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991.
Gupta Kalyan Kali Sen Begum Bilkis Ara Ghosh Sakti Prosad 《Transition Metal Chemistry》1998,23(3):295-299
Oxidations of glycolaldehyde (GA) to glyoxal by osmium(VIII), iridium(IV) and platinum(IV) have been investigated in aqueous
solution, and the orders with respect to each [reactant] determined. The reaction involving iridium(IV) takes place through
intermediate formation of free radicals in a MeCO2Na-MeCO2H buffer medium, whereas a one-step two-electron transfer process occurs in the oxidations by OsVIII and PtIV in an alkaline medium. Mechanisms consistent with the experimental findings are proposed.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
992.
An enantioselective first total syntheis of amphidinolide W (2) and a revision of its C6 absolute stereochemistry (1) are described. Amphidinolide W (1), a 12-membered macrolide isolated from Amphidinium sp., has shown potent antitumor properties against a variety of NCI tumor cell lines. The synthesis is convergent, and four of the five chiral centers were derived through asymmetric synthesis. The synthesis features Sharpless asymmetric dihydroxylation, diastereoselective alkylation, efficient cross metathesis of functionalized substrates, and novel functional group transformations using selective lipase-catalyzed hydrolysis of the primary acetate group. Of particular note, the C6 absolute stereochemistry of amphidinolide W (1) has now been revised through our current synthesis. 相似文献
993.
994.
The kinetics of oxidation of ethanol by cerium(IV) in presence of ruthenium(III) (in the order of 10?7 mol dm?3) in aqueous sulfuric acid media have been followed at different temperatures (25–40°C). The rate of disappearance of cerium(IV) in the title reaction increases sharply with increasing [C2H5OH] to a value independent of [C2H5OH] over a large range (0.2–1.0 mol dm?3) in which the rate law conforms to: where [Ru]T gives the total ruthenium (III) concentration. The values of 10?3kc and 10?3kd are 3.6 ± 0.1 dm3 mol?1 s?1 and 3.9 ± 0.2 s?1, respectively, at 40°C, I = 3.0 mol dm?3. The proposed mechanism involves the formation of ruthenium(III)? substrate complex which undergoes oxidation at the rate determining step by cerium(IV) to form ruthenium(IV)? substrate complex followed by the rapid red-ox decomposition giving rise to the catalyst and ethoxide radical which is oxidized by cerium(IV) rapidly. The mechanism is consistent with the existence of the complexes RuIII · (C2H5OH) and RuIII · (C2H5O?) and both are kinetically active. The overall bisulphate dependence conforms to: kobsd = A[Ru]T/{1 + C[HSO4?]} where A = 2.2 × 104 dm3 mol?1 s?1, C = 1.3 at 40°C, [H+] = 0.5 mol dm?3, and I = 3.0 mol dm?3. The observations are consistent with the Ce(SO4)2 as the kinetically active species. © 1995 John Wiley & Sons, Inc. 相似文献
995.
Ramakrishna G Jose DA Kumar DK Das A Palit DK Ghosh HN 《The journal of physical chemistry. B》2005,109(32):15445-15453
Dynamics of interfacial electron transfer (ET) in the ruthenium-polypyridyl complex [{bis(2,2'-bpy)-(4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol)} ruthenium(II) hexafluorophosphate] (Ru-cat)-sensitized TiO(2) nanoparticles has been investigated using femtosecond transient absorption spectroscopy detecting in the visible and near-infrared region. It has been observed that Ru-cat is coupled strongly with the TiO(2) nanoparticles through its pendant catechol moiety. Electron injection has been confirmed by direct detection of electrons in the conduction band, cation radical of the adsorbed dye, and a bleach of the dye in real time as monitored by transient absorption spectroscopy. A single-exponential and pulse width limited (<100 fs) electron injection has been observed, and the origin of it might have been from the nonthermalized excited states of the Ru-cat molecule. The result gave a strong indication that the electron injection competes with the thermalization of the photoexcited states due to large coupling elements for the forward ET reaction. Back-ET dynamics has been determined by monitoring the decay kinetics of the cation radical and injected electron and also from recovery kinetics of the bleach of the adsorbed dye. It has been fit with a multiexponential function, where approximately 30% of the injected electrons are recombined with a time constant of <2 ps, again indicating large coupling elements for the charge recombination reaction. However, our results have shown relatively long-lived charge separation in the Ru-cat/TiO(2) system as compared to other organic dye-sensitized TiO(2) nanoparticles with similar interactions. 相似文献
996.
Russell P. Newton Terence J. Walton Salem A. Basaif Andrea M. Jenkins A. Gareth Brenton Dipankar Ghosh Frank M. Harris 《Journal of mass spectrometry : JMS》1989,24(8):679-688
The syntheses of the dibutyryl derivatives of the 3′,5′-cyclic monophosphates of adenosine, guanosine and cytidine are described. The fast atom bombardment mass spectra of these compounds are discussed, together with the mass-analysed ion kinetic energy spectra of their protonated molecular ions and of diagnostic fragments. A protocol for the identification of the derivatives is reported which includes criteria for confirming retention of the cyclic phosphodiester moiety, substitution of both heterocyclic base and ribose ring, and butyrylation of the 2′-O-position. The origins of significant fragments in the spectra are discussed. 相似文献
997.
Summary Reaction of [OsX6]2– (X = Cl or Br) with HK [HK = PhC(=O)C(=NOH)R; R=H, Me or Ph] yielded osmium(III) complexes of the type [OsX2(K)(HK)]. The OsX2 group wastrans and the hydrogen-bonded (K)(HK) moiety behaved as a planar tetradentate N2O2 chelator. The complexes were one-electron paramagnets and exhibited characteristic osmium(III) e.p.r. spectra. Several spin-allowed and spin-forbidden charge-transfer transitions were observed in the 200–1300 nm region. 相似文献
998.
Premamoy Ghosh Partha Sarathi Mitra 《Journal of polymer science. Part A, Polymer chemistry》1977,15(7):1743-1758
Photopolymerization of MMA was carried out at 40°C in diluted systems by use of quinolinebromine (Q–Br2) charge-transfer complex as the initiator and chloroform, carbon tetrachloride, chlorobenzene, dioxane, THF, acetone, benzene, toluene, quinoline, and pyridine as solvents. The results showed variable monomer exponents ranging from 1 to 3. For chloroform, carbon tetrachloride, and chlorobenzene, the monomer exponent observed was unity; for other solvents used, the value of the same exponent was much higher (between 2 and 3). Initiation of polymerization is considered to take place through radicals generated in the polymerization systems by the photodecomposition of (Q–Br2)–monomer complex (C) formed instantaneously in situ on addition of the Q–Br2 complex in monomer. The kinetic feature of high monomer exponent is considered to be due to higher order of stabilization of the initiating complex (C) in presence of the respective solvents. In the presence of the retarding solvents, very low or zero initiator exponents were also observed, depending on the nature and concentration of the solvents used. The deviation from the square-root dependence of rate on initiator concentration becomes higher at high solvent and initiator concentrations in general. This novel deviation is explained on the basis of initiator termination, probably via degradative chain transfer involving the solvent-modified initiating complexes and the propagating radicals. 相似文献
999.
Sun P Jose DA Shukla AD Shukla JJ Das A Rathman JF Ghosh P 《Langmuir : the ACS journal of surfaces and colloids》2005,21(8):3413-3423
A set of covalently linked phenyl-amidophenyl-substituted porphyrin amphiphiles with n-C15H31 tails have been synthesized and completely characterized. These amphiphiles form good Langmuir-Blodgett (LB) films at the air/water interface. Mean molecular areas for the series were measured from the isotherms and found to increase as the number of aliphatic chains increased from one to four. No influence of the subphase pH was observed on the isotherms. LB films can be transferred successfully onto different solid surfaces. The LB films were characterized using tapping mode atomic force microscopy (AFM). Bis-, tris-, and tetra-substituted porphyrins were found to be fairly good film-forming amphiphiles, whereas irregular aggregates were seen in the case of the monosubstituted porphyrin amphiphile. Multilayers were also formed with tetra-substituted amphiphiles on mica. Detailed AFM studies of tetra-substituted amphiphiles have been carried out to investigate the effect of preparation procedure and solid substrates on film formation and transfer. The absorption and fluorescence spectra for the amphiphiles in solution and LB films deposited onto mica and glass were recorded, which demonstrated the successful transfer of LB films onto the substrates and provided more information about the arrangement of porphyrin molecules within the LB films. For comparison, self-assembled monolayers (SAMs) and the cast thin films of the amphiphiles were prepared and characterized. 相似文献
1000.
Ghosh S Gorelsky SI Chen P Cabrito I Moura JJ Moura I Solomon EI 《Journal of the American Chemical Society》2003,125(51):15708-15709
The tetranuclear CuZ cluster catalyzes the two-electron reduction of N2O to N2 and H2O in the enzyme nitrous oxide reductase. This study shows that the fully reduced 4CuI form of the cluster correlates with the catalytic activity of the enzyme. This is the first demonstration that the S = 1/2 form of CuZ can be further reduced. Complementary DFT calculations support the experimental findings and demonstrate that N2O binding in a bent mu-1,3-bridging mode to the 4CuI form is most efficient due to strong back-bonding from two reduced copper atoms. This back-donation activates N2O for electrophilic attack by a proton. 相似文献