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201.
Regioselective formylation behavior has been found in the reaction of pyrazolo[3,4-b]pyridines and pyrazolo[1,5-a]pyrimidines via Vilsmeier-Haack conditions. While the 4,5- and 6,7-dihydro derivatives afforded pyrazolo[3,4-b]pyridine-5-carbaldehydes and 4,7-dihydropyrazolo[1,5-a]pyrimidine-3,6-dicarbaldehydes, respectively, the aromatic analogs rendered the pyrazolo[1,5-a]pyrimidine-3-carbaldehyde only, and no reaction took place at the pyrazolopyridine derivatives. 相似文献
202.
Jesus Garcia-Diaz Jairo Sanchez-Hernandez Ricardo Menchaca-Mendez Rolando Menchaca-Mendez 《Journal of Heuristics》2017,23(5):349-366
The vertex k-center selection problem is a well known NP-Hard minimization problem that can not be solved in polynomial time within a \(\rho < 2\) approximation factor, unless \(P=NP\). Even though there are algorithms that achieve this 2-approximation bound, they perform poorly on most benchmarks compared to some heuristic algorithms. This seems to happen because the 2-approximation algorithms take, at every step, very conservative decisions in order to keep the approximation guarantee. In this paper we propose an algorithm that exploits the same structural properties of the problem that the 2-approximation algorithms use, but in a more relaxed manner. Instead of taking the decision that guarantees a 2-approximation, our algorithm takes the best decision near the one that guarantees the 2-approximation. This results in an algorithm with a worse approximation factor (a 3-approximation), but that outperforms all the previously known approximation algorithms on the most well known benchmarks for the problem, namely, the pmed instances from OR-Lib (Beasly in J Oper Res Soc 41(11):1069–1072, 1990) and some instances from TSP-Lib (Reinelt in ORSA J Comput 3:376–384, 1991). However, the \(O(n^4)\) running time of this algorithm becomes unpractical as the input grows. In order to improve its running time, we modified this algorithm obtaining a \(O(n^2 \log n)\) heuristic that outperforms not only all the previously known approximation algorithms, but all the polynomial heuristics proposed up to date. 相似文献
203.
Gold(I)‐Catalyzed Addition of Silylacetylenes to Acylsilanes: Synthesis of Indanones by CH Functionalization through a Gold(I) Carbenoid
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Jairo González Dr. Javier Santamaría Prof. Dr. Alfredo Ballesteros 《Angewandte Chemie (International ed. in English)》2015,54(46):13678-13681
A gold(I)‐catalyzed synthesis of indanones from trimethylsilylacetylenes and acylsilanes is presented. The reaction is initiated through a synergistic acylsilane activation–gold acetylide formation and involves consecutive alkyne σ‐gold(I) addition, π‐activation, and 1,2‐migration of a silyl group. Studies performed on the reaction mechanism allowed to establish the nature of the silyl migrating group and invoke the participation of a gold(I) carbenoid intermediate. The reaction is completed by a gold(I) C? H functionalization step. 相似文献
204.
Gas‐Phase Elimination Reaction of Ethyl (5‐cyanomethyl‐1,3,4‐thiadiazol‐2‐yl)carbamate: A Computational Study
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The gas‐phase elimination reaction of ethyl (5‐cyanomethyl‐1,3,4‐thiadiazol‐2‐yl)carbamate has been studied computationally at the MP2/6–31++G(2d,p) level of theory. The values of the activation parameters and rate constants for the thermal decomposition were evaluated over a temperature range from 405.0 to 458.0 K. The temperature dependence of the rate constants was used to deduce the modified Arrhenius expression: log k405–458 K = (9.01 ± 0.49) + (1.32 ± 0.16) log T – (6946 ± 30) 1/T, which is in good agreement with the expression obtained from experimental data. The results confirm that the mechanism is a cis‐concerted elimination that occurs in two steps: The first one corresponds to the formation of ethylene and an intermediate, 5‐(cyanomethyl)‐1,3,4‐thiadiazol‐2‐yl‐carbamic acid, via a six‐membered cyclic transition state, and the second one is the decarboxylation of this intermediate via a four‐membered cyclic transition step, leading to carbon dioxide and the corresponding 1,3,4‐thiadiazole derivative (5‐amino‐1,3,4‐thiadiazole‐2‐acetonitrile). The connectivity of transition states with their respective minima was verified through intrinsic reaction coordinate calculations, and the progress of the reaction was followed by means of Wiberg bond indices, resulting that both transition states have an “early” character, nearer to the reactants than to the products. 相似文献
205.
Jairo Quiroga Jorge Trilleras Justo Cobo Christopher Glidewell 《Acta Crystallographica. Section C, Structural Chemistry》2010,66(1):o47-o49
In the title compound, C11H11N3O·0.5H2O, the water molecule lies across a twofold rotation axis in the space group Pbcn. The bond distances in the organic component provide evidence for polarization of the electronic structure. The molecular components are linked into puckered sheets of R108(34) rings by a combination of O—H...N and N—H...O hydrogen bonds; adjacent sheets are weakly linked by an aromatic π–π stacking interaction. Comparisons are made with some fused‐ring analogues. 相似文献
206.
Jaime A. Glvez Jairo Quiroga Justo Cobo Christopher Glidewell 《Acta Crystallographica. Section C, Structural Chemistry》2010,66(10):o521-o523
Molecules of the title compound, C9H10N4O3, (I), are linked into complex sheets by a combination of one N—H...O hydrogen bond and two C—H...O hydrogen bonds. Comparisons are drawn between (I) and some related compounds in respect of both their molecular conformations and their hydrogen‐bonding arrangements. 相似文献
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210.
Braulio Insuasty Diana Becerra Jairo Quiroga Rodrigo Abonia Manuel Nogueras Justo Cobo 《Journal of heterocyclic chemistry》2013,50(3):506-512
The 5,6,7,8,9,10‐hexahydro‐2‐methylthiopyrimido[4,5‐b]quinolines 4a , 4b , 4c , 4d , 5a , 5b , 5c , 5d and their oxidized forms 6a , 6b , 6c , 6d , 7a , 7b , 7c , 7d were obtained from the reaction of 6‐amino‐2‐(methylthio)pyrimidin‐4(3H)‐one 2 or 6‐amino‐3‐methyl‐2‐(methylthio)pyrimidin‐4(3H)‐one 3 and α,β‐unsaturated ketones 1a , 1b , 1c , 1d using BF3.OEt2 as catalyst and p‐chloranil as oxidizing agent. Some of the new compounds were evaluated in the US National Cancer Institute (NCI), where compound 5a presented remarkable activity against 46 cancer cell lines, with the most important GI50 values ranging from 0.72 to 18.4 μM from in vitro assays. 相似文献