全文获取类型
收费全文 | 2125篇 |
免费 | 66篇 |
国内免费 | 14篇 |
专业分类
化学 | 1527篇 |
晶体学 | 12篇 |
力学 | 60篇 |
数学 | 355篇 |
物理学 | 251篇 |
出版年
2022年 | 15篇 |
2021年 | 34篇 |
2020年 | 22篇 |
2019年 | 25篇 |
2018年 | 25篇 |
2017年 | 16篇 |
2016年 | 32篇 |
2015年 | 55篇 |
2014年 | 42篇 |
2013年 | 110篇 |
2012年 | 119篇 |
2011年 | 137篇 |
2010年 | 76篇 |
2009年 | 61篇 |
2008年 | 107篇 |
2007年 | 111篇 |
2006年 | 126篇 |
2005年 | 89篇 |
2004年 | 90篇 |
2003年 | 80篇 |
2002年 | 65篇 |
2001年 | 39篇 |
2000年 | 20篇 |
1999年 | 13篇 |
1998年 | 20篇 |
1997年 | 17篇 |
1996年 | 35篇 |
1995年 | 19篇 |
1994年 | 18篇 |
1993年 | 28篇 |
1992年 | 14篇 |
1991年 | 23篇 |
1990年 | 19篇 |
1989年 | 19篇 |
1988年 | 21篇 |
1986年 | 13篇 |
1985年 | 28篇 |
1984年 | 24篇 |
1983年 | 14篇 |
1982年 | 27篇 |
1981年 | 26篇 |
1980年 | 15篇 |
1979年 | 33篇 |
1978年 | 20篇 |
1977年 | 28篇 |
1976年 | 22篇 |
1975年 | 23篇 |
1974年 | 24篇 |
1973年 | 18篇 |
1968年 | 12篇 |
排序方式: 共有2205条查询结果,搜索用时 31 毫秒
51.
D.?Mitra S.?Francis L.?VarshneyEmail author 《Journal of Thermal Analysis and Calorimetry》2004,78(3):821-829
DSC and TG studies were carried out on -radiation processed Indian natural products of medicinal importance, namely Ashwagandha (Withania Somnifera), Amla (Emblica Officinalis) and Hartiki (Terminalia chebula). DSC thermoanalytical curves were recorded from 35 to 400°C in air and nitrogen atmosphere. Similarly, TG thermoanalytical curves were taken from 35 to 700°C in air and nitrogen atmosphere. Irradiated products gave significantly different thermoanalytical profiles in comparison to non-irradiated samples. The differences were observed above decomposition temperature of 200°C and were non-linear with respect to radiation dose. Partial oxidation of the products during irradiation in air could be responsible for the observed differences. 相似文献
52.
Christophe Hoarau Alexis Du Fou de Kerdaniel Pierre Grandclaudon Francis Marsais 《Tetrahedron letters》2005,46(49):8573-8577
The efficient and regioselective palladium-catalyzed C-2 arylation of ethyl 4-oxazolecarboxylate 1 with phenyliodide is described. The different parameters (solvent, base, ligand and catalyst) for the optimal conditions of this arylation process have been screened. 相似文献
53.
al-Ktaifani MM Bauer W Bergsträsser U Breit B Francis MD Heinemann FW Hitchcock PB Mack A Nixon JF Pritzkow H Regitz M Zeller M Zenneck U 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(11):2622-2633
Several independent synthetic routes are described leading to the formation of a novel unsaturated tetracyclic phosphorus carbon cage compound tBu4C4P6 (1), which undergoes a light-induced valence isomerization to produce the first hexaphosphapentaprismane cage tBu4C4P6 (2). A second unsaturated isomer tBu4C4P6 (9) of 1 and the bis-[W(CO)5] complex 13 of 1 are stable towards similar isomerization reactions. Another starting material for the synthesis of the hexaphosphapentaprismane cage tBu4C4P6 (2) is the trimeric mercury complex [(tBu4C4P6)Hg]3 (11), which undergoes elimination of mercury to afford the title compound 2. Single-crystal X-ray structural determinations have been carried out on compounds 1, 2, 9, 11, and 13. 相似文献
54.
Yoshim Tokura Richard L. Edelson Francis P. Gasparro 《Photochemistry and photobiology》1993,58(6):822-826
Abstract The effects of cell differentiation and mitogen and phorbol ester stimulation on the formation of 8-methoxypsoralen (8-MOP)-DNA photoadducts in murine T lymphocytes were examined using 3 H-8-MOP. While there were no significant differences in 8-MOP photoadduct formation among BALB/c thymocytes, splenocytes, splenic T cells and MRL/1pr lymph node cells, BALB/c bone marrow cells showed fewer photoadducts than did the lymphocytes. This suggested that proliferating progenitor cells may be resistant to 8-MOP photoadduct formation. Incubation of purified splenic T cells with lectin mitogens for 2 h or with phorbol 12-myristate 13-acetate (PMA) for 2–43 h resulted in reduction of 8-MOP photoadduct formation in the DNA, whereas 64 h cultivation with these agents augmented the photoadduct formation. The reduction of photoadduct formation induced by phytohemagglutinin was restored by the further addition of a protein kinase C (PKC) inhibitor, H-7, to the culture. Thus, it is assumed that the reduction of adduct formation evoked by mitogens and PMA is mediated in part by the activation of PKC in the cells. On the other hand, the augmentation of the adduct formation induced by the longer-period cultures with mitogens and PMA appeared to be caused by down-regulation of PKC. The present study showed that the stimulatory signals in which PKC is presumably involved affect the ability of cells to form 8-MOP-DNA photoadducts. 相似文献
55.
Corine Simonnet-Jégat Francis Robert Francis Sécheresse 《Transition Metal Chemistry》1994,19(3):379-380
Summary The reaction of NH4VO3 with S
2
-2
in ammonia in the presence of 2,2-bipyridine (bipy) and [N(Bu)4]Br gives the mononuclear compound [N(Bu)4][V(O)(S2)2bipy] (1) isolated at room temperature in crystalline form. The X-ray crystal structure determination shows that the vanadium(V) centre is ligated by four sulphur atoms and a nitrogen atom of the bipy ligand forming the equatorial plane of pentagonal bipyramid, an oxygen and the remaining nitrogen atom of the bipy occupying the two apices of the bipyramid. 相似文献
56.
The coupling of subcritical water separation with flame ionization detection (FID) in the split mode has been investigated in this study. In order to keep the FID system stable during subcritical water separation, a Tee union was connected between the separation column and the FID system to split the water flow. The ratio of the water flow to the FID system over the flow-rate to a waste bottle varied depending on the dimension of capillary tubings and the total water flow-rate used. Separations of several carbohydrates, carboxylic acids, and amino acids were performed on commercially available columns using a laboratory-made subcritical water chromatography-FID system. The FID system was very stable in this split mode even at total flow-rate as high as 1.24 ml/min. The linear dynamic range was up to three orders of magnitude and the limit of detection (LOD) ranged from 38 to 111 ng (306-925 ng/microl injected) with split ratios of approximately 1:10 to approximately 1:17 (FID/waste bottle) for several analytes studied. However, the LOD can be significantly lowered by adjusting the dimensions of the restrictors to allow a higher percentage of the total flow to the FID system. 相似文献
57.
Optically detected magnetic resonance (ODMR) techniques are used to determine the orbital assignment of the phosphorescent triplet state of p-bezonquinone in the condensed phase. The principal spin—orbit coupling routes are discussed in terms of the observed spin—vibronic activity. 相似文献
58.
Koren AB Curtis MD Francis AH Kampf JW 《Journal of the American Chemical Society》2003,125(17):5040-5050
Syntheses are reported of new 4,4'-dialkyl-2,2'-bithiazole oligomers that have alkenoxy side chains that are capable of easy conversion to oligomers with functionalized side chains, e.g., terminally substituted hydroxy chains. The crystal structures of two representative oligomers (4,4',4' ',4' "-tetra-(2-propenoxymethyl)-2,2',5',5' ',2' ',2' "-quaterthiazole (3P2) and 4,4',4' ',4' "-tetra-(3-hydroxypropyloxymethyl)-2,2',5',5' ',2' ',2' "-quaterthiazole (3H2)) were determined; 3P2 crystallizes in a pi-stacked motif with two molecules per unit cell, whereas 3H2 forms pi-stacks that are linked with hydrogen bonds to form infinite two-dimensional sheets with one molecule per unit cell. A comparison of the UV-vis spectra of the compounds in solution and in the solid state provides unequivocal evidence for the presence of a Davydov splitting, W(D) approximately 0.2 eV, in solid 3P2. The spectra are interpreted in the framework of molecular exciton theory to extract a value of the intermolecular transfer integral, J approximately 0.2 eV, for a total exciton bandwidth of ca. 0.8 eV. Monte Carlo calculations were used to determine the density of states of the exciton band and the absorption and emission line shapes of the 0 <-- 0 transition. It is suggested that the "three-humped" absorption profile typical of partially crystalline, regioregular polymers is the "optical signature" of pi-stacking. 相似文献
59.
Marine de Person Aude Sevestre P. Chaimbault Laurent Perrot Francis Duchiron Claire Elfakir 《Analytica chimica acta》2004,520(1-2):149-158
This paper reports the use of an on-line LC–ESI–MS/MS method for the identification and quantification of di- and tripeptides in champagne wine without laborious sample pretreatment. The identification of these compounds, in their underivatised form, is based on identical retention times and ESI–MS spectra to those of reference standards. The presence of nine dipeptides (Arg–Ile, Ile–Arg, Ile–Val, Lys–Phe, Lys–Tyr, Phe–Lys, Tyr–Gln, Tyr–Lys, Val–Ile) and the absence of two tripeptides (Phe–Arg–Arg and Lys–Met–Asn) have been evidenced in the matrix. Calibration curves for each analyte were established using Phe–Arg as internal standard. The calibration curves were linear in the concentration range 0.1–10 mg L−1 with a determination coefficient, r2, better than 0.992. The accuracy for the calibration standard was estimated at between 92 and 102%. This method allows high recovery and satisfies the necessary requirements with respect to accuracy, repeatability and sensitivity. The first application of this analytical method to the measurement of di- and tri-peptides in different vintages of champagne wine is reported. Compositional changes in the peptides occurred depending on the vintage. 相似文献
60.
2,2′-Biquinoline dioxide and 4,4′-dichloro-2,2′-biquinoline have been used for the preparation of the following 4,4′-disubstituted-2,2′-biquinolines: dimethoxy, diethoxy, dihydroxy, dipiperidino, dipyrrolidino, dibromo, diphenoxy, dithiophenoxy, di(p-chloro-phenoxy), di(p-bromophenoxy), di(p-fluorophenoxy), di(β-naphthoxy) and the dinitro dioxide. Molar extinction coefficients have been determined for several of the copper (I) complexes of these compounds. 相似文献