全文获取类型
收费全文 | 376篇 |
免费 | 21篇 |
国内免费 | 2篇 |
学科分类
数理化 | 399篇 |
出版年
2024年 | 1篇 |
2023年 | 2篇 |
2022年 | 7篇 |
2021年 | 27篇 |
2020年 | 20篇 |
2019年 | 20篇 |
2018年 | 5篇 |
2017年 | 9篇 |
2016年 | 13篇 |
2015年 | 20篇 |
2014年 | 18篇 |
2013年 | 36篇 |
2012年 | 27篇 |
2011年 | 17篇 |
2010年 | 23篇 |
2009年 | 12篇 |
2008年 | 19篇 |
2007年 | 22篇 |
2006年 | 14篇 |
2005年 | 9篇 |
2004年 | 16篇 |
2003年 | 15篇 |
2002年 | 9篇 |
2001年 | 5篇 |
2000年 | 7篇 |
1999年 | 1篇 |
1998年 | 3篇 |
1997年 | 1篇 |
1996年 | 3篇 |
1995年 | 6篇 |
1994年 | 1篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有399条查询结果,搜索用时 149 毫秒
71.
The potential energy curves have been investigated for the 13 lowest sextet electronic states in the representation below 53,000 cm?1 of the molecule CrCl via CASSCF and MRCI (single and double excitation with Davidson correction) calculations. The harmonic frequency ωe, the internuclear distance re, the rotational constant Be, the electronic energy with respect to the ground state Te, and the permanent dipole moment μ have been calculated. By using the canonical functions approach, the eigenvalues Ev, the rotational constant Bv, and the abscissas of the turning points rmin and rmax have been calculated for the considered electronic states up to the vibrational level v = 16. Nine electronic states have been studied theoretically here for the first time. The comparison of these values with the theoretical and experimental results available in the literature shows a good agreement. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012 相似文献
72.
Alaa A. Hassan Kamal M. El‐Shaieb Raafat M. Shaker Dietrich Dpp 《Heteroatom Chemistry》2005,16(1):12-19
1,4‐Disubstituted thiosemicarbazides 1b–f reacted with ethenetetracarbonitrile ( 5 ) in di‐ methylformamide with formation of 2‐substituted 5‐phenyl‐1,3,4‐thiadiazoles 2a–f and 2‐substituted 5‐phenyl‐1,3,4‐oxadiazoles 4a–f . Upon addition of 5 to 1c–e in chlorobenzene, 3‐amino‐2‐benzoyl‐4,5,5‐tri‐ cyano‐2,5‐dihydro‐1H‐pyrazole‐1‐[N‐(4‐tricyanovi‐nyl)phenyl]carbothioamide ( 12 ), 5‐benzylamino‐, and 5‐allylamino‐4‐benzoyl‐2,3‐dihydro‐[1,3,4]thiadiazol‐ 2,2‐dicarbonitrile ( 13a,b ) and 5‐amino‐1‐benzoylpyrazole‐3,4‐dicarbonitrile ( 14 ) as well as 2‐phenyl‐ 4H‐[1,3,4]‐oxadiazine‐5,6‐dicarbonitrile ( 15 ) were formed. Rationales for the role of the solvent and the conversions observed are presented. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:12–19, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20071 相似文献
73.
Substituted thiosemicarbazones 7a–e reacted with ethenetetracarbonitrile (TCNE) in ethyl acetate with formation of 5‐amino‐3‐(substituted ben‐zylidene‐amino)‐2‐phenylimino‐2,3‐dihydrothiazole‐4‐carbonitrile 8a–e 2‐amino‐6‐phenyl‐imino‐1,6‐ dihydropyrrolo[1,3,4]thiadiazine‐3‐carbonitrile 9 , and phenyl‐(5‐{substituted phenyl}‐3H‐[1,3,4]thiadiazole‐2‐ylidene)amines 10a–e . Rationales for the observed conversations are presented. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:261–266, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20198 相似文献
74.
AbstractDiphenyl-4-thioxo-1,4-dihydropyrimidin-5-yl)ethan-1-one 4 was obtained upon the reaction of (phenylamino)but-2-enethioyl)benzamide 3 with sodium hydroxide. The reactions of 4 with several active species such as benzaldehyde, urea, malononitrile, ethyl cyanoacetate, hydrazine, and carbon disulfide were studied. The antitumor activities of some selective compounds were examined against two cell lines: Mammalian cell lines: A-549 cells (human lung cancer cell line) and MRC-5 cells (normal human lung fibroblast cell line); some of the compounds were highly efficient; compound 13 has the most effective anticancer activity. 相似文献
75.
Ahmed E. Allam Alaa M. Nafady Toshinori Nakagawa Naomichi Takemoto 《Natural product research》2018,32(16):1920-1925
Two new flavonoid glycosides, kaempferol 3-O-α-L-rhamnopyranosyl (1→6) (3′′-acetyl)-β-D-galactopyranoside 1 and kaempferol 3-O-α-L-arabinopyranosyl-5-O-α-L-rhamnopyranoside 2, along with six known ones 3–8 were isolated from the flowers of Vicia faba L. (Fabaceae). Methanol extract and the isolated compounds were tested against lipase and melanogenesis inhibition activities and resulted in that compound 2 showed 53 and 77% lipase inhibition activity in concentrations of 400 and 800 μg/mL, respectively. For melanogenesis, compounds 2, 3 and 4 exhibited potent melanogenesis inhibition activity where the melanin content in melanoma cells was decreased to be about 57.5, 56 and 61%, respectively, with no obvious melanocytotoxicity. The rest of compounds showed weak to moderate activity. The results of melanogenesis inhibition activity of this study suggested the potential use of Vicia faba flowers as a skin-whitening agent and reveal the flowers to be a rich source of important phytochemicals with antilipase and melanogenesis inhibitory activity. 相似文献
76.
Alaa S. Amin 《Mikrochimica acta》1997,126(1-2):105-108
A simple, rapid and accurate colorimetric method is proposed for the determination of tannins, based on the reduction of tetrazolium blue in alkaline medium by tannins at 90 ± 2 °C for 15 min, leading to the formation of a highly coloured formazan derivative. Absorbance measurements were made at 527 nm and the calibration graph was linear for 0.2-9.0 g/ml of tannic acid. For more accurate analysis, the Ringbom optimum concentration range was found to be 0.5–8.2 g/ml. The relative standard deviation for the determination in a tea sample containing 7.55% tannins was 1.65%. Most of the ingredients commonly found in tea samples do not interfere with the determination. Several tea samples were analysed using the proposed method. 相似文献
77.
Ching-Min Chang Marian W. Kemblowski Jagath J. Kaluarachchi Alaa Abdin 《Transport in Porous Media》1995,19(3):233-259
Stochastic analysis of steady-state two-phase (water and oil) flow in heterogeneous porous media is performed using the perturbation theory and spectral representation techniques. The governing equations describing the flow are coupled and nonlinear. The key stochastic input variables are intrinsic permeability,k, and the soil and fluid dependent retention parameter, . Three different stochastic combinations of these two imput parameters were considered. The perturbation/spectral analysis was used to develop closed-form expressions that describe stochastic variability of key output processes, such as capillary and individual phase pressures and specific discharges. The analysis also included the estimation of the effective flow properties. The impact of the spatial variability ofk and on the variances of pressures, effective conductivities, and specific discharges was examined. 相似文献
78.
Ahmed A. Abdel-Khalek Alaa E. D. M. Abdel-Hady Mohamed F. El-Shahat 《Transition Metal Chemistry》1995,20(5):430-434
Summary In aqueous solutions, [Cr(en)3]3+ aquates to [Cr(en)2-(H2O) 2]3+. A kinetic study of the oxidation of [Cr(en)3]3+ by N-bromosuccinimide (NBS) in aqueous solutions and water-alcohol solvent mixtures was performed. The reaction is first order with respect to both total [CrIII] and [NBS]. The rate is inversely dependent upon [H+] in the 7.0–7.9 pH range, and varies with the co-solvent according to the order: MeOH > EtOH > PrOH. An appropriate mechanism, in which the deprotonated [Cr(en)2(OH)(H2O)]2+ is the reactive species, is suggested. Thermodynamic activation parameters have been calculated.Abstracted from the PhD thesis (Ain Shams University) of A. E.- D. M. Abdel-Hady. 相似文献
79.
Synthesis of imidazo[1,2-b]pyridazines: Fenbendazole,oxifenbendazole analogs and related derivatives
Alaa E. Mourad Dean S. Wise Leroy B. Townsend 《Journal of heterocyclic chemistry》1993,30(5):1365-1372
A series of imidazo[1,2-b]pyridazines has been prepared and evaluated for macrofilaricidal activity against Brugia pahangi or Acanthocheilonema viteae infections in jirds. The imidazo[1,2-b]pyridazine analogs of fen-bendazole and oxifenbendazole are reported. In addition, several 6-aminoimidazo[1,2-b]pyridazine derivatives have been prepared. None of these compounds possessed significant activity against human cytomega-lovirus (HCMV) or filarial infections. 相似文献
80.
A spectrophotometric procedure for the determination of terfenadine and a number of its pharmaceutical preparations has been developed that offers advantages of simplicity, rapidity, sensitivity and stability indication over the official USP (1995) method. The proposed method is based on the formation of ion-pairs by the reaction of terfenadine with some chromotropic acid mono- and bis-azo dyes. Different variables affecting the ion-pair formation were studied and optimized. At the maximum absorption of 557, 521, 592 and 543 nm, Beer's law is obeyed in the range 0.2–18.6, 0.2–16.4, 0.2–25.0 and 0.2–22.2 g ml–1 on using reagents I, II, III and IV, respectively. The stoichiometric ratio and stability of each ion-pair were estimated and the mechanism of the reaction is discussed. The molar absorptivity and Sandell sensitivity of the produced ion-pairs were calculated in addition to Ringbom optimum concentration ranges. Statistical treatment of the experimental results indicates that the procedures are precise and accurate. Excipients used as additives in pharmaceutical formulations did not interfere in the proposed procedures. The reliability of the methods was established by parallel determination against the official USP method. The procedures described were successfully applied to the determination of the bulk drug and its pharmaceutical formulations by applying the standard addition technique. 相似文献