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991.
Lanina SA Toledo P Sampels S Kamal-Eldin A Jastrebova JA 《Journal of chromatography. A》2007,1157(1-2):159-170
ESI and APCI ionization techniques in both negative and positive ion modes were evaluated for simultaneous LC-MS analysis of the four tocopherol homologues (alpha, beta, gamma and delta). The ESI and APCI ionization of tocopherols in positive ion mode was not efficient and proceeded via two competitive mechanisms, with the formation of protonated pseudo-molecular ions and molecular ions, which adversely influenced the repeatability of MS signal. Ionization in negative ion mode in both ESI and APCI was more efficient as it only produced target deprotonated pseudo-molecular ions. The APCI in negative ion mode showed larger linearity range, lower detection limits and was less sensitive to the differences in chemical structure of analytes and nature of applied solvents than negative ion ESI. Negative ion APCI was, therefore, chosen for the development of LC-MS method for simultaneous determination of the four tocopherols in foods. A baseline separation of the tocopherols was achieved on novel pentafluorophenyl silica-based column Fluophase PFP. The use of methanol-water (95:5, v/v) as a mobile phase was preferable to the use of acetonitrile-water due to considerable gain in MS signal. The limits of quantifications were 9 ng/mL for alpha-tocopherol, 8 ng/mL for beta- and gamma- and 7.5 ng/mL for delta-tocopherol when 2 microL was injected. This method was successfully applied to determination of tocopherols in sunflower oil and milk. 相似文献
992.
Synthesis of thieno[2,3-d]pyrimidin-2-ylmethanamine combinatorial library with four diversity points
Bogolubsky AV Ryabukhin SV Stetsenko SV Chupryna AA Volochnyuk DM Tolmachev AA 《Journal of combinatorial chemistry》2007,9(4):661-667
The parallel solution-phase synthesis of more than 230 substituted thieno[2,3-d]pyrimidin-2-ylmethanamines has been accomplished. This strategy is based on the cyclization of 2-aminothiophen-3-carboxylates with chloroacetonitrile to construct the thieno[2,3-d]pyrimidine core with two diversity points. Derivatization of the active chlorine and functionalization of C-4 position of the pyrimidine ring allow the introduction of other diversity points. The products containing ester groups at the 6-position of the thieno[2,3-d]pyrimidine were used in amide synthesis. Simple manual techniques for parallel reactions, coupled with simple purification procedures, gave highly pure final products. The scope and limitations of the approach are discussed. 相似文献
993.
Pozharitskaya ON Ivanova SA Shikov AN Makarov VG Galambosi B 《Journal of separation science》2007,30(15):2447-2451
A new procedure has been developed to separate and quantify the free radical-scavenging activity of individual compounds from green, brown, and black leaves of Bergenia crassifolia based on the combination of high performance TLC (HPTLC) with a diode array detector (DAD) and postchromatographic 1,1-diphenyl-2-picrylhydrazyl radical (DPPH(*)) derivatization. Free gallic and ellagic acids, arbutin, hydroquinone, and bergenin in the B. crassifolia leaves' extracts were separated by HPTLC and identified. All compounds of the extract excluding bergenin were capable of scavenging DPPH * radicals. From the estimated ID(50) values, it can be seen that the increasing order of activity was gallic acid > arbutin > ellagic acid > hydroquinone > ascorbic acid. The antiradical activity of leaves of B. crassifolia is probably associated to the presence of phenol. 相似文献
994.
Shishkina SV Shishkin OV Desenko SM Leszczynski J 《The journal of physical chemistry. A》2007,111(12):2368-2375
The equilibrium geometry, ring-inversion barrier, and pathway for heterocyclic analogues of cyclohexene have been studied using the MP2/6-311G(d,p) level of theory. It is concluded that the replacement of one methylene group in cyclohexene by heteroatom results in significant changes in the character of the potential-energy surface in comparison with cyclohexene. The equilibrium conformation of ring strongly depends on the position of the heteroatom due to the existence of the n-pi conjugation. However, the character of the ring-inversion process is determined by the nature of the heteroatom. In the case of sulfur- and selenium-containing rings, the boat or twist-boat conformation corresponds to an additional minimum on the potential-energy surface. Moreover, the barriers of the conformational transition from this conformer to two different half-chair forms are significantly different. Nitrogen-containing heterocycles possess two pairs of minima corresponding to the different configurations of the nitrogen atom. However, the transition between the two minima with the same configuration of the heteroatom proceeds only in two steps that include ring inversion and nitrogen inversion. 相似文献
995.
Illner P Kern S Begel S van Eldik R 《Chemical communications (Cambridge, England)》2007,(45):4803-4805
Detailed kinetic studies on ligand substitution reactions of [M(II)(terpy)Cl](+) complexes (M = Pt, Pd; terpy = 2,2':6',2'-terpyridine) with thiourea as entering nucleophile were for the first time performed in the imidazolium based ionic liquid [emim][NTf(2)] using stopped-flow techniques, opening the route to study fast reactions of transition metal complexes in ionic liquids. 相似文献
996.
Baca SG Malaestean IL Keene TD Adams H Ward MD Hauser J Neels A Decurtins S 《Inorganic chemistry》2008,47(23):11108-11119
The synthesis, crystal structures and magnetic properties of five new manganese compounds are reported. These include a linear trinuclear cluster [Mn(II)(3)(O(2)CCHMe(2))(6)(dpa)(2)].2MeCN (1) (dpa = 2,2'-dipyridylamine), a tetranuclear cluster [Mn(II)(2)Mn(III)(2)O(2)(O(2)CCMe(3))(6)(bpy)(2)] (3) (bpy = 2,2'-bipyridine), and chain coordination polymers composed of cluster blocks such as Mn(3), Mn(3)O, and Mn(4)O(2) bridged by 2,2'-bipyrimidine (bpm) or hexamethylentetramine (hmta) ligands to give ([Mn(II)(3)(O(2)CCHMe(2))(6)(bpm)].2EtOH)(n) (2), [Mn(II)(2)Mn(III)(2)O(2)(O(2)CCHMe(2))(6)(bpm)(EtOH)(4)](n) (4), and (([Mn(II)Mn(III)(2)O(O(2)CCHMe(2))(6)(hmta)(2)].EtOH)(n) (5). The magnetic analysis of the compounds was achieved using a combination of vector coupling and full-matrix diagonalization methods. Susceptibility data for compound 1 was fitted using a vector coupling model to give g = 2.02(1) and 2J/k(B) = -5.38(2) K. To model the trimer chain, we used vector coupling for initial values of J(1) and then diagonalization techniques to estimate J(2) to give g = 1.98(1), 2J(1)/k(B) = -3.3(1) K and 2J(2)/k(B) = -1.0(1) K by approximating the system to a dimer of trimers. The analysis of 3 was made difficult by the mixture of polymorphs and the difficulties of a three-J model, while for 4 an analysis was not possible because of the size of the computation and the relative magnitudes of the three couplings. Compound 5 was modeled using the same techniques as 2 to give g = 1.99(1), 2J(1)/k(B) = +32.5(2) K, 2J(2)/k(B) = -16.8(1) K, and 2J(3)/k(B) = +0.4(1) K. The combination of techniques has worked well for compounds 2 and 5 and thus opens up a method of modeling complex chains. 相似文献
997.
Svetlana A. Kozlova Vladimir N. Emel’yanenko Miglena Georgieva Sergey P. Verevkin Yury Chernyak Benjamin Schäffner Armin Börner 《The Journal of chemical thermodynamics》2008,40(7):1136-1140
Molar enthalpies of vaporization of aliphatic alkyl carbonates: dimethyl carbonate [616-38-6], diethyl carbonate [105-58-8], di-n-propyl carbonate [623-96-1], di-n-butyl carbonate [542-52-9], and dibenzyl carbonate [3459-92-5] were obtained from the temperature dependence of the vapour pressure measured by the transpiration method. A large number of the primary experimental results on temperature dependences of vapour pressures have been collected from the literature and have been treated uniformly in order to derive vaporization enthalpies of dialkyl carbonates at the reference temperature 298.15 K. An internal consistency check was performed on enthalpy of vaporization values for dialkyl carbonates studied in this work. 相似文献
998.
Ashirov RV Shamov GA Lodochnikova OA Litvynov IA Appolonova SA Plemenkov VV 《The Journal of organic chemistry》2008,73(15):5985-5988
At elevated temperatures 3-methyl-cyclopropene-3-carbonitrile 1 was found to tetramerize giving compound 2 (3-methyl-2,3-bis(2-t-methyl-2-c-cyanocyclopropyl)-1-(2-t-methyl-2-c,3-c-dicyanocyclopropyl)-cyclopropene) in good yields. This is the first example of Alder-ene type oligomerization of a 3,3-disubstituted cyclopropene. On the basis of the product geometry and stereoselective character of the reaction, a mechanism of formation of 2 involving CN-Alder-ene reaction was proposed. DFT modeling of the mechanism has shown that the CN-Alder-ene reaction is possible as a stepwise process involving a biradical intermediate. 相似文献
999.
Sergey A. Katsyuba Elena E. Zvereva Alla V. Chernova Artur R. Shagidullin Svetlana E. Solovieva Igor S. Antipin Alexander I. Konovalov 《Journal of inclusion phenomena and macrocyclic chemistry》2008,60(3-4):281-291
It is demonstrated that the introduction of p-tert-butyl groups dramatically influences the conformational behaviour of the mercaptothiacalix[4]arene molecules. Quantum-chemical
computations in combination with IR and NMR spectroscopy prove that, in contrast to closely related calixarenes, the 1,3-alternate becomes a dominant conformer of p-tert-butyl-mercaptothiacalix[4]arene not only in crystal, but also in solutions and in vacuum. It is shown that the title molecules
form essentially non-cooperative intramolecular hydrogen bonds: their SH groups are intramolecularly H-bonded solely to the
sulfide groups bridging thiophenolic units. The enthalpy of this bonding, evaluated from Iogansen’s rule, amounts to ca. 1.5 kcal mol−1 per one SH···S bond, which about four times smaller than the enthalpies of cooperative intramolecular H-bonds formed by related
calixarenes and thiacalixarenes.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
1000.
A new silicated cyclotriphosphazene N3P3(O2C12H8)2(OC6H4Si(CH3)3)(OC6H4Br) 1 has been synthesized and characterized. The solid state pyrolysis of 1 in air gives a nanostructured SiP2O7 3D network. The morphology of the network strongly depends on the temperature of the pyrolysis. Spinal-like columns and ring-shaped
SiP2O7 are formed at 800 °C, while, at 600 °C, fused grains of about 300 nm were observed. Based on air TG and DSC thermal studies,
we propose the mechanism of formation for the nanostructured network. 相似文献