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961.
Calculating all one- and two-phonon contributions in fourth order to the isothermal desorption time, the validity of the relaxation time approach, and the adequacy of second-order perturbation theory to localized physisorption kinetics are examined. Explicit criteria are tested in numerical results. 相似文献
962.
The photoelectron spectrum of the title compound 2 is discussed and compared with the results of MO-calculations for the parent unsubstituted hydrocarbon 1 . The ground-state of 2+ is 2E″ which is Jahn-Teller (JT) active. Assuming the e′ C=C-stretch to be responsible for breaking its degeneracy, satisfactory agreement between experimental and theoretical band shape is obtained. The latter is calculated from the diabatic two-state approximation using MINDO/3-SCF results for the JT-stabilization energies. Comparison with Rydberg studies on 1, performed by other authors, yields further evidence for the A2″ – (not E′ –) symmetry of the Rydberg states. The next two observed higher lying states of 2+ are 2E′ and 2A2″, the former also being JT-active. Theoretical calculations of the corresponding band shape point toward the e′-ring stretching mode causing instability, an outcome reminiscent of the 2E′-groundstate of cyclopropane-ion. The energetic difference between the latter state and the 2E′-state of 2+ is discussed using a simple MO-model with methylidene and ethylidene carbene as interacting units. The results suggest that the unsaturated carbene is strongly stabilized by hyperconjugation. The chemistry of the two carbenes is compared on this basis. 相似文献
963.
Prof. Dr. Edwin Hengge Günther Bauer Ernst Brandstätter Georg Kollmann 《Monatshefte für Chemie / Chemical Monthly》1975,106(4):887-892
The removal of phenyl groups from disilane derivatives with HCl and HBr to form silicon-halogen bonds has been studied in a variety of methylphenyldisilanes to establish the possible range of application. Besides the preparation of several new compounds, a convenient method is described for the synthesis of certain disilane derivatives which have until now been accessible only with difficulty. 相似文献
964.
Chen WY Zheng JX Cheng SZ Li CY Huang P Zhu L Xiong H Ge Q Guo Y Quirk RP Lotz B Deng L Wu C Thomas EL 《Physical review letters》2004,93(2):028301
We proposed an approach to precisely control the density of tethered chains on solid substrates using PEO-b-PS and PLLA-b-PS. As the crystallization temperature Tx increased, the PEO or PLLA lamellar crystal thickness d(L) increased as well as the reduced tethering density sigma; of the PS chains. The onset of tethered PS chains overcrowding in solution occurs at sigma(*) approximately 3.7-3.8 as evidenced by an abrupt change in the slope between (d(L))(-1) and Tx. This results from the extra surface free energy created by the tethered chain that starts to affect the growth barrier of the crystalline blocks. 相似文献
965.
A new design of pulse-contrast-measuring correlator, based on optical parametric amplification, is presented. It amplifies the wings of the pulse and allows high-contrast measurement with a low-sensitivity detector. The test system demonstrates an ability to measure a dynamic range of >10(11):1. 相似文献
966.
Zhou Tang Dr. Edwin Otten Prof. Dr. Joost N. H. Reek Dr. Ir. Jarl Ivar van der Vlugt Prof. Dr. Bas de Bruin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(36):12683-12693
Ligand cooperativity provides (transition) metal complexes with new reactivities in substrate activation and catalytic reactions, but usually the ligand acts as an internal (Brønsted) base, while the metal acts as a (Lewis) acid. We describe the synthesis and stepwise activation of a new phosphane‐pyridine‐amide ligand PNNH2 in combination with RhI. The ligand is susceptible to stepwise proton and hydride loss from the nitrogen arm (imine formation) and deprotonation at the pyridylphosphine arm (dearomatization), giving rise to amine complex 1 , amido species 2 , imine complex 3 and dearomatized compound 4 . Complex 4 bears a dual‐mode cooperative PNN′ ligand containing both a (nucleophilic) basic methine fragment and a reactive (electrophilic) imine moiety. The basic ligand arm enables substrate deprotonation while the imine ligand arm enables reversible “storage” of the activated (nucleophilic) form of a sulfonamide substrate at the ligand. In combination with metal‐based reactivity, this allows for the mono‐alkylation of o‐toluenesulfonamide with iodomethane. Compounds 1 , 3 and 4 are structurally characterized. We also report the first structurally characterized example of an aminal in the coordination sphere of rhodium, complex 5 , [Rh(CO)( PNN′′ )], formed by sequential N?H activation of sulfonamide by the dearomatized ligand PNN′ and follow‐up nucleophilic attack of anionic sulfonamide onto the imine fragment. 相似文献
967.
Cyclic anhydrides such as naphtho[2,3-c]furan-1,3-dione derivatives were synthesized from the reaction of arylpropiolic acids and 2-chloro-4,6-dimethoxy-1,3,5-triazine in the presence of N-methylmorpholine at room temperature. This mild condition provided the naphtho[2,3-c]furan-1,3-dione derivatives in good yields. Spectroscopic analysis suggested that the formation of arylpropiolate is the rate-determining step. 相似文献
968.
Hendrik Klien Wilhelm Seichter Konstantinos Skobridis Edwin Weber 《Acta Crystallographica. Section C, Structural Chemistry》2015,71(9):768-775
Having reference to an elongated structural modification of 2,2′‐bis(hydroxydiphenylmethyl)biphenyl, (I), the two 1,1′:4′,1′′‐terphenyl‐based diol hosts 2,2′′‐bis(hydroxydiphenylmethyl)‐1,1′:4′,1′′‐terphenyl, C44H34O2, (II), and 2,2′′‐bis[hydroxybis(4‐methylphenyl)methyl]‐1,1′:4′,1′′‐terphenyl, C48H42O2, (III), have been synthesized and studied with regard to their crystal structures involving different inclusions, i.e. (II) with dimethylformamide (DMF), C44H34O2·C2H6NO, denoted (IIa), (III) with DMF, C48H42O2·C2H6NO, denoted (IIIa), and (III) with acetonitrile, C48H42O2·CH3CN, denoted (IIIb). In the solvent‐free crystals of (II) and (III), the hydroxy H atoms are involved in intramolecular O—H...π hydrogen bonding, with the central arene ring of the terphenyl unit acting as an acceptor. The corresponding crystal structures are stabilized by intermolecular C—H...π contacts. Due to the distinctive acceptor character of the included DMF solvent species in the crystal structures of (IIa) and (IIIa), the guest molecule is coordinated to the host via O—H...O=C hydrogen bonding. In both crystal structures, infinite strands composed of alternating host and guest molecules represent the basic supramolecular aggregates. Within a given strand, the O atom of the solvent molecule acts as a bifurcated acceptor. Similar to the solvent‐free cases, the hydroxy H atoms in inclusion structure (IIIb) are involved in intramolecular hydrogen bonding, and there is thus a lack of host–guest interaction. As a result, the solvent molecules are accommodated as C—H...N hydrogen‐bonded inversion‐symmetric dimers in the channel‐like voids of the host lattice. 相似文献
969.
970.
Highly substituted, tethered alkyne dipolarophiles participate in the internal 2 + 3 cycloaddition with azomethine ylides generated by treatment of oxazolium salts with cyanide ion. Starting from oxazole 26, a sequence of N-methylation, cyanide addition, and electrocyclic ring opening of a 4-oxazoline intermediate affords the indoloquinone 31 in a one-pot process. A similar reaction from the protected alkynol derivative 25 affords the sensitive, but isolable, enone 32, and subsequent oxidation affords 31 and the deprotected quinone alcohol 34. Related azomethine cycloaddition methodology via intramolecular oxazolium salt formation from 43 or 46 is also demonstrated and allows the synthesis of quinone 45 and derived structures having the substitution pattern of aziridinomitosene A. Removal of the N-trityl protecting group could not be achieved without aziridine cleavage. 相似文献