全文获取类型
收费全文 | 414篇 |
免费 | 8篇 |
专业分类
化学 | 365篇 |
晶体学 | 4篇 |
力学 | 3篇 |
数学 | 15篇 |
物理学 | 35篇 |
出版年
2022年 | 12篇 |
2021年 | 14篇 |
2020年 | 2篇 |
2019年 | 2篇 |
2018年 | 2篇 |
2016年 | 8篇 |
2015年 | 2篇 |
2014年 | 5篇 |
2013年 | 29篇 |
2012年 | 16篇 |
2011年 | 15篇 |
2010年 | 10篇 |
2009年 | 13篇 |
2008年 | 21篇 |
2007年 | 20篇 |
2006年 | 19篇 |
2005年 | 25篇 |
2004年 | 23篇 |
2003年 | 16篇 |
2002年 | 19篇 |
2001年 | 9篇 |
2000年 | 2篇 |
1999年 | 5篇 |
1998年 | 5篇 |
1997年 | 12篇 |
1996年 | 8篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 8篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 9篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1976年 | 7篇 |
1974年 | 4篇 |
1972年 | 2篇 |
1968年 | 3篇 |
1965年 | 1篇 |
1960年 | 2篇 |
1937年 | 1篇 |
排序方式: 共有422条查询结果,搜索用时 15 毫秒
321.
The reaction of 5,10,15,20-tetrakis(4-carboxylphenyl)porphyrin (TCPP) with Cd(II), Pb(II), Hg(II) and Zn(II) was studied spectrophotometrically and kinetics, equilibrium constants as well as photodecomposition of complexes were determined. It was verified that these metal ions with large radius accelerate the incorporation reaction of zinc into TCPP. On the basis of the mechanism and kinetics of this reaction, a sensitive method for the spectrophotometric determination of trace amounts of Zn(II) has been developed. The molar absorptivity of examined Zn-TCPP complex and Sandell's sensitivity at 423 nm were 3.5×105 M−1 cm−1 and 18.3 ng cm−2. The detection limit for the recommended procedure was 1.4×10−9 M (0.9 ng ml−1) and precision in range 20-100 ng ml−1 not exceeds 2.7% RSD. The proposed method applied for zinc determination in natural waters and nutritional supplement was compared with AAS results and declared value. 相似文献
322.
The reactivity of iron(III) tetraphenylporphyrin pi-cation radical (TPP(*))Fe(III)(ClO(4))(2), (1-1) iron(III) tetra-p-tolylporphyrin pi-cation radical (TTP(*))Fe(III)(ClO(4))(2) (1-2) and iron(III) tetramesitylporphyrin pi-cation radical (TMP(*))Fe(III)(ClO(4))(2) (1-3) complexes with 2,4,6-collidine, 2,3,6-collidine, 2-picoline, 2,6-di-tert-butylpyridine, and 2,6-dibromopyridine has been examined by (1)H NMR spectroscopy in dichloromethane-d(2) solution at low temperatures. These complexes undergo hydration processes which are essential in the generation of highly oxidized species via acid base/equilibria of coordinated water followed by disproportionation pathway, giving as sole stable products [(TPP(*))Fe(III)OFe(III)(TPP)](+) (4-1), [(TTP(*))Fe(III)OFe(III)(TTP)](+) (4-2), and (TMP)Fe(III)(OH) (6) respectively. The sterically hindered pyridines act as efficient proton scavengers. Two novel highly oxidized iron complexes have been detected by (1)H NMR spectroscopy after addition of 2,4,6-collidine to (TTP(*))Fe(III)(ClO(4))(2) or (TPP(*))Fe(III)(ClO(4))(2) in dichloromethane-d(2) solution at 202 K. New intermediates have been identified as iron porphyrin N-oxide complexes, i.e., iron(III) porphyrin N-oxide cation radical (2-n) and iron(IV) porphyrin N-oxide radical (3-n). The (1)H NMR results indicate that the D(4)(h)() symmetry of the parent iron(III) pi-cation radical is drastically reduced upon disproportionation in the presence of proton scavengers. Both species are very unstable and were observed from 176 to 232 K. The intermediate 2-2 has a (1)H NMR spectrum which demonstrates large hyperfine shifts (ppm) for the meso p-tolyl substituents (ortho 98.0, 94.8, 92.9, 91.7; meta -34.8, -38.7, -41.5, -42.3; p-CH(3) -86.3, -88.0) which are consistent with presence of an N-substituted iron porphyrin radical in the product mixture. The characteristic (1)H NMR spectrum of 2-2 includes six pyrrole resonances at 149.6, 118.2, 115.4, 88.3, 64.6, and 55.7 ppm at 202 K, i.e., in the positions corresponding to iron(III) high-spin porphyrins. On warming to 222 K, the pyrrole resonances broaden and then coalesce pairwaise. Such dynamic behavior is accounted for by a rearrangement mechanism which involves an inversion of the porphyrin puckering. The pattern of p-tolyl resonances revealed the cation radical electronic structure of 3-2. The p-tolyl resonances are divided in two distinct sets showing opposite direction of the isotropic shift for the same ring positions. The pyrrole resonances of 3-2 also demonstrated downfield and upfield shifts. A disproportionation mechanism of the hydrated iron porphyrin cation radicals to generate 2 and 3 has been proposed. Both intermediates react with triphenylphosphine to produce triphenylphosphine oxide and high-spin iron porphyrins. Addition of 2,4,6-collidine to (TMP(*))Fe(III)(ClO(4))(2) does not produce analogs of 2 and 3 found for sterically unprotected porphyrins. It results instead in the formation of a variety of X(TMP(*))Fe(IV)O (5) complexes also accounted for by the disproportionation process. 相似文献
323.
Krystyna Bugajska 《International Journal of Theoretical Physics》1979,18(2):95-106
Geometrical properties of elements of the unique representation of the Clifford algebra of quadratic form (+, –, –, –) are investigated. A connection between horospheres on the positive Lobatschevsky space of timelike directions and spinors is established.Partly supported by the Polish Government under the Research Program MR I.7. 相似文献
324.
Mieczys?aw M?kosza Krystyna Kamieńska-Trela Maciej Paszewski Ma?gorzata Bechcicka 《Tetrahedron》2005,61(50):11952-11964
Oxidative nucleophilic substitution of hydrogen (ONSH) in nitroarenes with carbanion of isopropyl phenyl acetate gives various products depending on the conditions and oxidant. The reaction carried out in liquid ammonia and KMnO4 oxidant gives iso-propyl α-hydroxy-α-nitroarylphenylacetates formed via hydroxylation of the initial ONSH products. In some cases additionally dimeric, trimeric and tetrameric products are formed. In THF and Bu4N+MnO4− or DDQ oxidants simple ONSH products are formed whereas oxidation by dimethyl dioxirane (DMD) gave iso-propyl hydroxyaryl phenyl acetates. The dimeric and trimeric products are apparently formed via coupling of nitrobenzylic radicals generated in course of oxidation with nitrobenzylic carbanions of the ONSH products. 相似文献
325.
Rachlewicz K Latos-Grazyński L Vogel E Ciunik Z Jerzykiewicz LB 《Inorganic chemistry》2002,41(7):1979-1988
Five-coordinate iron(III) 2,7,12,17-tetrapropylporphycene (TPrPc)Fe(III)X (X = C(6)H(5)O(-), Cl(-), Br(-), I(-), ClO(4)(-)) complexes have been investigated. The (1)H NMR spectra demonstrate downfield shifts for pyrrole resonances [(TPrPc)Fe(III)(C(6)H(5)O), 65.3 ppm; (TPrPc)Fe(III)Cl, 28.5 ppm] but large upfield ones for (TPrPc)Fe(III)Br (-7.8 ppm), (TPrPc)Fe(III)I (-49.4 ppm), and (TPrPc)Fe(III)ClO(4) (-77.1 ppm) (294 K, CD(2)Cl(2)). The pyrrole chemical shifts span the remarkable +70 to -80 ppm range. The variable-temperature (1)H NMR spectra of (TPrPc)Fe(III)X demonstrate anti-Curie behavior with a sign reversal for (TPrPc)Fe(III)Cl. These behaviors are consistent with the admixed S = 3/2, 5/2 ground electronic state with a dominating contribution of the S = 3/2 one. In terms of the chemical shift, (TPrPc)Fe(III)(ClO(4)) can be considered as an example of the purest S = 3/2 state in the investigated series. The extent of the S = 5/2 contribution in the admixed S = 3/2, 5/2 ground electronic state, as gradated solely the basis of the pyrrole proton paramagnetic shifts, is controlled by the strength of the axial ligand, following the magnetochemical series (Evans, D. R.; Reed, C. A. J. Am. Chem. Soc. 2000, 122, 4660). Significantly iron(III) 2,7,12,17-tetrapropylporphycene, soluble in typical organic solvents, can be considered as a universal framework to classify the ligand strength in a magnetochemical series, consistently using the beta-H pyrrole paramagnetic shifts as a fundamental criterion. The structure of (TPrPc)Fe(III)Cl has been determined by X-ray crystallography. The iron is five-coordinate with bonds of nearly equal length to the four pyrrole nitrogen atoms (Fe-N in the range 1.983(5)-2.006(6) A). The iron lies 0.583(1) A out of the mean plane of the macrocycle and 0.502(5) A out of the mean N(4) plane. In the solid, pairs of molecules are positioned about the center of symmetry so there is face-to-face pi-pi contact. The mean plane separation is 3.38 A, and the lateral shift of the porphycene center along the Fe-N bond is 4.490 A. The distance from one porphycene center to the other is 5.62 A, and the iron-iron separation is 6.304(2) A. 相似文献
326.
Piotr Kowalski Krystyna Nowak Barbara Szpakiewicz 《Journal of heterocyclic chemistry》2005,42(5):883-888
Syntheses of 1‐propyl derivatives of 2‐methyl‐5‐nitroimidazole ( 11‐17 ), containing various azanaphtha‐lene systems attached at 3‐position of the propyl group have been described. Due to structural similarity of the derivatives 11‐17 with metronidazole, an antibacterial and antiprotozoal drug, pharmacological properties of the target compounds were predicted using the PASS program. Polar surface area (PSA) parameter was also applied for estimation of the physical properties of 11‐17 as drug‐like candidates. 相似文献
327.
Isocyanate-terminated six armed star shaped macromers with a statistical copolymer backbone consisting of 80% EO and 20% PO have previously demonstrated excellent protein and cell repellence as nano-layered surfaces. In this study, various macromers are mixed with water and provide a spectrum of materials that range from particles to uniform hydrogels. Due to hydrophobic end groups, 3 kDa molecular weight macromers result in micro and nano-particles, while 18 kDa macromers completely dissolve and consequently uniform, transparent, high water content hydrogels are formed. Oriented channels may be induced into these hydrogels through the controlled freezing of water in the preformed hydrogel. 相似文献
328.
Dabrowski JM Pereira MM Arnaut LG Monteiro CJ Peixoto AF Karocki A Urbańska K Stochel G 《Photochemistry and photobiology》2007,83(4):897-903
A water-soluble halogenated porphyrin, namely 5,10,15,20-tetrakis(2-chloro-3-sulfophenyl)porphyrin (TCPPSO(3)H), was prepared and evaluated as sensitizer for photodynamic therapy (PDT). Photophysical properties of TCPPSO(3)H, such as high photostability, long triplet lifetime and high singlet oxygen quantum yield suggest high effectiveness of this class of halogenated porphyrins in PDT. TCPPSO(3)H is non-toxic in the dark and causes a significant photodynamic effect examined against MCF7 (human breast carcinoma), SKMEL 188 (human melanoma) and S91(mouse melanoma) cell lines upon red light irradiation (cutoff < 600 nm) at low light doses. Time-dependent cellular uptake of TCPPSO(3)H reached plateau at 120 min and was the highest for S91, 20% lower for MCF7 and 70% lower for SKMEL 188. Our results show that this halogenated water-soluble porphyrin is an efficient photosensitizer and reveal the potential of this class of compounds as PDT agents. 相似文献
329.
Microparticles of poly(1,8-diaminonapthalene) (PDAN) were prepared by chemically oxidative polymerization by (NH4)2S2O8. The effect of pH on the sorption of Cd(II), Cu(II), Ni(II), Mn(II), Zn(II) and Pb(II) on PDAN was examined by the batch procedure. PDAN showed good sorption capacity and high selectivity towards Cu(II) in comparison with the very popular chelating sorbent Chelex 100 containing iminodiacetic functional groups. The on-line preconcentration system containing the knotted reactor with the obtained polymer was examined for the sorption and desorption processes of copper ions. The applicability of this system was checked by analysis of Cu(II) content in standard reference material (NIST 1643e) and some natural water samples. 相似文献
330.
High‐performance countercurrent chromatography separation of Peucedanum cervaria fruit extract for the isolation of rare coumarin derivatives 下载免费PDF全文
Krystyna Skalicka‐Woźniak Tomasz Mroczek Ewelina Kozioł 《Journal of separation science》2015,38(2):179-186
For the first time, rare major and minor compounds from fruits of Peucedanum cervaria were isolated. High‐performance countercurrent chromatography with two different solvent systems, heptane/ethyl acetate/methanol/water (3:2:3:2 and 2:1:2:1, v/v), was successfully used in the reversed‐phase mode. A scale‐up process from analytical to semipreparative in a very short time was developed. The structures of isolated compounds were evaluated by high‐performance liquid chromatography with diode array detection and electrospray ionization mass spectrometry, gas chromatography with mass spectrometry, and one‐ and two‐dimensional NMR spectroscopy. (8S,9R)‐9‐(3‐Methylbutenoyloxy)‐O‐acetyl‐8,9‐dihydrooroselol (compound B), (8S,9R)‐9‐(2‐methyl‐Z‐butenoyloxy)‐O‐acetyl‐8,9‐dihydrooroselol (edultin, compound C), and (8S,9R)‐9‐acetoxy‐O‐(2α‐methylbutyryl)‐8,9‐dihydrooroselol (compound D) were obtained using heptane/ethyl acetate/methanol/water (2:1:2:1, v/v) in <40 min. The method yielded 4.6 mg of a mixture of compounds B and C (11:89) and 3.7 mg of compound D. These amounts were obtained from the crude extract (0.5 g) in a single run. Although the compounds are known, their isolation by countercurrent chromatography and the analysis of their relative stereochemistry by two‐dimensional NMR spectroscopy have been performed for the first time. Additionally, heptane/ethyl acetate/methanol/water (3:2:3:2, v/v) led to the isolation of oxypeucedanin (1.2 mg; compound A). This is the first time that angular dihydrofuranocoumarin was isolated from plant extract by countercurrent chromatography. 相似文献