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排序方式: 共有491条查询结果,搜索用时 31 毫秒
241.
Srivari Chandrasekhar Chennamaneni Lohitha Rao Chada Raji Reddy 《Tetrahedron letters》2007,48(33):5869-5872
The Cu(I)-catalyzed cycloaddition of azido-alkynes (click reaction) of furanose sugar substrates provides a facile approach for the construction of macrocyclic molecules via an inter and/or intramolecular cycloaddition based on the functionality between alkyne and azide. 相似文献
242.
Günther W. Spitznagel Timothy Clark Jayaraman Chandrasekhar Paul Von Ragu Schleyer 《Journal of computational chemistry》1982,3(3):363-371
The entire set of methyl anions, XCH2?, substituted by first-row substituents, Li, BeH, BH2, CH3, NH2, OH, and F, was examined at various ab initio levels. Diffuse orbital-augmented basis sets, such as 4?31+G and 6?31+G *, are needed to describe the energies of these anions adequately. Estimates of proton affinities are further improved by second-order Møller–Plesset (MP 2) electron correlation corrections, but relative energies are less affected. The methyl group in the ethyl anion is destabilizing, the amino substituent is borderline, but all other groups are stabilizing. Very large π effects are exhibited by BH2 and BeH groups; inductive stabilization by the electronegative F and OH groups is less effective. Lithium also is stabilizing, but the best singlet geometry of CH2Li? is not planar. A planar CH2Li? triplet with a π1 configuration may be lower in energy. 相似文献
243.
Total synthesis of (+)-boronolide, (+)-deacetylboronolide, and (+)-dideacetylboronolide has been achieved from a single intermediate 26, which was synthesized in 11 steps from a d-mannitol-derived intermediate 8 in an overall yield of 10%. The key steps in the synthesis are inversion of a chiral center by taking an advantage of the inherent mechanism involved in the ring closing to an epoxide via intramolecular S(N)2 reaction and lactonization of a diol using Fetizons reagent. The strategy is amenable to preparation of analogues of (+)-boronolide in sufficient amount for further screening of biological activity. 相似文献
244.
Jayaraman Chandrasekhar Paul Von Ragu Schleyer Karsten Krogh-Jespersen 《Journal of computational chemistry》1981,2(3):356-360
At high levels of ab initio theory (6-31G*//4-31G), the most stable C4H isomer is indicated to be the nonplanar cyclobutadiene dication ( 1a ); the planar form, 1b , is indicated to be 7.5 kcal/mol less stable. The second most stable C4H isomer, the methylenecyclopropene dication, is indicated to prefer the perpendicular ( 2a ) over the planar ( 2b ) arrangement by 7 kcal/mol. The “anti van't Hoff” cyclo-(HB)2C?CH2 system ( 4 ), isoelectronic with 2 , also prefers the perpendicular conformation ( 4a ), and retains the C?C double bond. The linear butatriene dication ( 3 ) is the least stable C4H species investigated. The perpendicular (D2d) arrangement ( 3a ), permitting double allyl cationlike conjugation, is preferred over the planar D2h form ( 3b ) by 26 kcal/mol. The heat of formation of the most stable form of C4H, 1a , is estimated to be 623–640 kcal/mol. This species should be thermodynamically stable toward dissociation into smaller charged fragments. 相似文献
245.
Sosale Chandrasekhar 《Research on Chemical Intermediates》2005,31(9):779-783
The 'duplication' strategy for the further enrichment of an already enantiomerically-enriched mixture consists of the formation
of all the three possible 'dimeric' diastereomers, i.e., (R)–X–(R), (R)–X–(S) and (S)–X–(S), where X is an appropriate spacer that can be readily cleaved to yield the original enantiomers. The mixture of (R)–X–(R) and (S)–X–(S) thus obtained would be of higher enantiomeric excess (e.e.) as compared to the original mixture, on the basis of a simple
kinetic scheme. The success of the strategy is experimentally well-established, but is apparently based on the (unproven)
assumption that the theoretically-derived rate ratios are identical to the experimentally observed product ratios. Although
the detailed kinetic treatment for a system such as the above is extremely complex, it is possible to show (mathematically)
that the above assumption is indeed justified when all the three diastereomers are formed without chiral discrimination (as
assumed in the strategy). 相似文献
246.
247.
248.
Dr. Mingchao Wang Dr. Gang Wang Dr. Chandrasekhar Naisa Dr. Yubin Fu Dr. Sai Manoj Gali Dr. Silvia Paasch Dr. Mao Wang Haiko Wittkaemper Dr. Christian Papp Prof. Eike Brunner Dr. Shengqiang Zhou Dr. David Beljonne Prof. Hans-Peter Steinrück Prof. Renhao Dong Prof. Xinliang Feng 《Angewandte Chemie (International ed. in English)》2023,62(46):e202310937
Electrochemical proton storage plays an essential role in designing next-generation high-rate energy storage devices, e.g., aqueous batteries. Two-dimensional conjugated covalent organic frameworks (2D c-COFs) are promising electrode materials, but their competitive proton and metal-ion insertion mechanisms remain elusive, and proton storage in COFs is rarely explored. Here, we report a perinone-based poly(benzimidazobenzophenanthroline) (BBL)-ladder-type 2D c-COF for fast proton storage in both a mild aqueous Zn-ion electrolyte and strong acid. We unveil that the discharged C−O− groups exhibit largely reduced basicity due to the considerable π-delocalization in perinone, thus affording the 2D c-COF a unique affinity for protons with fast kinetics. As a consequence, the 2D c-COF electrode presents an outstanding rate capability of up to 200 A g−1 (over 2500 C), surpassing the state-of-the-art conjugated polymers, COFs, and metal–organic frameworks. Our work reports the first example of pure proton storage among COFs and highlights the great potential of BBL-ladder-type 2D conjugated polymers in future energy devices. 相似文献
249.
Chandrasekhar V Sasikumar P Boomishankar R 《Dalton transactions (Cambridge, England : 2003)》2008,(38):5189-5196
The reaction of ArPO(3)H(2) (Ar = 2,4,6-iPr(3)-C(6)H(2)) with Cd(CH(3)COO)(2).2H(2)O using various co-ligands such as methanol, dimethylformamide (DMF) and 3,5-dimethylpyrazole (DMPZH) resulted in the formation of tetranuclear assemblies [Cd(4)(ArPO(3))(2)(ArPO(3)H)(4)(CH(3)OH)(4)].3(CH(3)OH) (1), [Cd(4)(ArPO(3))(2)(ArPO(3)H)(4)(DMF)(4)].3(DMF) (2) and [Cd(4)(ArPO(3))(2)(ArPO(3)H)(4)(DMF)(2)(DMPZH)(2)].2(DMF).2(H(2)O) (3). In all of these compounds the tetranuclear cadmium array, containing two five-coordinate and two six-coordinate cadmium atoms, is held together by two mu(4) capping [ArPO(3)](2-) and four anisobidentate mu(2) [ArPO(2)(OH)](-) ligands. Each cadmium atom is bound to an additional ancillary ligand. The reaction of ArPO(3)H(2) with Cd(CH(3)COO)(2).2H(2)O in the presence of the chelating ligand 2,2'-bipyridine (bipy) leads to the exclusive formation of the dinuclear assembly [Cd(2)(ArPO(3)H)(4)(bipy)(2)].(CH(3)OH)(H(2)O) (4). The latter contains an eight-membered Cd(2)P(2)O(4) inorganic ring formed as a result of the bridging coordination action of two anisobidentate mu(2) [ArPO(2)(OH)](-) ligands. Each cadmium atom is bound by one chelating bipy and one monodentate [ArPO(2)(OH)](-) ligands. Use of four equivalents of 3,5-dimethylpyrazole leads to the formation of the mononuclear derivative [Cd(ArPO(3)H)(2)(DMPZH)(4)] (5). The molecular structure of the latter comprises of a central cadmium atom surrounded by six monodentate ligands. Four of these are neutral pyrazole ligands that occupy the equatorial plane; the remaining two are anionic phosphinate ligands which are present trans to each other. The thermal analysis of 1 and 4 reveals that the char residue obtained at 600 degrees C consists predominantly of Cd(2)P(2)O(7). 相似文献
250.
The synthesis of capped α-Fe2O3 nanoparticles from thermal treatment of iron (III) hydroxy-oleate in boiling organic solvents around 250 °C with retention of the integrity of the oleate units during the reaction process is reported. The formation of capped iron oxide particles is accomplished under aerobic conditions while the solvents used in the synthesis have strong influence on the nature and morphology of nanoparticles. These nanoparticles are studied by means of X-ray powder diffraction, IR and XPS while the morphology and particle size of nanocrystals are evaluated using SEM and TEM analysis suggesting the formation of monocrystalline α-Fe2O3 particles having cubical and spherical morphologies with sizes ranging from 20 to 30 nm. This organophilic material with oleate capping around the surfaces can be readily dispersed in organic solvents thus forming organosols. These organosols exhibit band-edge emission photoluminescence band both in toluene as well as in solid state while FT-IR analysis reveals formation oleate capped nanoparticles The XPS data indicate ferric state having doublet from Fe 2p3/2 and Fe 2p1/2 core-level electrons. 相似文献