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991.
We prove that every one-parameter family of complex rationally connected varieties has a section.
992.
Jason Fulman 《Journal of Algebra》2002,250(2):731
The study of asymptotic properties of the conjugacy class of a random element of the finite affine group leads one to define a probability measure on the set of all partitions of all positive integers. Four different probabilistic understandings of this measure are given—three using symmetric function theory and one using Markov chains. This leads to non-trivial enumerative results. Cycle index generating functions are derived and are used to compute the large dimension limiting probabilities that an element of the affine group is separable, cyclic, or semisimple and to study the convergence to these limits. The semisimple limit involves both Rogers–Ramanujan identities. This yields the first examples of such computations for a maximal parabolic subgroup of a finite classical group. 相似文献
993.
Hideo Makimura Tooru M Mizuno Jason W Mastaitis Reuven Agami Charles V Mobbs 《BMC neuroscience》2002,3(1):18-6
Background
Several lines of evidence strongly suggest that agouti-related peptide (AGRP) plays a key role in the regulation of metabolic function but ablation of the AGRP gene has no apparent effect on metabolic function. Since specific pharmacological antagonists of AGRP do not presently exist, we assessed if reduction of hypothalamic AGRP mRNA by RNA interference (RNAI) would influence metabolic function, an outcome suggesting that pharmacological antagonists might constitute useful reagents to treat obesity. 相似文献994.
The ropelength of a knot is the quotient of its length by its thickness, the radius of the largest embedded normal tube around
the knot. We prove existence and regularity for ropelength minimizers in any knot or link type; these are C
1,1 curves, but need not be smoother. We improve the lower bound for the ropelength of a nontrivial knot, and establish new ropelength
bounds for small knots and links, including some which are sharp.
Oblatum 11-IV-2001 & 20-III-2002?Published online: 17 June 2002 相似文献
995.
Manganese(I)‐Catalyzed C−H Activation: The Key Role of a 7‐Membered Manganacycle in H‐Transfer and Reductive Elimination
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Dr. Nasiru P. Yahaya Kate M. Appleby Magdalene Teh Conrad Wagner Erik Troschke Joshua T. W. Bray Prof. Simon B. Duckett L. Anders Hammarback Dr. Jonathan S. Ward Jessica Milani Dr. Natalie E. Pridmore Dr. Adrian C. Whitwood Dr. Jason M. Lynam Prof. Ian J. S. Fairlamb 《Angewandte Chemie (International ed. in English)》2016,55(40):12455-12459
Manganese‐catalyzed C?H bond activation chemistry is emerging as a powerful and complementary method for molecular functionalization. A highly reactive seven‐membered MnI intermediate is detected and characterized that is effective for H‐transfer or reductive elimination to deliver alkenylated or pyridinium products, respectively. The two pathways are determined at MnI by judicious choice of an electron‐deficient 2‐pyrone substrate containing a 2‐pyridyl directing group, which undergoes regioselective C?H bond activation, serving as a valuable system for probing the mechanistic features of Mn C?H bond activation chemistry. 相似文献
996.
Reagent‐Controlled α‐Selective Dehydrative Glycosylation of 2,6‐Dideoxy‐ and 2,3,6‐Trideoxy Sugars
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Dr. Jason M. Nogueira Marissa Bylsma Danielle K. Bright Prof. Clay S. Bennett 《Angewandte Chemie (International ed. in English)》2016,55(34):10088-10092
We have found that activating either 2,3‐bis(2,3,4‐trimethoxyphenyl)cyclopropenone or 2,3‐bis(2,3,4‐trimethoxyphenyl)cyclopropene‐1‐thione with oxalyl bromide results in the formation of a species that promotes the glycosylation between 2,6‐dideoxy‐sugar hemiacetals and glycosyl acceptors in good yield and high α‐selectivity. Both reactions are mild and tolerate a number of sensitive functional groups including highly acid‐labile 2,3,6‐trideoxy‐sugar linkages. 相似文献
997.
Dr. Jason Greenwald Michael P. Friedmann Prof. Roland Riek 《Angewandte Chemie (International ed. in English)》2016,55(38):11609-11613
Current theories on the origin of life reveal significant gaps in our understanding of the mechanisms that allowed simple chemical precursors to coalesce into the complex polymers that are needed to sustain life. The volcanic gas carbonyl sulfide (COS) is known to catalyze the condensation of amino acids under aqueous conditions, but the reported di‐, tri‐, and tetra‐peptides are too short to support a regular tertiary structure. Here, we demonstrate that alanine and valine, two of the proteinogenic amino acids believed to have been among the most abundant on a prebiotic earth, can polymerize into peptides and subsequently assemble into ordered amyloid fibers comprising a cross‐β‐sheet quaternary structure following COS‐activated continuous polymerization of as little as 1 mm amino acid. Furthermore, this spontaneous assembly is not limited to pure amino acids, since mixtures of glycine, alanine, aspartate, and valine yield similar structures. 相似文献
998.
Jason L. Ormsby 《Tetrahedron》2008,64(50):11370-11378
Rearrangements readily occur in Scholl oxidations and interfere with the construction of certain molecular architectures. 3,3?,4,4′,4″,4?,5′,5″-Octamethoxy-1,1′,2′,1″,2″,1?-quaterphenyl and 3,3?,4,4′,4′′,4?,5′,5′-octamethyl-1,1′,2′,1″,2″,1?-quaterphenyl, which were conceived as precursors to benzenoid strips, rearranged under Scholl conditions to the unexpected C2v-substituted products 1,2,5,6,9,10,12,13-octamethoxydibenzo[fg,op]naphthacene and 1,2,5,6,9,10,12,13-octamethyldibenzo[fg,op]naphthacene. This corrects a widely propagated error in the literature in which the assignments of 1,2,5,6,9,10,12,13-octamethoxydibenzo[fg,op]naphthacene (C2v) and 1,2,5,6,8,9,12,13-octamethoxydibenzo[fg,op] naphthacene (C2h) are crossed. A mechanism involving the migration of an aryl ring on an arenium cation intermediate is proposed. 相似文献
999.
Nilsson BL Overman LE Read de Alaniz J Rohde JM 《Journal of the American Chemical Society》2008,130(34):11297-11299
Total syntheses of (+)-nankakurine A (2) and (+)-nankakurine B (3) were accomplished by a sequence that employs an intramolecular dipolar cycloaddition of an azomethine imine intermediate to form the azatricyclic moiety and establish the relative configuration of the spiropiperidine ring. These syntheses, together with the synthesis of the originally purported structure 1 of nankakurine A, rigorously establish the relative and absolute configuration of these structurally unusual Lycopodium alkaloids. The syntheses are sufficiently concise that gram quantities of (+)-nankakurine A (2) and (+)-nankakurine B (3) will be available for further biological studies. 相似文献
1000.
Photochemistry of adsorbed nitrate 总被引:2,自引:0,他引:2
Schuttlefield J Rubasinghege G El-Maazawi M Bone J Grassian VH 《Journal of the American Chemical Society》2008,130(37):12210-12211
In the atmosphere, gas-phase nitrogen oxides including nitric acid react with particle surfaces (e.g., mineral dust and sea salt aerosol) to yield adsorbed nitrate, yet little is known about the photochemistry of nitrate on the surface of these particles. In this study, nitrate adsorbed on alumina surfaces, a surrogate for mineral dust aerosol, is irradiated with broadband light (lambda > 300 nm) in the absence and presence of coadsorbed water, at <1% and 45 +/- 2% relative humidity (%RH), respectively, and molecular oxygen. Upon irradiation, the nitrate ion readily undergoes photolysis to yield nitrogen-containing gas-phase products, NO2, NO, and N2O. Although NO2, NO, and N2O form under the different conditions investigated, both coadsorbed water and molecular oxygen change the gas-phase product distribution, with NO being the major product under dry and humid conditions in the absence of molecular oxygen and NO2 the major product in the presence of molecular oxygen. To the best of our knowledge, this is the first study to investigate the role of solvation by coadsorbed water in the photochemistry of adsorbates at solid interfaces and the roles that molecular oxygen, adsorbed water, and relative humidity may have in photochemical processes on aerosol surfaces that have the potential to alter the chemical balance of the atmosphere. 相似文献