全文获取类型
收费全文 | 2132篇 |
免费 | 67篇 |
国内免费 | 15篇 |
专业分类
化学 | 1541篇 |
晶体学 | 28篇 |
力学 | 68篇 |
数学 | 185篇 |
物理学 | 392篇 |
出版年
2023年 | 11篇 |
2022年 | 23篇 |
2021年 | 52篇 |
2020年 | 46篇 |
2019年 | 56篇 |
2018年 | 47篇 |
2017年 | 48篇 |
2016年 | 91篇 |
2015年 | 60篇 |
2014年 | 118篇 |
2013年 | 181篇 |
2012年 | 146篇 |
2011年 | 175篇 |
2010年 | 98篇 |
2009年 | 71篇 |
2008年 | 122篇 |
2007年 | 125篇 |
2006年 | 101篇 |
2005年 | 105篇 |
2004年 | 61篇 |
2003年 | 52篇 |
2002年 | 50篇 |
2001年 | 16篇 |
2000年 | 20篇 |
1999年 | 14篇 |
1998年 | 23篇 |
1997年 | 16篇 |
1996年 | 17篇 |
1995年 | 16篇 |
1994年 | 14篇 |
1993年 | 9篇 |
1992年 | 10篇 |
1991年 | 18篇 |
1990年 | 11篇 |
1989年 | 11篇 |
1988年 | 16篇 |
1987年 | 14篇 |
1986年 | 6篇 |
1985年 | 16篇 |
1984年 | 14篇 |
1983年 | 7篇 |
1982年 | 9篇 |
1981年 | 14篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 9篇 |
1976年 | 10篇 |
1975年 | 7篇 |
1964年 | 6篇 |
排序方式: 共有2214条查询结果,搜索用时 545 毫秒
51.
Summary The absorption spectra (in the visible and ultraviolet) of the complexes formed in absolute methanol between Zr4+, V4+, Y3+ and Sb3+ with 3-hydroxyflavone, apigenin, hesperidin, naringenin, morin, kaempferol, quercetin, rutin and myricetin have been studied. Zr4+ and Sb3+ form complexes that are stable in acid medium. The stoichiometry of the vairous Zr4+-flavonoid complexes formed in methanol and methanol/HClO4 media has been determined by the molar ratio method. The preferred sites for complex formation of flavonoids with Zr4+ are postulated. Zr4+ forms chelates with the 3-hydroxy-4-keto and 5-hydroxy-4-keto systems simultaneously. Sb3+ forms complexes only with the 3,5-dihydroxy system in flavonoids. The results permit estimation of the relative order of chelating power of each type of binding site and are useful in elucidation of the structure of the flavonoids.
Verwendung von Zirkonium(III) und Antimon(III) für die Erforschung der Flavonoide
Zusammenfassung Die Absorptions-Spektren (im sichtbaren und UV-Bereich) der in absolutem Methanol hergestellten Komplexe von Zr4+, V4+, Y3+ und Sb3+ mit 3-Hydroxyflavon, Apigenin, Hesperidin, Naringenin, Morin, Kaempferol, Quercetin, Rutin und Myricetin wurden untersucht. Zr4+ und Sb3+ bilden in saurem Milieu beständige Komplexe. Die Stöchiometrie der verschiedenen Zr4+-Flavonoid-Komplexe, die in Methanol bzw. Methanol/ HClO4 hergestellt wurden, wurde durch Bestimmung der Molarverhältnisse ermittelt. Die für die Komplexbildung bevorzugten Stellen (im Molekül) wurden angegeben. Zr4+ bildet Chelate mit der 3-Hydroxy-4-keto- und mit der 5-Hydroxy~4-ketogruppe. Sb3+ bildet solche Komplexe nur mit der 3,5-Dihydroxy-4-keto-Gruppe in Flavonoiden. Die Untersuchungsergebnisse ermöglichen die Abschätzung der Komplexbildungsfähigkeit der verschiedenen Gruppierungen und sind für die Strukturaufklärung von Nutzen.相似文献
52.
B. Venkateswara Reddy K. V. N. Suresh Reddy J. Sreeramulu G. V. Kanumula 《Chromatographia》2007,66(1-2):111-114
A rapid, specific reversed phase HPLC method has been developed for simultaneous determination of olanzapine and fluoxetine
in their formulations. Chromatographic separation of these two pharmaceuticals was carried out on an Inertsil C18 reversed phase column (150 mm × 4.6 mm, 5 μm) with a 40:30:30 (v/v/v) mixture of 9.5 mM sodium dihydrogen phosphate (pH adjusted to 6.8 ± 0.1 with triethylamine), acetonitrile and methanol as
mobile phase. The flow rate 1.2 mL min−1 and the analytes are monitored at 225 nm. Paroxetine was used as internal standard. The assay results were linear from 25
to 75 μg mL−1 for olanzapine (r
2 ≥ 0.995) and 100–300 μg mL−1 for fluoxetine (r
2 ≥ 0.995), showed intra- and inter-day precision less than 1.0%, and accuracy of 97.7–99.1% and 97.9–99.0%. LOQ was 0.005
and 0.001 μg mL−1 for olanzapine and fluoxetine, respectively. Separation was complete in less than 10 min. Validation of the method showed
it to be robust, precise, accurate and linear over the range of analysis. 相似文献
53.
[structures: see text] A range of cis-fused hexahydro-1-benzofuran-3(2H)-one or tetrahydro-2H-cyclopenta[b]furan-3(3aH)-one ring systems was synthesized by the rhodium(II) acetate catalyzed reaction of alpha-diazo carbonyl compounds in the presence of various oxygen, nitrogen, and sulfur nucleophiles. A double-nucleophilic addition was possible by using an excess of alpha-diazo ketone. These reactions proceeded with complete diastereoselectivity, and the stereochemistry was confirmed by the single-crystal X-ray analysis. This process discloses the first example of tandem cyclization-nucleophilic addition reaction. 相似文献
54.
55.
The ab initio calculation of the title compound was carried out at HF as well as DFT level of theory. The full geometry optimization of the ligand was carried out using 6-31G(d) basis set. The results obtained were correlated with the single crystal X-ray data, also reported in this paper, shows close resemblance between these two. The influence of electron correlation effects also was studied by carrying out geometry optimization at the MP2 level. The attempts were also made to ascertain the most stable tautomer of the said compound. 相似文献
56.
KailasaSuresh Kumar Kanchi Suvardhan Dasari Rekha B. Jayaraj Pattium Chiranjeevi 《Helvetica chimica acta》2005,88(5):1022-1027
A facile, rapid, and sensitive spectrophotometric method for the determination of propoxur in insecticidal formulations, fortified water, vegetables, agricultural wastewater, and agricultural soil samples has been elaborated. The proposed method is based on the hydrolysis of propoxur under basic conditions, followed by instantaneous azo coupling of the resulting 2‐isopropoxyphenol with the anilines 2a – c . This yielded the orange‐red chromophore 3a (λmax=at 470 nm), the pale‐red coupling product 3b (490 nm), or the red derivative 3c (478 nm), which are stable for 46 h, 38 h, and 24 h, respectively, and could be readily analyzed spectrophotometrically. 相似文献
57.
Devendra K. Rastogi Suresh K. Sahni Vidya B. Rana Satendra K. Dua 《Transition Metal Chemistry》1978,3(1):56-60
Summary Dimeric manganese(II)and iron(II)complexes, (ML)2, derived from benzoyl hydrazones ofo-hydroxyaryl aldehydes and ketones arc described and characterised by elemental analyses and by conductance, molecular weight, magnetic, electronic and i.r. spectral measurements. The dimeric nature of the complexes is revealed by i.r. spectra which show bands atca. 885 Mn2+ and 820 cm–1 Fe2+, characteristic of ring vibrations. The i.r. spectra reveal the terdentate nature of the ligands. The electronic spectra in dimethylformamide are consistent with the tetrahedral nature of the complexes. The appreciable lowering in
eff is attributed to the presence of exchange interactions between two paramagnetic atomsvia oxygen bridges.Reprints of this article are not available. 相似文献
58.
D'Souza F Chitta R Gadde S Zandler ME McCarty AL Sandanayaka AS Araki Y Ito O 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(15):4416-4428
Two types of structurally well-defined, self-assembled zinc porphyrin-fullerene conjugates were formed by "two-point" binding strategies to probe the effect of axial ligation or pi-pi-type interactions on the photochemical charge stabilization in the supramolecular dyads. To achieve this, meso-tetraphenylporphyrin was functionalized to possess one or four [18]crown-6 moieties at different locations on the porphyrin macrocycle while fullerene was functionalized to possess an alkyl ammonium cation, and a pyridine or phenyl entities. As a result of the crown ether-ammonium cation complexation, and zinc-pyridine coordination or pi-pi-type interactions, stable zinc porphyrin-fullerene conjugates with defined distance and orientation were formed. Evidence for the zinc-pyridine complexation or pi-pi-type interactions was obtained from the spectral and computational studies. Steady-state and time-resolved emission studies revealed efficient quenching of the zinc-porphyrin singlet excited state in these dyads, and the measured rates of charge separation, k(CS) were found to be slightly better in the case of the dyads held by axial coordination and crown ether-cation complexation. Nanosecond transient absorption studies provided evidence for the electron transfer reactions, and these studies also revealed charge stabilization in these dyads. The lifetimes of the radical ion pairs were found to depend upon the type of porphyrins utilized to form the dyads, that is, porphyrin possessing the crown ether moiety at the ortho position of one of the phenyl rings yielded prolonged charge stabilized states. Addition of pyridine to the supramolecular dyads eliminated the zinc-pyridine coordination or pi-pi-type interactions of the "two-point" bound systems due to the formation of a new zinc-pyridine axial bond thus giving a unique opportunity to probe the effect of axial coordination or pi-pi interactions on k(CS) and k(CR). Under these conditions, the measured electron transfer rates revealed faster k(CS) and slower k(CR) as compared to those obtained in the absence of added pyridine. The evaluated lifetimes of the radical ion-pairs were found to be hundreds of nanoseconds and were longer in the presence of pyridine. 相似文献
59.
Bhatia SK Tran K Nguyen TX Nicholson D 《Langmuir : the ACS journal of surfaces and colloids》2004,20(22):9612-9620
We present new simulation results for the packing of single-center and three-center models of carbon dioxide at high pressure in carbon slit pores. The former shows a series of packing transitions that are well described by our density functional theory model developed earlier. In contrast, these transitions are absent for the three-center model. Analysis of the simulation results shows that alternations of flat-lying molecules and rotated molecules can occur as the pore width is increased. The presence or absence of quadrupoles has negligible effect on these high-density structures. 相似文献
60.
Suresh Chandra Rastogi 《Journal of separation science》1995,18(10):653-658
A gas chromatographic (GC)-mass spectrometric (GC-MS) method has been developed for the routine analysis of 11 fragrance substances in cosmetics: cinnamic alcohol, cinnamic aldehyde, eugenol, hydroxy citronellal, α-amyl cinnamic aldehyde, geraniol, isoeugenol, coumarin, dihydrocoumarin, citronellal and citral. Methods for sample preparation of various types of cosmetic products, prior to GC analysis, have also been developed and proved to be rugged. Detection limits of all of target fragrance substances were approximately 1 ppm. Calibration curves of the target fragrance substances analyzed by GC were found to be linear in the investigated concentration range, 0.005% – 0.50%. The recoveries of the target fragrances from various types of cosmetic products were 80% – 116% and the relative standard deviations of the quantitative analysis of the target fragrance substances were within 5%. 相似文献