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111.
Rotationally resolved, velocity distributions for desorbed O2 molecules formed by O-atom recombination on the surface of a polycrystalline Ag surface are reported. Surface O atoms are generated by oxygen permeation through a 0.25-mm-thick Ag foil heated to 1020 K. Desorbing O2 molecules are probed by (2 + 1) resonant multiphoton ionization via the C 3Pig (3ssigma), v' = 2 <-- <-- X 3Sigmag-, v" = 0 transition and time-of-flight mass spectrometry. Measured velocity distributions are near Maxwell-Boltzmann and yield average translational energies which are significantly lower than the surface temperature ([Et]/2kB approximately 515 K) and essentially independent of rotational excitation. Comparison of the observed C-X (2,0) resonantly enhanced multiphoton ionization spectrum with spectral simulations suggests that the v" = 0 rotational state distribution is more consistent with the surface temperature, but spectral congestion and apparent intensity perturbations prevent a more quantitative analysis. The calculated, sticking curves show a small barrier energy barrier (approximately 10 meV) beyond which sticking decreases. These observations are consistent with low energy desorption and adsorption pathways involving a weakly bound molecular O2 precursor. 相似文献
112.
Urs D. Wermuth Graham Smith Jonathan M. White 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(6):o353-o357
The structures of two brucinium (2,3‐dimethoxy‐10‐oxostrychnidinium) salts of the α‐hydroxy acids l ‐malic acid and l ‐tartaric acid, namely brucinium hydrogen (S)‐malate pentahydrate, C23H27N2O4+·C4H5O5−·5H2O, (I), and anhydrous brucinium hydrogen (2R,3R)‐tartrate, C23H27N2O4+·C4H5O6−,(II), have been determined at 130 K. Compound (I) has two brucinium cations, two hydrogen malate anions and ten water molecules of solvation in the asymmetric unit, and forms an extensively hydrogen‐bonded three‐dimensional framework structure. In compound (II), the brucinium cations form the common undulating brucine sheet substructures, which accommodate parallel chains of head‐to‐tail hydrogen‐bonded tartrate anion species in the interstitial cavities. 相似文献
113.
K. I. White 《Optical and Quantum Electronics》1979,11(2):185-196
Both the theoretical basis and experimental realization of the refracted near-field technique for the direct measurement of optical fibre profiles are presented. The technique requires minimal sample preparation, no computation and is applicable to both single and multimode fibres. Both the core and the cladding are profiled. After outlining the problems associated with other techniques, the use of this method for the measurement of fibre profile, numerical aperture and geometry is discussed. Leaky mode rejection and resolution are treated in detail. A fitting procedure for determining the-value of a profile is given. The experimental apparatus is fully discussed. Results are presented to illustrate both the applicability of the technique to single and multimode fibres and also the rejection of leaky modes. The experimental sensitivity is shown sufficient to reveal an index fluctuation having a wavelength < 1m and an amplitude of < 0.0001. 相似文献
114.
115.
Lai Hong Wong Asier Unciti-Broceta Michaela Spitzer Rachel White Mike Tyers Lea Harrington 《Chemistry & biology》2013,20(3):333-340
Highlights? Growth arrest induced by human telomerase in yeast is chemically reversible ? Readout is sensitive to telomerase catalytic activity and telomere recruitment ? Three cell-permeable compounds also inhibit purified human telomerase ? Yeast can be successfully used to screen for human telomerase inhibitors 相似文献
116.
Sesquifulvalene (1) is a substance of considerable theoretical interest1 which is expected to have a significant contribution to its structure from the dipolar resonance form 1b,2 and hence an exceptionally low bond order for the central double bond. Although a wide variety of annelated and otherwise substituted sesquifulvalenes have been prepared and characterized,3 the parent substance has proved remarkably elusive. Very recently, a preparation of 1 in crystalline form has been reported, based upon elimination of an acetoxy-11,12-dihydrosesquifulvalene derivative.4 相似文献
117.
The syntheses, characterization and metal ion complexation and extraction capabilities of six new calix[4]arene Schiff base compounds, 5–10, are reported. The preparation of the compounds was achieved by the condensation of 5,17-diamino-11,23-di-tert-butyl-25,27-di-n-butoxy-26,28-dihydroxycalix[4]arene with the appropriate aldehyde (5-bromosalicylaldehyde for 5, 4-anisaldehyde for 6, 4-(dimethylamino)benzaldehyde for 7, 9-anthracenecarboxaldehyde for 8, 1-pyrenecarboxaldehyde for 9, and 9-fluorenecarboxaldehyde for 10) in refluxing ethanol. The compounds were characterized by 1H and 13C NMR spectroscopy, IR spectroscopy, high-resolution mass spectrometry and elemental analysis. The X-ray crystal structures of 7, 8 and 9 (as dichloromethane solvates) revealed that the calixarene molecules adopt H-bond stabilized, distorted-cone conformations and form centrosymmetric dimers in the solid state. Compounds 5–10 did not form host–guest complexes with NEt4[(bdt)MoO2(OSiPh3)] (bdt2–=benzene-1,2-dithiolate), a potential precursor for biologically relevant oxosulfido-Mo(VI/V) enzyme models; such host–guest complexes have the potential to stabilize these sought-after but highly reactive model compounds. In addition, the capabilities of 5–10 to extract selected metal ions (Ni2+, Co2+, Cu2+, Zn2+, Ag+, Pb2+, Cd2+ and Hg2+) from an aqueous into an organic phase have been assessed by picrate extraction experiments. Compound 5 displayed exceptional selectivity towards Ni2+, compound 7 exhibited enhanced extraction towards all of the metal ions tested and compounds 6, 9 and 10 showed very high selectivity towards Hg2+. On the other hand, compound 8 exhibited negligible capacity to extract any of the metal ions tested. 相似文献
118.
Thomas Behrsing Glen B. Deacon Peter C. Junk Brian W. Skelton Alexandre N. Sobolev Allan H. White 《无机化学与普通化学杂志》2013,639(1):41-48
Reaction of LnCl3 with Na2(PDDA) (PDDA = phenylene‐1, 2‐dioxydiacetate) in a 1 to 2 mol ratio in aqueous solution yielded [Ln2(PDDA)3(H2O)6] · 2H2O, structurally characterized for Ln = Ce ( 1 ), Sm ( 2 ) (redetermination), Tb ( 3 ) and Y ( 4 ) in a monoclinic C2/c array, a second related structural form [orthorhombic, Pbcn] being obtained for Tb ( 5 ), Ho ( 6 ) and Er ( 7 ). The ‘domains of existence' of these two previously described forms are now extended to Ce–Dy, Y, and Eu–Er, respectively. Reaction under the same conditions for the heavier Yb3+ ion yielded [Yb2(PDDA)3(H2O)6](∞|∞) · 4H2O ( 8 ), orthorhombic, Pbca. In the case of Ln = La the bimetallic species [NaLa(PDDA)2(H2O)2](∞|∞) · 4H2O ( 9 ) was obtained, while reaction of LnCl3 with Na2(PDDA) in a 1 to 3 mol ratio led to the isolation of the isotypic (monoclinic, P21/c) [NaLn(PDDA)2(H2O)2](∞|∞) · 4H2O) for Ln = Ce ( 10 ) and Sm ( 12 ). With the smaller Ln = Yb, the more definitively bimetallic [NaYb(PDDA)2(H2O)2](∞|∞) · 3H2O ( 13 ) (triclinic, P$\bar{1}$ )) was obtained, the trihydrate solvation ascribed differing from that recorded (dihydrate) in a cosynchronous report. 相似文献
119.
Rohan J. Williams Nathan W. McGill Jonathan M. White Spencer J. Williams 《Journal of carbohydrate chemistry》2013,32(5):236-263
Variable yields and glycosylation stereoselectivity were obtained for NIS/TfOH-medi- ated reaction of 4-methoxyphenyl 2,4,6-tetra-O-acetyl-β-D-galactopyranoside and thiogalactosides bearing acetyl, benzoyl, 2,6-dimethoxylbenzoyl, 2,4,6-trimethylbenzoyl, or 2,6-dichlorobenzoyl groups at the 2-positions and acetyl at the remainder. X-ray structures of 4-methylphenyl 2,3,4,6-tetra-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyr anoside and 4-methylphenyl 3,4-O-isopropylidene-2,6-di-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyranoside revealed slightly distorted 4 C 1 chair conformations. Variable temperature NMR revealed that activation of 4-methylphenyl 2,3,4,6-tetra-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyranoside afforded only dioxolenium ion, whereas 4-methylphenyl 3,4,6-tri-O-acetyl-2-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyranoside gave a 1:1 mixture of dioxolenium ion and glycosyl triflate. However, the reaction intermediates formed from these deactivated donors do not influence the glycosylation stereoselectivity; instead, it is influenced by steric and electronic interactions at the transition states. 相似文献
120.
We report the first measurements and calculations of the intrinsic mobility of bridge-bonded oxygen (BBO) vacancies on a rutile TiO2(110). The sequences of isothermal (340-420 K) scanning tunneling microscope images show that BBO vacancies migrate along BBO rows. The hopping rate increases exponentially with increasing temperature with an experimental activation energy of 1.15 eV. Density functional theory calculations are in very good agreement giving an energy barrier for hopping of 1.03 eV. Both theory and experiment indicate repulsive interactions between vacancies on a given BBO row. 相似文献