments are based on the nature of the aldehydic residue and effects of solvent, concentration, and temperature on both the position and absorptivity of the CT complex-band. It is concluded that the CT complex formed will be highly solvated inDMF, DMSO, ethanol and methanol relative to in CHCl3, dioxane and acetone. The formation constant of the CT complex in solutions of different polarities is determined at different temperatures. Furthermore, the thermodynamic parameters H
o, G
o and S
o for complex formation are calculated and discussed.
Absorptionsspektren von 2-Styryl-4-phenyl-thiazol-ethiodiden in verschiedenen Lösungsmitteln und Bestimmung der Bildungskonstanten der Charge-Transfer-Komplexe
Zusammenfassung Die Elektronenanregungsspektren einiger substituierter 2-Styryl-4-phenyl-thiazol-ethiodide wurden in einigen Lösungsmitteln unterschiedlicher Polarität untersucht. Die Absorption bei kürzerer Wellenlänge wird einem intramolekularen Charge-Transfer (CT)-Übergang zugeordnet, die langwellige Bande einem intermolekularen CT-Übergang (Jodid—organ. Kation). Die Diskussion erfolgt basierend auf Substitutions-, Lösungsmittel-, Konzentrations-, und Temperatur-Effekten. Die Komplexbildungskonstanten und die thermodynamischen Parameter H o, G o und S o werden angegeben.相似文献
23.
Mohamed R. Mahmoud Ahmed A. Abdel Gaber Ahmed A. Boraei Elham M. Abdalla 《Transition Metal Chemistry》1994,19(4):435-438
Summary Mixed ligand metal complexes of CoII, NiII and CuII with dicarboxylic aliphatic acids, H2L (succinic, malic and tartaric) as primary ligands and with imidazoles, L (imidazole and 2-methylimidazole) as secondary ligands were prepared and characterized. MLL2 and ML4 molecular formulae were suggested for these complexes were Formation constants of the different complexes were determined pH-metrically at T = 25 ± 0.1°C and = 0.1 mol dm–3 (NaClO4). The stability of the mixed ligand complexes increased as the effective basicity of the dicarboxylic aliphatic acid anion increased, namely, tartarate < malate < succinate acid. 相似文献
24.
Mahmoud M. Al Omari Mohammad B. Zughul J. Eric D. Davies Adnan A. Badwan 《Journal of inclusion phenomena and macrocyclic chemistry》2006,54(3-4):159-164
Ternary complexes exploiting solubility synergism (SSn) between basic drugs and β-cyclodextrin (β-CD) in the presence of an organic hydoxy acid have been reported to provide the pharmaceutical technology with highly soluble ternary complexes, even with the least soluble β-CD. In this work, phase solubility techniques were used to study factors affecting SSn in aqueous solution, which may help in understanding the mechanism involved in ternary complex formation in solution, under equilibrium conditions. The equilibrium solubility of both β-CD and each of 8 structurally unrelated drugs were measured in tandem in the presence of different acid types at low and high pHs, and at different time intervals over a period of 1–40 days. The results indicate that SSn is evident regardless of acid type (organic and inorganic) at low pH, but the extent of SSn is acid type dependant and is limited by the drug salt solubility product constant (pK
sp). Among different drugs, no apparent trend exists between drug salt solubility and the extent of SSn, but lowering drug salt solubility by increasing pH depresses SSn. The results also reveal no apparent trend between the magnitude of the complex formation constant (K
ij) and SSn. For example, drugs of low K
ij values such as astemizole, cisapride and sildenafil do not show any SSn, yet ketotifen and pizotifen, which also have low K
ij values, exhibit substantial SSn. However, the solublizing power of β-CD represented by the slope of phase solubility diagram can be used as a marker for SSn (slopes exceeding 0.4 induce SSn). 相似文献
25.
A series of binary complexes of Zn(II), Hg(II), Pb(II), La(III), Ce(III), Th(IV) and UO2(II) with 3,5-dinitrosalicylic acid and 5-sulphosalicylic acid have been isolated and characterized. The solution equilibria of these complexes have been studied pH-metrically. The formation constants (log K) at 25±1°C and ionic strength of 0.2M NaClC4 have been calculated. Stability of the complexes formed in relation to ligand molecular structure and nature of the metal ion has been examined and discussed. 相似文献
26.
Spectroscopic characterization of human and mouse primary cells,cell lines and malignant cells 总被引:3,自引:0,他引:3
Erukhimovitch V Talyshinsky M Souprun Y Huleihel M 《Photochemistry and photobiology》2002,76(4):446-451
Fourier transform infrared (FTIR) spectroscopy is currently being developed as a new optical approach to the diagnosis and characterization of cell or tissue pathology. The advantage of FTIR microspectroscopy over conventional FTIR spectroscopy in the diagnosis of malignancies is that it facilitates inspection of restricted regions of the cell culture or tissue. In this study, we set out to evaluate FTIR microspectroscopy as a diagnostic tool for identifying retrovirus-induced malignancies. Our study showed significant and consistent differences between cultures of different types of cells of both mouse and human origin, i.e. primary fibroblast cells (one to two passages in cell culture), fibroblast cell lines and malignant cells transformed by murine sarcoma virus. An impressive decrease in the levels of phosphate and other metabolites was seen in malignant cells compared with primary cells. The levels of these metabolites in the cell lines were significantly lower than in the primary cells but higher than in the malignant cells. In addition, the peak attributed to the PO2- symmetric stretching mode at 1082 cm(-1) in primary cells shifted significantly to 1085 cm(-1) for the cell line and to 1087 cm(-1) for the malignant cells. These differences taken together with differences in the shapes of various bands throughout the spectrum strongly support the possibility of developing FTIR microspectroscopy for the detection and study of malignant--and possibly premalignant--cells. 相似文献
27.
Mohamed Ibrahim Younis Xiaofeng Ren Azalldeen Kazal Alzubaidi Khaled Fahmy Mahmoud Ammar B. Altemimi Francesco Cacciola Husnain Raza Anubhav Pratap-Singh Tarek Gamal Abedelmaksoud 《Molecules (Basel, Switzerland)》2022,27(14)
The total phenolic content (TPC) from Cassia javanica L. petals were extracted using ethanolic solvent extraction at concentrations ranging from 0 to 90% and an SCF-CO2 co-solvent at various pressures. Ultrasound-assisted extraction parameters were optimized using response surface methodology (RSM). Antioxidant and anticancer properties of total phenols were assessed. An SCF-CO2 co-solvent extract was nano-encapsulated and applied to sunflower oil without the addition of an antioxidant. The results indicated that the best treatment for retaining TPC and total flavonoids content (TFC) was SCF-CO2 co-solvent followed by the ultrasound and ethanolic extraction procedures. Additionally, the best antioxidant activity by β-carotene/linoleic acid and DPPH free radical-scavenging test systems was observed by SCF-CO2 co-solvent then ultrasound and ethanolic extraction methods. SCF-CO2 co-solvent recorded the highest inhibition % for PC3 (76.20%) and MCF7 (98.70%) and the lowest IC50 value for PC3 (145 µ/mL) and MCF7 (96 µ/mL). It was discovered that fortifying sunflower oil with SCF-CO2 co-solvent nanoparticles had a beneficial effect on free fatty acids and peroxide levels. The SCF-CO2 method was finally found to be superior and could be used in large-scale processing. 相似文献
28.
A new Schiff base ligand (HL) was prepared via a condensation reaction of quinoline‐2‐carboxaldhyde with 2‐aminophenol in a molar ratio of 1:1. Its transition metal mixed ligand complexes with 1,10‐phenanthroline (1,10‐phen) as co‐ligand were also synthesized in a 1:1:1 ratio. HL and its mixed ligand complexes were characterized using elemental analysis, infrared, 1H NMR, mass and UV–visible spectroscopies, molar conductance, magnetic measurements, solid reflectance, thermal analysis, electron spin resonance and X‐ray diffraction. Molar conductance measurements showed that all complexes have an electrolytic nature, except Cd(II) complex. From elemental and spectral data, the formulae [M(L)(1,10‐phen)(H2O)]Clx?nH2O (where M = Cr(III) (x = n = 2), Mn(II) and Ni(II) (x = 1, n = 2), Fe(III) (x = n = 2), Co(II), Cu(II) and Zn(II) (x = 1, n = 2)) and [Cd(L)(1,10‐phen)Cl]?3H2O for the metal complexes have been proposed. The geometric structures of complexes were found to be octahedral. Powder X‐ray diffraction reflected the crystalline nature of the complexes; however, the Schiff base is amorphous. HL and its mixed ligand complexes were screened against Gram‐positive bacteria (Streptococcus pneumoniae and Bacillus subtilis) and Gram‐negative bacteria (Pseudomonas aeruginosa and Escherichia coli). Antifungal activity was determined against Aspergillus fumigatus and Candida albicans, the data showing that most complexes had activity less than that of the Schiff base while Mn(II), Fe(III) and Ni(II) complexes showed no significant antifungal activity. The anticancer activity of HL and its metal complexes was also studied against breast and colon cell lines. The metal complexes showed IC50 higher than that of HL, especially the Cu(II) complex which showed the highest IC50 against breast cell line. 相似文献
29.
30.
Nonlinear Dynamics - We investigate tunneling behavior between two bright solitons in a Bose–Einstein condensate with attractive contact interactions between atoms. The explicit tunneling... 相似文献