全文获取类型
收费全文 | 1136篇 |
免费 | 38篇 |
国内免费 | 6篇 |
专业分类
化学 | 958篇 |
晶体学 | 3篇 |
力学 | 10篇 |
数学 | 122篇 |
物理学 | 87篇 |
出版年
2023年 | 7篇 |
2022年 | 12篇 |
2021年 | 19篇 |
2020年 | 15篇 |
2019年 | 14篇 |
2018年 | 5篇 |
2017年 | 15篇 |
2016年 | 34篇 |
2015年 | 29篇 |
2014年 | 38篇 |
2013年 | 33篇 |
2012年 | 67篇 |
2011年 | 81篇 |
2010年 | 40篇 |
2009年 | 37篇 |
2008年 | 76篇 |
2007年 | 68篇 |
2006年 | 83篇 |
2005年 | 65篇 |
2004年 | 59篇 |
2003年 | 41篇 |
2002年 | 45篇 |
2001年 | 16篇 |
2000年 | 16篇 |
1999年 | 13篇 |
1998年 | 16篇 |
1997年 | 17篇 |
1996年 | 12篇 |
1995年 | 19篇 |
1994年 | 20篇 |
1993年 | 17篇 |
1992年 | 11篇 |
1991年 | 7篇 |
1990年 | 14篇 |
1989年 | 12篇 |
1988年 | 11篇 |
1987年 | 14篇 |
1986年 | 11篇 |
1985年 | 8篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 6篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1973年 | 3篇 |
1968年 | 2篇 |
排序方式: 共有1180条查询结果,搜索用时 0 毫秒
81.
Wiberg J Guo L Pettersson K Nilsson D Ljungdahl T Mårtensson J Albinsson B 《Journal of the American Chemical Society》2007,129(1):155-163
Optimizing the ratio of the rates for charge separation (CS) over charge recombination (CR) is crucial to create long-lived charge-separated states. Mastering the factors that govern the electron transfer (ET) rates is essential when trying to achieve molecular-scale electronics, artificial photosynthesis, and also for the further development of solar cells. Much work has been put into the question of how the donor-acceptor distances and donor-bridge energy gaps affect the electronic coupling, V(DA), and thus the rates of ET. We present here a unique comparison on how these factors differently influence the rates for CS and CR in a porphyrin-based donor-bridge-acceptor model system. Our system contains three series, each of which focuses on a separate charge-transfer rate-determining factor, the donor-acceptor distance, the donor-bridge energy gap, and last, the influence of the electron acceptor on the rate for charge transfer. In these three series both CS and CR are governed by superexchange interactions which make a CR/CS comparative study ideal. We show here that the exponential distance dependence increases slightly for CR compared to that for CS as a result of the increased tunneling barrier height for this reaction, in accordance with the McConnell superexchange model. We also show that the dependence on the tunneling barrier height is different for CS and CR. This difference is highly dependent on the electron acceptor and thus cannot solely be explained by the differences in the frontier orbitals of the electron donor in these porphyrin systems. 相似文献
82.
In this work the three dipeptides, Z-l-alanyl-l-glutaric acid (Z-l-ala-l-glu), Z-l-phenylanyl-l-glutaric acid (Z-l-phe-l-glu) and Z-glycyl-l-glutaric acid (Z-gly-l-glu) were tested as chiral counter ions for enantiomeric resolution of amino alcohols. The influence of solute and counter
ion structure upon retention and enantioselectivity was evaluated. The chiral counter ions were dissolved in a mixture of
polar solvents, i.e., ethyl acetate, methanol and acetonitrile and the achiral solid phase used was porous graphitic carbon,
marketed as Hypercarb. The enantioselectivities observed for the tested solutes were highly influenced by the used chiral
counter ion structure. For example no enantioselectivity was observed for (R,S)-alprenolol using Z-l-ala-l-glu while a separation factor (α) of 1.59 was obtained using Z-l-phe-l-glu as chiral counter ion. High selectivity factors (α > 2.7) were observed between enantiomers of tertiary amines using Z-l-phe-l-glu as counter ion. Interestingly, the structure of the counter ion, as well as the charge on Z-l-phe-l-glu and the mobile phase solvent composition, influenced the retention order of the enantiomers. 相似文献
83.
84.
Dominik Jacob Kathrin Thüring Aurellia Galliot Virginie Marchand Adeline Galvanin Akif Ciftci Karin Scharmann Michael Stock Jean‐Yves Roignant Sebastian A. Leidel Yuri Motorin Raffael Schaffrath Roland Klassen Mark Helm 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(28):9666-9670
85.
Hansen Karin Vels Kreka Kosova Jacobsen Torben 《Journal of Solid State Electrochemistry》2019,23(3):811-822
Journal of Solid State Electrochemistry - Local cathodic polarizations of yttria-stabilized zirconia were carried out with a PtIr probe as the working electrode in a controlled atmosphere high... 相似文献
86.
David C. Green Ulrich Englich Karin Ruhlandt‐Senge 《Angewandte Chemie (International ed. in English)》1999,38(3):354-357
An unprecedented ligand bending mode is displayed by the acetylide ligands in the first structurally characterized σ‐bound organometallic strontium and barium complexes [M([18]crown‐6)(CCSiPh3)2] (M=Sr, Ba). Furthermore, the observed decrease of the angle at the sp‐hybridized C atom on descending Group 2 (see structures depicted) affords new information that will lead to a better understanding of the bonding in alkaline earth metal compounds. 相似文献
87.
Kudoh T Fukuoka M Ichikawa S Murayama T Ogawa Y Hashii M Higashida H Kunerth S Weber K Guse AH Potter BV Matsuda A Shuto S 《Journal of the American Chemical Society》2005,127(24):8846-8855
We previously developed cyclic ADP-carbocyclic ribose (cADPcR, 2) as a stable mimic of cyclic ADP-ribose (cADPR, 1), a Ca(2+)-mobilizing second messenger. A series of the N1-ribose modified cADPcR analogues, designed as novel stable mimics of cADPR, which were the 2"-deoxy analogue 3, the 3"-deoxy analogue 4, the 3"-deoxy-2"-O-(methoxymethyl) analogue 5, the 3"-O-methyl analogue 6, the 2",3"-dideoxy analogue 7, and the 2",3"-dideoxydidehydro analogue 8, were successfully synthesized using the key intramolecular condensation reaction with phenylthiophosphate-type substrates. We investigated the conformations of these analogues and of cADPR and found that steric repulsion between both the adenine and N9-ribose moieties and between the adenine and N1-ribose moieties was a determinant of the conformation. The Ca(2+)-mobilizing effects were evaluated systematically using three different biological systems, i.e., sea urchin eggs, NG108-15 neuronal cells, and Jurkat T-lymphocytes. The relative potency of Ca(2+)-mobilization by these cADPR analogues varies depending on the cell-type used: e.g., 3"-deoxy-cADPcR (4) > cADPcR (2) > cADPR (1) in sea urchin eggs; cADPR (1) > cADPcR (2) approximately 3"-deoxy-cADPcR (4) in T-cells; and cADPcR (2) > cADPR (1) > 3"-deoxy-cADPcR (4) in neuronal cells, respectively. These indicated that the target proteins and/or the mechanism of action of cADPR in sea urchin eggs, T-cells, and neuronal cells are different. Thus, this study represents an entry to cell-type selective cADPR analogues, which can be used as biological tools and/or novel drug leads. 相似文献
88.
An extended family of aryl-substituted alkaline earth metal silylamides M{N(2,4,6-Me3C6H2)(SiMe3)}donor(n) was prepared using alkane elimination (Mg), salt elimination (Ca, Sr, Ba), and direct metalation (Sr, Ba). Three different donors, THF, TMEDA (TMEDA = N,N,N',N'-tetramethylethylenediamine), and PMDTA (PMDTA = N,N,N',N',N'-pentamethyldiethylenetriamine) were employed to study their influence on the coordination chemistry of the target compounds, producing monomeric species with the composition M{N(2,4,6-Me3C6H2)(SiMe3)}2(THF)2 (M = Mg, Ca, Sr, Ba), M{N(2,4,6-Me3C6H2)(SiMe3)}2TMEDA (M = Ca, Ba), and M{N(2,4,6-Me3C6H2)(SiMe3)}2PMDTA (M = Sr, Ba). For the heavier metal analogues, varying degrees of agostic interactions are completing the coordination sphere of the metals. Compounds were characterized using IR and NMR spectroscopy in addition to X-ray crystallography. 相似文献
89.
90.
A laboratory system for the on-line monitoring of important lactic acid fermentation variables is described. The system contains flow-injection analysers for glucose, lactose, galactose, lactate and protein and a continuous-flow analyser for the biomass concentration. The sugar and lactate analysers are based on enzymatic reactions involving oxidases followed by chemiluminescence detection of the hydrogen peroxide formed. The protein analyser is based on the biuret reaction. The system has been used to monitor many fermentation experiments, and some results are presented as examples. 相似文献