全文获取类型
收费全文 | 592篇 |
免费 | 18篇 |
国内免费 | 3篇 |
专业分类
化学 | 416篇 |
晶体学 | 2篇 |
力学 | 11篇 |
数学 | 90篇 |
物理学 | 94篇 |
出版年
2022年 | 9篇 |
2021年 | 9篇 |
2020年 | 8篇 |
2019年 | 10篇 |
2018年 | 8篇 |
2017年 | 4篇 |
2016年 | 20篇 |
2015年 | 17篇 |
2014年 | 18篇 |
2013年 | 39篇 |
2012年 | 38篇 |
2011年 | 36篇 |
2010年 | 23篇 |
2009年 | 26篇 |
2008年 | 32篇 |
2007年 | 28篇 |
2006年 | 34篇 |
2005年 | 30篇 |
2004年 | 27篇 |
2003年 | 27篇 |
2002年 | 17篇 |
2001年 | 7篇 |
2000年 | 5篇 |
1999年 | 8篇 |
1998年 | 9篇 |
1997年 | 4篇 |
1996年 | 7篇 |
1995年 | 10篇 |
1994年 | 7篇 |
1993年 | 8篇 |
1992年 | 11篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1973年 | 5篇 |
1972年 | 2篇 |
1938年 | 2篇 |
排序方式: 共有613条查询结果,搜索用时 15 毫秒
591.
Catalyst Activation,Deactivation, and Degradation in Palladium‐Mediated Negishi Cross‐Coupling Reactions 下载免费PDF全文
Katharina Böck Dr. Julia E. Feil Prof. Konstantin Karaghiosoff Prof. Konrad Koszinowski 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(14):5548-5560
Pd‐mediated Negishi cross‐coupling reactions were studied by a combination of kinetic measurements, electrospray‐ionization (ESI) mass spectrometry, 31P NMR and UV/Vis spectroscopy. The kinetic measurements point to a rate‐determining oxidative addition. Surprisingly, this step seems to involve not only the Pd catalyst and the aryl halide substrate, but also the organozinc reagent. In this context, the ESI‐mass spectrometric observation of heterobimetallic Pd–Zn complexes [L2PdZnR]+ (L=S‐PHOS, R=Bu, Ph, Bn) is particularly revealing. The inferred presence of these and related neutral complexes with a direct Pd–Zn interaction in solution explains how the organozinc reagent can modulate the reactivity of the Pd catalyst. Previous theoretical calculations by González‐Pérez et al. (Organometallics 2012 , 31, 2053) suggest that the complexation by the organozinc reagent lowers the activity of the Pd catalyst. Presumably, a similar effect also causes the rate decrease observed upon addition of ZnBr2. In contrast, added LiBr apparently counteracts the formation of Pd–Zn complexes and restores the high activity of the Pd catalyst. At longer reaction times, deactivation processes due to degradation of the S‐PHOS ligand and aggregation of the Pd catalyst come into play, thus further contributing to the appreciable complexity of the title reaction. 相似文献
592.
Robustness of a method based on the serial coupling of reversed‐phase and zwitterionic hydrophilic interaction LC–MS for the analysis of phenols 下载免费PDF全文
We have recently proved that the serial coupling of RP and zwitterionic hydrophilic interaction LC with mass spectrometric detection is a versatile and reliable technique to simultaneously separate polar and apolar compounds in complex samples, for example, phenols in wine. In order to evaluate the system suitability for long‐term usage, the robustness of a method based on the serial coupling of RP and zwitterionic hydrophilic interaction LC was evaluated after one year of analyses comprising >1100 injections. The retention time and peak shape of phenol standards and phenols in a red wine were compared to the values previously published. Phenol retention times were shifted <30 s. However, the peak widths were increased, partially due to the deterioration of the stationary phases. Nevertheless, the method was still highly reliable for the analysis of phenols in wine. 相似文献
593.
Bedford RB Hazelwood SL Limmert ME Albisson DA Draper SM Scully PN Coles SJ Hursthouse MB 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(14):3216-3227
Bulky triarylphosphite ligands undergo facile orthometallation reactions with palladium and platinum precursors. The crystal structure of an example of the resultant palladacycles has been determined. The reactivity of some of the metallacycles with HCl, monodentate and bidentate phosphines and sodium diethyldithiocarbamate has been investigated, and the crystal structure of a diethyldithiocarbamate adduct of a palladacycle is presented. The palladacyclic complexes prove to be extremely active catalysts for the Suzuki coupling of aryl bromides with aryl boronic acids. They can also be used as catalysts for the coupling of alkylboronic acids. Meanwhile di- and trialkyl phosphine adducts of one of the palladacycles shows very high activity in the Suzuki coupling of aryl chlorides and can also be used to good effect for the Stille coupling of these substrates. The role of the phosphite ligand in the Suzuki coupling of aryl chlorides seems to be one of increasing catalyst longevity by stabilisation of the Pd(0) resting state. 相似文献
594.
Florian Feil 《Journal of organometallic chemistry》2003,683(1):56-63
α-Methyl-benzylcalcium complexes were prepared analogue to α-Me3Si-benzylcalcium complexes for which procedures were reported earlier. The crystal structures of homoleptic bis(2-Me2N-α-Me-benzyl)calcium·(THF)2 and heteroleptic (9-Me3Si-fluorenyl)(2-Me2N-α-Me-benzyl)calcium·(THF) were determined. For both compounds only one of the two diastereomers crystallized. Barriers for inversion of the chiral benzylic carbon were estimated by variable temperature NMR spectroscopy. The α-methyl-benzylcalcium compounds are less stable and show a higher reactivity and faster initiation of styrene polymerization than the analogue α-Me3Si-benzylcalcium complexes. Intramolecular CH activation in a heteroleptic α-methyl-benzylcalcium complex was observed and the product, a calcium complex with a dianionic alkyl/fluorenyl ansa-ligand, was characterized by crystal structure determination. 相似文献
595.
596.
Gesa Schfer Jelena Mili Adeeb Eldahshan Frank Gtz Kerstin Zühlke Christian Schillinger Annika Kreuchwig Jonathan M. Elkins Kamal R. AbdulAzeez Andreas Oder Marie C. Moutty Nanako Masada Monika Beerbaum Brigitte Schlegel Sylvia Niquet Peter Schmieder Gerd Krause Jens Peter vonKries Dermot M. F. Cooper Stefan Knapp Jrg Rademann Walter Rosenthal Enno Klussmann 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2013,125(46):12409-12413
597.
Yue Lu Dr. Niamh McGoldrick Dr. Frances Murphy Dr. Brendan Twamley Xiaoneng Cui Dr. Colm Delaney Dr. Gearóid M. Ó Máille Junsi Wang Prof. Dr. Jianzhang Zhao Prof. Dr. Sylvia M. Draper 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(32):11349-11356
A series of IrIII complexes, based on 1,10‐phenanthroline featuring aryl acetylene chromophores, were prepared and investigated as triplet photosensitizers. The complexes were synthesized by Sonogashira cross‐coupling reactions using a “chemistry‐on‐the‐complex” method. The absorption properties and luminescence lifetimes were successfully tuned by controlling the number and type of light‐harvesting group. Intense UV/Vis absorption was observed for the IrIII complexes with two light‐harvesting groups at the 3‐ and 8‐positions of the phenanthroline. The asymmetric IrIII complex (with a triphenylamine (TPA) and a pyrene moiety attached) exhibited the longest lifetime. Red emission was observed for all the complexes in deaerated solutions at room temperature. Their emission at low temperature (77 K) and nanosecond time‐resolved transient difference absorption spectra revealed the origin of their triplet excited states. The singlet‐oxygen (1O2) sensitization and triplet‐triplet annihilation (TTA)‐based upconversion were explored. Highly efficient TTA upconversion (ΦUC=28.1 %) and 1O2 sensitization (ΦΔ=97.0 %) were achieved for the asymmetric IrIII complex, which showed intense absorption in the visible region (λabs=482 nm, ?=50900 m ?1 cm?1) and had a long‐lived triplet excited state (53.3 μs at RT). 相似文献
598.
Giambattista Consiglio Sylvia C. A. Nefkens Carmelina Pisano Fritz Wenzinger 《Helvetica chimica acta》1991,74(2):323-325
The regioselectivity of the hydro-alkoxycarbonylation of methyl acrylate, methacrylate, and crotonate catalyzed by [PdCl2L2] complexes (L = phosphine ligands) can be largely controlled by variation of the ligands. PPh3, promotes preferential carbonylation at the α-position, whereas with [(2,2-dimethyl-1,3-dioxolane-4,5-diyl)bis-(methylene)]bis[diphenylphosphine] as ligand, the β-position is overwhelmingly carbonylated. 相似文献
599.
The intramolecular Diels-Alder reactions of hydrazones derived from methacrolein or ethylacrolein and terminally unsaturated N-acyl-N-methylhydrazines have been investigated. The hydrazones 7b and 7c derived from N-methyl-N-pent-4-enoylhydrazine 3b were found to undergo intramolecular [4 + 2] cycloaddition above 140 °C and the pyridopyridazines 12 were isolated. The corresponding hydrazones 8b and 8c from N-methyl N-pent-4-ynoylhydrazone 4a reacted similarly and gave as the final products the pyridines 13. The scope of the reaction is limited, as was shown by the failure of several other terminally unsaturated hydrazones of β-unsaturated aldehydes to undergo intramolecular cycloaddition. These hydrazones did, however, undergo intermolecular [4 + 2] cyctoaddition to N-phenylmaleimide. Other hydraiones 15 of methacrolein. including the benzoylhydrazone and the phenylhydrazone, also reacted with N-phenylmaleimide to give the pyridine 14b by way of an isolable dihydropyridine 16. 相似文献
600.
A voltammetric stripping procedure is described for the determination of arsenic(V) in a mannitol-sulphuric acid medium.
The arsenic is coprecipitated with copper and selenium and reduced to arsine at the hanging mercury drop electrode. Using
an accumulation time of 240 s, the detection limit is 0.52 μg L–1, the determination limit is 0.9 μg L–1. The method has been applied to the determination of arsenic in water samples. By varying the composition of the supporting
electrolyte it is possible to differentiate between arsenic(III) and arsenic(V). As both oxidation states have different toxicological
characteristics, the ability to discriminate between both is an distinct advantage of the proposed method.
Received: 25 October 1996 / Revised: 7 February 1997 / Accepted: 12 February 1997 相似文献