全文获取类型
收费全文 | 453篇 |
免费 | 27篇 |
国内免费 | 1篇 |
专业分类
化学 | 352篇 |
晶体学 | 9篇 |
力学 | 7篇 |
数学 | 73篇 |
物理学 | 40篇 |
出版年
2024年 | 2篇 |
2023年 | 5篇 |
2022年 | 12篇 |
2021年 | 20篇 |
2020年 | 26篇 |
2019年 | 31篇 |
2018年 | 18篇 |
2017年 | 18篇 |
2016年 | 34篇 |
2015年 | 18篇 |
2014年 | 19篇 |
2013年 | 49篇 |
2012年 | 42篇 |
2011年 | 27篇 |
2010年 | 26篇 |
2009年 | 14篇 |
2008年 | 11篇 |
2007年 | 26篇 |
2006年 | 12篇 |
2005年 | 11篇 |
2004年 | 15篇 |
2003年 | 6篇 |
2002年 | 6篇 |
2001年 | 4篇 |
2000年 | 1篇 |
1999年 | 1篇 |
1998年 | 1篇 |
1997年 | 2篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1975年 | 1篇 |
1969年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有481条查询结果,搜索用时 15 毫秒
31.
Fayez H.?OsmanEmail author Fatma A.?El-Samahy I. S.?Ahmed Farag 《Monatshefte für Chemie / Chemical Monthly》2004,135(7):823-831
Summary. The reaction of (E)-(2-oxo-1,2-dihydroindol-3-ylidene)acetic acid esters with dry acetone at 70°C in presence of aluminum oxide as a catalyst led to the formation of an addition product as a mixture of two isomers. When the same reaction was carried out in presence of morpholine as a base, the unexpected spiro-product was obtained beside the two isomers. Methylation of the latter with methyl iodide in acetone in presence of anhydrous potassium carbonate yielded the corresponding methylated products, whereas methylation of the starting material by the same procedure gave an N-methylated product and the unexpected dispiro-compound. The reaction mechanisms are considered and the structural assignments of the new compounds are based on the chemical and spectroscopic evidences. The structure of the dispiro-product was derived by X-ray analysis. 相似文献
32.
In this paper we report the effect of dicationic ‘gemini’ surfactants (CH3)2C16H33N+? (CH2)m? N+C16H33(CH3)2, 2Br? (where m = 4, 5, 6) on the reaction of ninhydrin with DL ‐tryptophan. The gemini surfactant micellar media are comparatively more effective than their conventional monomeric counterpart cetyltrimethylammonium bromide (CTAB) micelles. Also, whereas typical rate constant (kψ) increase and leveling‐off regions, just like CTAB, are observed with geminis, the latter produce a third region of increasing kψ at higher concentrations. These subsequent increases are ascribed to changes in micellar morphologies, consistent with changes in 1H NMR line widths. Quantitative kinetic analysis of the rate constant–[surfactant] data has been performed on the basis of modified pseudophase model. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
33.
Synthesis of three new stable spirodifuranose derivatives (3, 5, and 7), which cannot be obtained easily using ordinary synthetic methods, has been achieved by reduction of 3-O-acetyl and 3-O-methyl derivatives of (4R)-1,2-O-alkylidene-5-eno-4,7-epidioxy-5,6,8-trideoxy-α-D-threo-1,4-furano-4,7-diulo-octoses (1, 4, and 6). 相似文献
34.
Yusuf Kurtulmus T. Tanyalçın Giray Bozkaya Ogˇuzhan Gündüz Ömür Çerçi Fatma Z. Kutay Biltan Ersöz 《Accreditation and quality assurance》2001,6(9-10):424-426
The aim of this study was to compare two immunoassay measurement methods, the ACS:180 Plus (Chiron Diagnostics) kit and the Immulite sPSA (DPC) kit. Method comparison analysis was performed according to EP9-A; approved guideline of NCCLS 51. Serum samples having a wide range of total prostate-specific antigen (PSA) concentrations were evaluated in split-sample analysis. F-test, t-test analysis and regression statistics were performed. In Deming regression analysis the coefficients were as follows; the slope=0.967; y-intercept=-0.148, r=0.989. An acceptable bias was seen since the systematical error was calculated to have a value less than the total allowable error calculated from biological variations. Non-parametric evaluation of the area under ROC curves for ACS:180 Plus and Immulite sPSA were 0.997 and 0.987, respectively. Diagnostic accuracy was at the level of p= 0.000 and no statistical difference was found between the two assay methods. 相似文献
35.
Gunjan Pandey Tasneem Fatma Sudha M. Cowsik Sneha Sudha Komath 《Journal of photochemistry and photobiology. B, Biology》2009,97(2):87-93
Recent research has shown that, like porphyrins, phycocyanin (PC) too can produce singlet oxygen upon excitation with the appropriate radiation and hence could be useful in photodynamic therapy (PDT) for cancer. Unlike porphyrins, PC has the advantage of being a non-toxic, non-carcinogenic, soluble protein. However, the challenge would be to target the fluorescent phycobiliprotein to malignant cells. We report here that the tumor-specific lectin, jacalin, binds PC specifically in a carbohydrate-independent manner and with affinities better than that for porphyrins. Hence the lectin could prove to be a useful carrier for targeted delivery of PC. The interaction involves both ionic and hydrophobic interactions and more than one contact site. 相似文献
36.
A series of homo‐, heterodinuclear and homotrinuclear copper(II) complexes containing a new Schiff base ligand and 1,10‐phenanthroline were synthesized. Based on results of elemental analyses, FTIR, 1H‐ and 13C‐NMR spectra, conductivity measurements and magnetic susceptibility measurements, the complexes had general compositions {[Cu(L)(H2O)M(phen)2](ClO4)2 [M = Cu(II), Mn(II), Co(II)]} and {[Cu3(L)2(H2O)2](ClO4)2}. The metal:L:phen ratio is 2:1:2 for the dinuclear copper(II) complexes and the metal:L ratio was 3:2 for the trinuclear copper(II) complex. The liquid–liquid extraction of various transition metal cations [Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Pb(II), Cd(II), Hg(II)] from the aqueous phase to the organic phase was carried out using the diimine–dioxime ligand. It was concluded that the ligand can effectively be used in solvent extraction of copper(II) from the aqueous phase to the organic phase. Furthermore, catalytic activitiy of the complexes for the disproportionation of hydrogen peroxide was also investigated in the presence of imidazole. Dinuclear copper(II)–manganese(II) complex has some similarity to manganese catalase in structure and activity. The interaction between these complexes and DNA has also been investigated by agarose gel electrophoresis; we found that the homo‐ and heterodinuclear copper complexes can cleave supercoiled pBR322 DNA to nicked and linear forms in the presence of H2O2. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
37.
This research work is concerned with the use of N‐benzoyl cyanoacetylhydrazine ( 3 ) in synthesizing several new heterocyclic compounds with potential biological activity, via its reaction with various chemical reagents. The synthesized derivatives have actually exhibited, upon screening, antibacterial and antifungal activities. 相似文献
38.
Emine Özge Karaca Mitat Akkoç Muhammad Nawaz Tahir Cengiz Arıcı Fatma İmik Nevin Gürbüz Sedat Yaşar İsmail Özdemir 《Tetrahedron letters》2017,58(36):3529-3532
A series of seven-membered ditopic ring-expanded N-heterocyclic carbene (dre-NHC) precursors, bearing sterically demanding and electron-rich aryl groups, were synthesised in moderate yields via the reaction of 1,2,4,5-tetrakis(bromomethyl)benzene with the corresponding N,N′-diarylformamidines in the presence of K2CO3 in acetonitrile under an air atmosphere. All new compounds were characterised by HRMS, NMR spectroscopy, and microanalysis, as well as X-ray crystallography for compound 1c. The development of an efficient catalytic system for the Suzuki-Miyaura coupling reaction of aryl chlorides with various boronic acids was also investigated using the in situ generated dre-NHC ligands. 相似文献
39.
Eno E. Ebenso Taner Arslan Fatma Kandemi̇rlı Ian Love Cemi̇l Öğretır Murat Saracoğlu Savıour A. Umoren 《International journal of quantum chemistry》2010,110(14):2614-2636
Density functional theory (DFT) at the B3LYP/6‐31G (d,p) and BP86/CEP‐31G* basis set levels and ab initio calculations using the RHF/6‐31G (d,p) methods were performed on four sulfonamides (namely sulfaacetamide (SAM), sulfapyridine (SPY), sulfamerazine (SMR), and sulfathiazole (STI)) used as corrosion inhibitors for mild steel in acidic medium to determine the relationship between molecular structure and their inhibition efficiencies (%IE). The order of inhibition efficiency obtained was SMR > SPY > STI > SAM which corresponded with the order of most of the calculated quantum chemical parameters namely EHOMO (highest occupied molecular orbital energy), ELUMO (lowest unoccupied molecular orbital energy), the energy gap (ΔE), the Mulliken charges on the C, O, N, S atoms, hardness (η), softness (S), polarizability (α), dipole moment (μ), total energy change (ΔET), electrophilicity (ω), electron affinity (A), ionization potential (I), the absolute electronegativity (χ), and the fraction of electrons transferred (ΔN). Quantitative structure activity relationship (QSAR) approach has been used and a correlation of the composite index of some of the quantum chemical parameters was performed to characterize the inhibition performance of the sulfonamides studied. The results showed that the %IE of the sulfonamides was closely related to some of the quantum chemical parameters but with varying degrees/order. The calculated %IE of the sulfonamides studied was found to be close to their experimental corrosion inhibition efficiencies. The experimental data obtained fits the Langmuir adsorption isotherm. The negative sign of the EHOMO values and other thermodynamic parameters obtained indicates that the data obtained supports physical adsorption mechanism. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 相似文献
40.
Fatma Karipcin Bülent Dede Mustafa Cengiz 《Russian Journal of Inorganic Chemistry》2010,55(4):530-540
Four different types of new ligands Ar[COC(NOH)R]
n
(Ar=biphenyl, n = 1 H2L1; Ar=biphenyl, n = 2 H4L2; Ar=diphenylmethane, n = 1 H2L3; Ar=diphenylmethane, n = 2 H4L4; R=2-amino-4-chlorophenol in all ligands) have been obtained from 1 equivalent of chloroketooximes Ar[COC(NOH)Cl]
n
(HL1-H2L4) and 1 equivalent of 2-amino-4-chlorophenol (for H2L1 and H2L3) or 2 equivalent of 2-amino-4-chlorophenol (for H4L2 and H4L4). (Mononuclear or binuclear cobalt(II), nickel(II), copper(II) and zinc(II) complexes were synthesized with these ligands.)
These compounds have been characterized by elemental analyses, AAS, infra-red spectra and magnetic susceptibility measurements.
The ligands have been further characterized by 1H NMR. The results suggest that the dinuclear complexes of H2L1 and H2L3 have a metal:ligand ratio of 1:2; the mononuclear complexes of H4L2 and H4L4 have a metal:ligand ratio of 1:1 and dinuclear complexes H4L2 and H4L4 have a metal:ligand ratio of 2:1. The binding properties of the ligands towards selected transition metal ions (MnII, CoII, NiII, CuII, ZnII, PbII, CdII, HgII) have been established by extraction experiments. The ligands show strong binding ability towards mercury(II) ion. In addition,
the thermal decomposition of some complexes is studied in nitrogen atmosphere. 相似文献