全文获取类型
收费全文 | 2977篇 |
免费 | 101篇 |
国内免费 | 22篇 |
专业分类
化学 | 1869篇 |
晶体学 | 20篇 |
力学 | 59篇 |
数学 | 584篇 |
物理学 | 568篇 |
出版年
2023年 | 14篇 |
2022年 | 22篇 |
2021年 | 72篇 |
2020年 | 70篇 |
2019年 | 81篇 |
2018年 | 67篇 |
2017年 | 58篇 |
2016年 | 121篇 |
2015年 | 107篇 |
2014年 | 129篇 |
2013年 | 224篇 |
2012年 | 192篇 |
2011年 | 205篇 |
2010年 | 143篇 |
2009年 | 147篇 |
2008年 | 180篇 |
2007年 | 173篇 |
2006年 | 141篇 |
2005年 | 115篇 |
2004年 | 93篇 |
2003年 | 71篇 |
2002年 | 78篇 |
2001年 | 32篇 |
2000年 | 38篇 |
1999年 | 28篇 |
1998年 | 17篇 |
1997年 | 21篇 |
1996年 | 36篇 |
1995年 | 27篇 |
1994年 | 27篇 |
1993年 | 20篇 |
1992年 | 29篇 |
1991年 | 16篇 |
1990年 | 26篇 |
1989年 | 13篇 |
1988年 | 19篇 |
1987年 | 14篇 |
1986年 | 8篇 |
1985年 | 19篇 |
1984年 | 25篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 14篇 |
1980年 | 18篇 |
1979年 | 10篇 |
1978年 | 21篇 |
1977年 | 21篇 |
1976年 | 12篇 |
1970年 | 6篇 |
1968年 | 8篇 |
排序方式: 共有3100条查询结果,搜索用时 328 毫秒
991.
Cassandra Millet-Boureima Roman Rozencwaig Felix Polyak Chiara Gamberi 《Molecules (Basel, Switzerland)》2020,25(22)
Autosomal dominant polycystic kidney disease (ADPKD) causes progressive cystic degeneration of the renal tubules, the nephrons, eventually severely compromising kidney function. ADPKD is incurable, with half of the patients eventually needing renal replacement. Treatments for ADPKD patients are limited and new effective therapeutics are needed. Melatonin, a central metabolic regulator conserved across all life kingdoms, exhibits oncostatic and oncoprotective activity and no detected toxicity. Here, we used the Bicaudal C (BicC) Drosophila model of polycystic kidney disease to test the cyst-reducing potential of melatonin. Significant cyst reduction was found in the renal (Malpighian) tubules upon melatonin administration and suggest mechanistic sophistication. Similar to vertebrate PKD, the BicC fly PKD model responds to the antiproliferative drugs rapamycin and mimics of the second mitochondria-derived activator of caspases (Smac). Melatonin appears to be a new cyst-reducing molecule with attractive properties as a potential candidate for PKD treatment. 相似文献
992.
Roman Ezhov Alireza Karbakhsh Ravari Yulia Pushkar 《Angewandte Chemie (International ed. in English)》2020,59(32):13502-13505
Hypervalent FeV=O species are implicated in a multitude of oxidative reactions of organic substrates, as well as in catalytic water oxidation, a reaction crucial for artificial photosynthesis. Spectroscopically characterized FeV species are exceedingly rare and, so far, were produced by the oxidation of Fe complexes with peroxy acids or H2O2: reactions that entail breaking of the O?O bond to form a FeV=O fragment. The key FeV=O species proposed to initiate the O?O bond formation in water oxidation reactions remained undetected, presumably due to their high reactivity. Here, we achieved freeze quench trapping of six coordinated [FeV=O,(OH)(Pytacn)]2+ (Pytacn=1‐(2′‐pyridylmethyl)‐4,7‐dimethyl‐1,4,7‐triazacyclononane) ( 2 ) generated during catalytic water oxidation. X‐ray absorption spectroscopy (XAS) confirmed the FeV oxidation state and the presence of a FeV=O bond at ≈1.60 Å. Combined EPR and DFT methods indicate that 2 contains a S=3/2 FeV center. 2 is the first spectroscopically characterized high spin oxo‐FeV complex and constitutes a paradigmatic example of the FeV=O(OH) species proposed to be responsible for catalytic water oxidation reactions. 相似文献
993.
Gerald Platzer Moriz Mayer Andreas Beier Sven Brüschweiler Julian E. Fuchs Harald Engelhardt Leonhard Geist Gerd Bader Julia Schrghuber Roman Lichtenecker Bernhard Wolkerstorfer Dirk Kessler Darryl B. McConnell Robert Konrat 《Angewandte Chemie (International ed. in English)》2020,59(35):14861-14868
While CH–π interactions with target proteins are crucial determinants for the affinity of arguably every drug molecule, no method exists to directly measure the strength of individual CH–π interactions in drug–protein complexes. Herein, we present a fast and reliable methodology called PI (π interactions) by NMR, which can differentiate the strength of protein–ligand CH–π interactions in solution. By combining selective amino‐acid side‐chain labeling with 1H‐13C NMR, we are able to identify specific protein protons of side‐chains engaged in CH–π interactions with aromatic ring systems of a ligand, based solely on 1H chemical‐shift values of the interacting protein aromatic ring protons. The information encoded in the chemical shifts induced by such interactions serves as a proxy for the strength of each individual CH–π interaction. PI by NMR changes the paradigm by which chemists can optimize the potency of drug candidates: direct determination of individual π interactions rather than averaged measures of all interactions. 相似文献
994.
Roman Abrams Jonathan Clayden 《Angewandte Chemie (International ed. in English)》2020,59(28):11600-11606
We report tandem alkyl‐arylations and phosphonyl‐arylations of vinyl ureas by way of a photocatalytic radical‐polar crossover mechanism. Addition of photoredox‐generated radicals to the alkene forms a new C?C or C?P bond and generates a product radical adjacent to the urea function. Reductive termination of the photocatalytic cycle generates an anion that undergoes a polar Truce–Smiles rearrangement, forming a C?C bond. The reaction is successful with a range of α‐fluorinated alkyl sodium sulfinate salts and diarylphosphine oxides as radical precursors, and the conformationally accelerated Truce–Smiles rearrangement is not restricted by the electronic nature of the migrating aromatic ring. Formally the reaction constitutes an α,β‐difuctionalisation of a carbon–carbon double bond, and proceeds under mild conditions with visible light and a readily available organic photocatalyst. The products are α,α‐diaryl alkylureas typically functionalized with F or P substituents that may be readily converted into α,α‐diaryl alkylamines. 相似文献
995.
The original Kissinger, Friedman, and modified Kissinger–Akahira–Sunose (KAS) methodologies were used to evaluate the apparent activation energy of theoretically simulated complex processes consisting of two overlapping independent JMA-like (Johnson–Mehl–Avrami-like) subprocesses. Three overlay series were studied, each representing one of the basic conceptual types of peak overlap. It was shown that, in the case of complex processes, both the tested isoconversion methods (KAS and Friedman) provide good qualitative information about the activation energies of the involved overlapping signals. However, from the quantitative point of view, the data are not easy to interpret, and deconvolution procedures are necessary for meaningful results to be obtained. On the other hand, in most cases, the apparent activation energy determined by the original Kissinger equation for the overlapped dominant peak corresponded very well to the true values determined for the sole processes. This suggests large robustness of the Kissinger methodology that even nowadays may be considered very advantageous and utilized in kinetic analysis of complex processes. 相似文献
996.
Patrycjusz Mańkowski Adam Dominiak Roman Domański Mirosław J. Kruszewski Łukasz Ciupiński 《Journal of Thermal Analysis and Calorimetry》2014,116(2):881-885
The thermal diffusivity (TD) and thermal conductivity (TC) of Cu–Cr–diamond composite materials were examined in the temperature range from 50 to 300 °C for diamond volume fractions of 22, 40, 50, 55, and 60 %. The samples were fabricated by the plasma pulse sintering (PPS) method. TC does not increase proportionally with the diamond fraction in the particular composite materials. The highest TD was determined for 50 % diamond volume fraction, and the evaluated TC reached 658 W m?1 K?1 at 50 °C. This article complements earlier articles concerning synthesis and characterization of the diamond–copper composites produced by the PPS method. 相似文献
997.
A New Type of Donor–Acceptor Cyclopropane Reactivity: The Generation of Formal 1,2‐ and 1,4‐Dipoles 下载免费PDF全文
Roman A. Novikov Anna V. Tarasova Victor A. Korolev Vladimir P. Timofeev Yury V. Tomilov 《Angewandte Chemie (International ed. in English)》2014,53(12):3187-3191
A new type of donor–acceptor cyclopropane reactivity has been discovered. On treatment with anhydrous GaCl3, they react as sources of even‐numbered 1,2‐ and 1,4‐dipoles instead of the classical odd‐numbered 1,3‐dipoles due to migration of positive charge from the benzyl center. This type of reactivity has been demonstrated for new reactions, namely, cyclodimerizations of donor–acceptor cyclopropanes that occur as [2+2]‐, [3+2]‐, [4+2]‐, [5+2]‐, [4+3]‐, and [5+4]‐annulations. The [4+2]‐annulation of 2‐arylcyclopropane‐1,1‐dicarboxylates to give polysubstituted 2‐aryltetralins has been developed in a preparative version that provides exceedingly high regio‐ and diastereoselectivity and high yields. The strategy for selective hetero‐combination of donor–acceptor cyclopropanes was also been developed. The mechanisms of the discovered reactions involving the formation of a comparatively stable 1,2‐ylide intermediate have been studied. 相似文献
998.
Palladium‐Catalyzed Chain‐Growth Polycondensation of AB‐type Monomers: High Catalyst Turnover and Polymerization Rates 下载免费PDF全文
Dr. Roman Tkachov Dr. Volodymyr Senkovskyy Dr. Tetyana Beryozkina Dr. Kseniya Boyko Prof. Dr. Vasiliy Bakulev Dr. Albena Lederer Dr. Karin Sahre Prof. Dr. Brigitte Voit Dr. Anton Kiriy 《Angewandte Chemie (International ed. in English)》2014,53(9):2402-2407
Chain‐growth catalyst‐transfer polycondensations of AB‐type monomers is a new and rapidly developing tool for the preparation of well‐defined π‐conjugated (semiconducting) polymers for various optoelectronic applications. Herein, we report the Pd/PtBu3‐catalyzed Negishi chain‐growth polycondensation of AB‐type monomers, which proceeds with unprecedented TONs of above 100 000 and TOFs of up to 280 s?1. In contrast, related AA/BB‐type step‐growth polycondensation proceeds with two orders of magnitude lower TONs and TOFs. A similar trend was observed in Suzuki‐type polycondensation. The key impact of the intramolecular (vs. intermolecular) catalyst‐transfer process on both polymerization kinetics and catalyst lifetime has been revealed. 相似文献
999.
Hydrosilylation Induced by N→Si Intramolecular Coordination: Spontaneous Transformation of Organosilanes into 1‐Aza‐Silole‐Type Molecules in the Absence of a Catalyst 下载免费PDF全文
Miroslav Novák Dr. Libor Dostál Dr. Mercedes Alonso Prof. Frank De Proft Dr. Aleš Růžička Prof. Antonín Lyčka Dr. Roman Jambor 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(9):2542-2550
Our attempts to synthesize the N→Si intramolecularly coordinated organosilanes Ph2L1SiH ( 1 a ), PhL1SiH2 ( 2 a ), Ph2L2SiH ( 3 a ), and PhL2SiH2 ( 4 a ) containing a CH?N imine group (in which L1 is the C,N‐chelating ligand {2‐[CH?N(C6H3‐2,6‐iPr2)]C6H4}? and L2 is {2‐[CH?N(tBu)]C6H4}?) yielded 1‐[2,6‐bis(diisopropyl)phenyl]‐2,2‐diphenyl‐1‐aza‐silole ( 1 ), 1‐[2,6‐bis(diisopropyl)phenyl]‐2‐phenyl‐2‐hydrido‐1‐aza‐silole ( 2 ), 1‐tert‐butyl‐2,2‐diphenyl‐1‐aza‐silole ( 3 ), and 1‐tert‐butyl‐2‐phenyl‐2‐hydrido‐1‐aza‐silole ( 4 ), respectively. Isolated organosilicon amides 1 – 4 are an outcome of the spontaneous hydrosilylation of the CH?N imine moiety induced by N→Si intramolecular coordination. Compounds 1–4 were characterized by NMR spectroscopy and X‐ray diffraction analysis. The geometries of organosilanes 1 a – 4 a and their corresponding hydrosilylated products 1 – 4 were optimized and fully characterized at the B3LYP/6‐31++G(d,p) level of theory. The molecular structure determination of 1 – 3 suggested the presence of a Si?N double bond. Natural bond orbital (NBO) analysis, however, shows a very strong donor–acceptor interaction between the lone pair of the nitrogen atom and the formal empty p orbital on the silicon and therefore, the calculations show that the Si?N bond is highly polarized pointing to a predominantly zwitterionic Si+N? bond in 1 – 4 . Since compounds 1 – 4 are hydrosilylated products of 1 a – 4 a , the free energies (ΔG298), enthalpies (ΔH298), and entropies (ΔH298) were computed for the hydrosilylation reaction of 1 a – 4 a with both B3LYP and B3LYP‐D methods. On the basis of the very negative ΔG298 values, the hydrosilylation reaction is highly exergonic and compounds 1 a – 4 a are spontaneously transformed into 1 – 4 in the absence of a catalyst. 相似文献
1000.
Miroslav Janicek Martina Polaskova Radek Holubar Roman Cermak 《Cellulose (London, England)》2014,21(6):4039-4048
Cellulose powders hydrophobized by surface esterification with carboxylic acids with different chain lengths (3, 10 and 18 carbons) were dispersed in a polypropylene matrix. Quality of the dispersion and nucleation activity of the filler were investigated by means of differential scanning calorimetry and optical microscopy. The results showed that the esterification decreases the crystallization rate in case of cellulose esterified with propionic or decanoic acid. On the other hand, the oleic acid ester demonstrated slightly higher crystallization rates than the unmodified cellulose, which was ascribed primarily to the newly arisen non-esterified surface after disintegration of the filler. Optical microscopy with hot stage showed the high nucleation ability of the natural cellulose fiber and its suppression in case of esterified surfaces. A complete inability to nucleate polypropylene crystallization was observed in case of decanoyl ester, while the other two retained some activity, but lower than that of the natural fiber. Finally, analysis of the filler dispersion and distribution revealed that the decanoyl and octadecanoyl esters disintegrate during melt mixing, while both dispersion and distribution of the fibers modified with propionic acid are poor. 相似文献