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81.
The carbohydrate chains decorating cell membranes and secreted proteins participate in a range of important biological processes. However, their ultimate significance and possible therapeutic potential have not been fully explored due to the lack of economical methods for their production. This study is an example of the use of a genetically engineered bacterial strain in the preparation of diverse oligosaccharides. Based on an ex vivo biosynthetic pathway, an artificial gene cluster was constructed by linking the genes of five associated enzymes on a plasmid vector. This plasmid was inserted into the E. coli NM522 strain to form globotriose-producing cells ('superbug' pLDR20-CKTUF). The specific strain was conveniently applied to the synthesis of globotriose trisaccharide and its derivatives, as potential neutralizers for Shiga toxin. This work demonstrates a novel and economical method for generating ligand diversity for carbohydrate drug development. 相似文献
82.
A semiclassical transition amplitude is derived for electronically inelastic molecular collisions in the presence of an intense electromagnetic field. The calculation of this amplitude involves the integration of classical trajectories for nuclei moving on effective potential energy surfaces. These surfaces, which exhibit an avoided crossing due to the coupling through the dipole interaction around the resonance nuclear configuration, are analytically continued to their complex intersection points, where trajectories switch surfaces. Other parts of these surfaces are similar to the field-free adiabatic surfaces, where one is shifted by ?ω. Metastable states, formed at some collision energies, are suggested in the field which are absent in the field-free system. 相似文献
83.
The reaction of cycloheptatriene (CHT) with dibenzoylacetylene (DBA), in refluxing toluene gave a mixture of products consisting of 6,7-dibenzoyltricyclo[3.2.2.02.4]nona-6,8-diene 6 and 3,4-dibenzoylbicyclo [3.2.2]nona-2,6,8-triene 7. Neat heating of a mixture of CHT and DBA around 95–100° gave besides, 6 and 7, small amounts of two dimers of DBA. Photolysis of 6 in benzene solution gave a lactonic, rearrangement product. 25. The structures of the various products have been established and reasonable mechanisms for their formation have been suggested. 相似文献
84.
Michael I. Bruce Andrew Catlow Mark G. Humphrey George A. Koutsantonis Michael R. Snow Edward R.T. Tiekink 《Journal of organometallic chemistry》1980,338(1):59-80
A reinvestigation of the reaction between C2(CO2Me)2 and RuH(PPh3)2(η-C5H5) and some related complexes is reported. Initial cis addition is followed by conversion into the trans isomer. In the case of the bis-(PPh3) complex, isomerisation is followed by chelation of the ester CO group with concomitant displacement of one PPh3ligand. The resulting chelate complex reacts with CO or CNBut to give the (Z)-RuC(CO2Me)CH(CO2Me) complexes; the (E)-isomer of the carbonyl complex is obtained by addition of C2(CO2Me)2to RuH(CO)(PPh3)(η-C5H5). The 1Hand 13C NMR spectra are not a reliable guide to assignment of the stereochemistry of the vinyl group. Other products isolated from the initial reaction are the bis-insertion product CH(CO2Me)} -(PPh3)(η-C5H5) and the 1/2 PPh3/C2(CO2Me)2 adduct. The molecular structures of Ru{(Z)-C(CO2Me)CH(CO2Me)}(CO)(PPh3(η-C5H5) · 0.5EtOH, Ru{(E)-C(C2Me)CH(CO2Me)}(dppe)(η-C5H5) and CH(CO2Me)}(PPh3)(η-C5H5) have been determined. The cis isomer is monoclinic, space group P21,with a 9.328(8), b 17.385(10), c 10.356(7) Å, β 101.78(3)° and Z = 2; 2107 data with I ≥ 2.5σ(I) were refined to R = 0.076 Rw = 0.085. The trans isomer is triclinic, space group P, with a 10.404(7) b 11.221(6), c 13.230(9) Å, α 92.67(5), β 110.56(5), γ 106.21(5)° and Z = 2; 2520 data with I ≥ 2.5σ(I) were refined to R = 0.055 Rw = 0.068. The butadienyl complex is monoclinic, space group P21/a, with a 19.655(8), b 8.674(4), c 21.060(5) Å, β 116.22(3)° and Z = 4; 2724 data with I ≥ 2.5σ(I) were refined to R = 0.042, Rw = 0.047. 相似文献
85.
Two methods for implementing angular momentum decoupling approximations in quantum mechanical reactive scattering examined. Applications of both reactive and nonreactive H + H2 collisions indicate that for the most intense individual reactive transitions and for all degeneracy-averaged ones, these decoupling methods (especially the “proper” decoupling method) yield results in good agreement with those of fully-coupled calculations. However, for the less intense reactive transitions and all individual nonreactive transitions, very large errors can result from use of these approximate methods. 相似文献
86.
Michael R. Detty James W. Hassett Bruce J. Murray George A. Reynolds 《Tetrahedron》1985,41(21):4853-4859
The O-, S-, Se- and Te-containing Δ4,4' -2,2?,6,6'-tetramethyl-, -tetra-t-butyl- and -tetraphenyl-4-(chalcogenapyranyl)-4H-chalcogenapyrans were prepared from the corresponding chalcogenapyran-4-ones. The thia-, selena- and tellurapyran-4-ones were prepared by sodium borohydride reduction of the appropriate chalcogen and addition of the disodium chalcogenide to a 1,4-diyn-3-one. The comparative electrochemistry of all three series was examined by cyclic voltammetry. Within each scries the first oxidation (E1) became more difficult with increasing chalcogen size. The radical-cation states of the tetra-t-butyl analogues were examined by ESR. The O and S analogues gave well-defined five-line spectra, but the Se and Te analogues gave broad single lines. The g value increased with increasing size of the heteroatom. A linear correlation exists between g value and spin-orbit coupling constants λ for the chalcogens, suggesting that the electron densities on the heteroatoms are similar in each heteroatom analogue. 相似文献
87.
Ehmke Pohl George M. Sheldrick Johann J. Blsterli Jrg Kallen Ren Traber Malcolm D. Walkinshaw 《Helvetica chimica acta》1996,79(6):1635-1642
The crystal structure of isocyclosporin A ( 1 ), a rearrangement product of the immunosuppressant drug cyclosporin A, has been determined at 193 (2) K. Crystals are orthorhombic with cell dimensions a = 26.684 (7), b = 26.936 (3) Å, c = 28.549 (7) Å, space group C2221. The structure was solved by direct methods and refined by full-matrix least-squares methods to a conventional R value of 0.110. In contrast to the structure of cyclosprin A in solution and in the crystal, isocyclosporin A ( 1 ) has no regular secondary structural elements. The backbone adopts an open, irregular conformation with cis amide bonds between residue 2 and 3, and 3 and 4, respectively. All the other amide bonds and the ester linkage are trans. Contrary to crystal structures of cyclosporin derivatives, this crystal structure is stabilized by two transannular and four intermolecular H-bonds. 相似文献
88.
Charles W. Bock Mendel Trachtman Philip George 《Journal of computational chemistry》1980,1(2):167-177
The geometry, harmonic and anharmonic force fields, and fundamental vibrational frequencies of cis- and trans-thiolformic acid are studied ab initio in the 4-31G basis set. An extensive comparison is made between changes in diagonal and off-diagonal quadratic and cubic force constants and diagonal stretching quartic constants in going from the chain to the ring structure in thiolformic acid and formic acid. The changes in the force constants are indicative of a much weaker interaction in the trans conformer between S? H and O?C, compared with O–H and O?C, in keeping with the weaker hydrogenbonding property of the S? H group in general. 相似文献
89.
Wen-Ping Wang Robert G. Parr Danny R. Murphy George A. Henderson 《Chemical physics letters》1976,43(3):409-412
It is demonstrated that the ground-state atomic kinetic energy functional T[?], where ? is the electron density, can be computed to surprising accuracy from the truncated gradient expansion: T[?] = + T2[?] + T4[?], with To[?] = (3π2) ∫ ? dτ, T2 [?] = ∫ (??)2??1 dτ, and T4 [?] given by the formula of Hodges. Calculations of T0, T2 and T4 are reported for He with ? both the Hartree—Fock and a very accurate density, and for Ne, Ar and Kr with ? the Hartree—Fock density. For Kr, T0 + T2 + T4 is within 0.3% of the exact Hartree—Fock T, with T2/T0 = 0.05, T4/T2 = 0.17. 相似文献
90.
George R. Clark Brian W. Skelton T.Nell Waters 《Journal of organometallic chemistry》1975,85(3):375-394
The crystal structure of tetrakis(methyldiphenylphosphine)iridium(I) tetrafluoroborate with cyclohexane of solvation, [Ir(PPh2Me)4]BF4·C6H12, has been determined from a three-dimensional X-ray analysis. The compound has been analysed in space group C2/c of the monoclinic system. There are twelve molecules (i.e. 1.5 molecules per asymmetric unit) in a cell of dimensions a = 36.804(8), b = 22.93(2), c = 21.676(4) Å, β = 121.41(1)°. Block-diagonal least-squares refinement has given a final R-factor of 0.060 for 7905 reflections having I > 3σ(I).The structure consists of two crystallographically distinct, but structurally similar molecules, one on a general position and one on a crystallographic two-fold axis. The phosphine ligands around the iridium atoms are in a very distorted square-planar arrangement. The reactions of the cation axe discussed in terms of this structure. 相似文献