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排序方式: 共有560条查询结果,搜索用时 31 毫秒
101.
Brandon Carter Richard A. Battye Jean–Philippe Uzan 《Communications in Mathematical Physics》2003,235(2):289-311
The computation of long range linear self-interaction forces in string and higher dimensional brane models requires the evaluation
of the gradients of regularised values of divergent self-interaction potentials. It is shown that the appropriately regularised
gradient in directions orthogonal to the brane surface will always be obtainable simply by multiplying the regularised potential
components by just half the trace of the second fundamental tensor, except in the hypermembrane case for which the method
fails. Whatever the dimension of the background this result is valid provided the codimension is two (the hyperstring case)
or more, so it can be used for investigating brane-world scenarios with more than one extra space dimension.
Received: 15 April 2002 / Accepted: 23 October 2002 Published online: 25 February 2003
Communicated by H. Nicolai 相似文献
102.
Brandon EF van Ooijen RD Sparidans RW Lázaro LL Heck AJ Beijnen JH Schellens JH 《Journal of mass spectrometry : JMS》2005,40(6):821-831
The cyclic depsipeptide aplidine is a new anti-cancer drug of marine origin. Four metabolites of this compound were found after incubation with pooled human microsomes using gradient high-performance liquid chromatography with ultraviolet detection. After chromatographic isolation, the metabolites have been identified using nano-electrospray triple quadrupole mass spectrometry. A highly specific sodium-ion interaction with the cyclic structure opens the depsipeptide ring, and cleavage of the amino acid residues gives sequence information when activated by collision-induced dissociation in the second quadrupole. The aplidine molecule could undergo the following metabolic reactions: hydroxylation at the isopropyl group (metabolites apli-h 1 and apli-h 2); C-dealkylation at the N(Me)-leucine group (metabolite apli-da); hydroxylation at the isopropyl group and C-dealkylation at the N(Me)-leucine group (metabolite apli-da/h), and C-demethylation at the threonine group (metabolite apli-dm). The identification of these metabolites formed in vitro may greatly aid the elucidation of the metabolic pathways of aplidine in humans. 相似文献
103.
Cinchona alkaloids catalyze the enantioselective Mannich reaction of beta-keto esters with acyl aryl imines. The reaction requires 10 mol % of cinchonine or cinchonidine. The reaction products are obtained in good yields (81-99%), high enantioselectivities (80-96% ee), and in diastereoselectivities that range from 1:1 to >95:5. The cinchonine-catalyzed reaction provides access to highly functionalized building blocks used in the asymmetric synthesis of a dihydropyrimidone and beta-amino alcohol. 相似文献
104.
Prolonged, extensive exposure to asphalt fume has been associated with several adverse health effects. Inhaled polycyclic aromatic hydrocarbons (PAHs) from asphalt fume exposure have been suspected of inducing such effects. In this study, a bioanalytical method was proposed and evaluated to identify and quantify benzo[a]pyrene and its hydroxy-metabolites. This method is based on coupling a microflow liquid chromatography (LC) to a hybrid quadrupole orthogonal acceleration time-of-flight mass spectrometry (Q-TOFMS). In the experiment, thirty-two B6C3FI mice were exposed to asphalt fume in a whole body inhalation chamber for 10 days (4 h day(-1)) and twelve other mice were used as controls. The asphalt fume was generated at 180 degrees C and the concentrations in the animal exposure chamber ranged 175-182 mg m(-3). Benzo[a]pyrene and its metabolites of 3-hydroxybenzo[a]pyrene, benzo[a]pyrene-7,8-dihydrodiol(+/-), benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide(+/-), and benzo[a]pyrene-7,8,9,10-tetrahydrotetrol(+/-) in the urine of asphalt fume exposed mice were identified and found at 3.18 ng 100 mL(-1), 31.36 ng 100 mL(-1), 11.56 ng 100 mL(-1), 54.92 ng 100 mL(-1), and 45.23 ng 100 mL(-1) respectively. The results revealed that the urinary benzo[a]pyrene and its hydroxy-metabolites from exposed mice were at significantly higher levels (p < 0.001) than those from the control groups. Compared with several other technologies such as HPLC-UV and HPLC-fluorescence, the new method is more sensitive and selective, and it can also provide additional useful information on the structures of the metabolites. Hence, this method can be used to perform the assessment and to study the mechanisms of the adverse health effects. The fragmentation patterns established in this study can also be used to identify and quantify PAH metabolites in other biological fluids. 相似文献
105.
Weeks BL Vaughn MW Deyoreo JJ 《Langmuir : the ACS journal of surfaces and colloids》2005,21(18):8096-8098
Environmental scanning electron microscopy was used to image meniscus formation between an AFM tip and a surface. At high relative humidity, 70%-99%, the meniscus formed is 100 to 1200 nm in height, orders of magnitude larger than predicted by the Kelvin equation using spherical geometry. The height of the meniscus also demonstrates hysteresis associated with increasing or decreasing relative humidity. 相似文献
106.
Multifunctional Tricarbazolo Triazolophane Macrocycles: One‐Pot Preparation,Anion Binding,and Hierarchical Self‐Organization of Multilayers 下载免费PDF全文
Dr. Semin Lee Dr. Brandon E. Hirsch Yun Liu James R. Dobscha David W. Burke Dr. Steven L. Tait Dr. Amar H. Flood 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(2):560-569
Programming the synthesis and self‐assembly of molecules is a compelling strategy for the bottom‐up fabrication of ordered materials. To this end, shape‐persistent macrocycles were designed with alternating carbazoles and triazoles to program a one‐pot synthesis and to bind large anions. The macrocycles bind anions that were once considered too weak to be coordinated, such as PF6?, with surprisingly high affinities (β2=1011 M ?2 in 80:20 chloroform/methanol) and positive cooperativity, α=(4 K2/K1)=1200. We also discovered that the macrocycles assemble into ultrathin films of hierarchically ordered tubes on graphite surfaces. The remarkable surface‐templated self‐assembly properties, as was observed by using scanning tunneling microscopy, are attributed to the complementary pairing of alternating triazoles and carbazoles inscribed into both the co‐facial and edge‐sharing seams that exist between shape‐persistent macrocycles. The multilayer assembly is also consistent with the high degree of molecular self‐association observed in solution, with self‐association constants of K=300 000 M ?1 (chloroform/methanol 80:20). Scanning tunneling microscopy data also showed that surface assemblies readily sequester iodide anions from solution, modulating their assembly. This multifunctional macrocycle provides a foundation for materials composed of hierarchically organized and nanotubular self‐assemblies. 相似文献
107.
Anniefer N. Magpusao Kelli Rutledge Dr. Trevor A. Hamlin Jean‐Marc Lawrence Dr. Brandon Q. Mercado Prof. Nicholas E. Leadbeater Prof. Mark W. Peczuh 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(17):6001-6011
Shape is an inherent trait of a molecule that dictates how it interacts with other molecules, either in binding events or intermolecular reactions. Large‐ring macrocyclic compounds in particular leverage their shape when they are selectively bound by biomolecules and also when they exhibit macrocyclic diastereoselectivity. Nonetheless, rules that link structural parameters to the conformation of a macrocycle are still rudimentary. Here we use a structural investigation of a family of [13]‐macrodilactones as a case study to develop rules that can be applied generally to macrocycles of different sizes and with a variety of functionality. A characteristic “ribbon” shape is adopted by the [13]‐macrodilactones in the absence of stereogenic centres, which exhibits planar chirality. When one stereogenic centre at key positions on the backbone is incorporated into the structure, the planar chirality is dictated by the configuration of the centre. In cases where two stereogenic centres are present, their relationships can either reinforce the characteristic ribbon shape or induce alternative shapes to be adopted. The rules established in the case study are then applied to the analysis of a structure of the natural product migrastatin. They lay the groundwork for the development of models to understand macrocycle‐biomolecule interactions and for the preparation of macrocycles with designed properties and activities. 相似文献
108.
109.
Mickhail V. Ryzhkov Alexei Mirmelstein Sung‐Woo Yu Brandon W. Chung James G. Tobin 《International journal of quantum chemistry》2013,113(16):1957-1965
Calculations of the electronic structure of clusters of plutonium have been performed, within the framework of the relativistic discrete‐variational method. These theoretical results and those calculated earlier for related systems have been compared to spectroscopic data produced in the experimental investigations of bulk systems, including photoelectron spectroscopy. Observation of the changes in the Pu electronic structure as a function of size provides powerful insight for aspects of bulk Pu electronic structure. © 2013 Wiley Periodicals, Inc. 相似文献
110.