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961.
Brown DJ Chisholm MH Gribble CW 《Dalton transactions (Cambridge, England : 2003)》2007,(18):1793-1801
The compounds M(2)(mhp)(4), where M = Mo or W and mhp is the anion formed from deprotonation of 2-hydroxy-6-methylpyridine, are shown to react with carboxylic acids RCOOH to give an equilibrium mixture of products M(2)(O(2)CR)(n)(mhp)(4-n) where R = 2-thienyl and phenyl. The equilibrium can be moved in favor of M(2)(O(2)CR)(4) by the addition of excess acid or by the favorable crystallization of these products. The latter provides a facile synthesis of the W(2)(O(2)CR)(4) compound where R = 9-anthracene. Reactions involving 2,4,6-triisopropyl benzoic acid, TiPBH, yield M(2)(TiPB)(2)(mhp)(2) compounds as thermodynamic products. Reactions involving Me(3)OBF(4) (1 and 2 equiv.) yield the complexes Mo(2)(mhp)(3)(CH(3)CN)(2)BF(4) and Mo(2)(mhp)(2)(CH(3)CN)(4)(BF(4))(2), respectively. The latter compound has been structurally characterized and shown to have mirror symmetry with two cis mhp ligands: MoMo = 2.1242(5) A, Mo-O = 2.035(2) A, Mo-N(mhp) = 2.161(2) A, and Mo-N(CH(3)CN) = 2.160(3) and 2.170(3) A. Reactions involving Mo(2)(mhp)(3)(CH(3)CN)(2)(2+) and Mo(2)(mhp)(2)(CH(3)CN)(4)(2+) with (n)Bu(4)NO(2)CMe (1 and 2 equiv.) yield the complexes Mo(2)(mhp)(3)(O(2)CMe) and Mo(2)(mhp)(2)(O(2)CMe)(2) which are shown to be kinetically labile to ligand scrambling. Reactions between Mo(2)(mhp)(3)(CH(3)CN)(2)(+)BF(4)(-) (2 equiv.) and [(n)Bu(4)N(+)](2)[O(2)C-X-CO(2)](2-) yielded dimers of dimers [Mo(2)(mhp)(3)](2)(micro-O(2)C-X-CO(2)] where X = nothing, 2,5- or 3,4-thienyl and 1,4-C(6)H(4). Reactions between Mo(2)(mhp)(2)(CH(3)CN)(4)(2+)(BF(4)(-))(2) and tetra-n-butylammonium oxalate and terephthalate yield compounds [Mo(mhp)(2)bridge](n) which by MALDI-TOF MS are proposed to be a mixture of molecular squares (n = 4) and triangles (n = 3) along with minor products of [Mo(2)(mhp)(3)](2)(bridge) and Mo(2)(mhp)(4) that arise from ligand scrambling. 相似文献
962.
Let β
0=0.308443… denote the Littlewood-Salem-Izumi number, i.e., the unique solution of the equation
In this paper it is proved that if a
0≥a
1≥⋅⋅⋅≥a
n
>0 and
, k≥1 then for all θ∈(0,π)
and furthermore, the number β
0 is best possible in the sense that for any β∈(0,β
0)
where the coefficients c
k
(β) are defined as
Results for the sine sums are obtained as well.
These results generalize and sharpen the well known trigonometric inequalities of Vietoris.
This research was supported by a grant from the Australian Research Council. The second author was also supported in part
by the NSERC Canada under grant G121211001. The third author was also supported in part by the NSFC of China under grant 10471010. 相似文献
963.
A. B. D. Brown C. Ferrero T. Narayanan A. R. Rennie 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(3):481-489
The structure and phase behaviour of a colloidal dispersion of plate-like particles are described. The plates are nickel (II)
hydroxide and have short-range, repulsive interactions and a low polydispersity. As the concentration of the plates is increased,
an equilibrium phase separation between a columnar phase and a less ordered phase is observed. Complementary measurements
using small-angle neutron and small-angle X-ray scattering have been used to distinguish the columnar phase from other possible
ordered structures. Previously isotropic-nematic phase transitions have been observed [#!ref1!#], however this dispersion
forms the more highly ordered columnar phase, due to the aspect ratio and the low polydispersity of the plate-like particles.
The concentration at which phase separation occurs, increases as the range of the particle interactions is reduced. This system
provides an interesting model for comparison with theory and calculations of structures in liquid crystal and mesophase in
which the particle interactions can be altered.
Received 24 February 1999 相似文献
964.
The objective of this case study was to examine the economics of three lignocellulose-to-ethanol conversion technologies:
fast pyrolysis integrated with a fermentation step, simultaneous saccharification and fermentation (SSF), and dilute sulfuric
acid hydrolysis and fermentation. All technologies were assumed to have an annual production rate of 25 million gallons of
ethanol. The three technologies were compared in terms of capital costs, operating costs, and ethanol production costs. Sensitivity
analyses were carried out to study the uncertainties of wood costs and ethanol production rates on ethanol production costs.
Final economic analysis showed that fast pyrolysis integrated with a fermentation step is comparable with the other two processes
and suggests that it should be considered for further development. 相似文献
965.
The effects of complex formation between sodium dodecylsulfate (SDS) and the positively charged (3-(2-methylpropionamide)propyl)trimethyl-ammonium
chloride-acrylamide (MAPTAC-AM) copolymer have been studied in dilute and semidilute aqueous solution in the presence of 10 mM
NaCl. Two different charge densities of the copolymer have been used in the study: 0.31 and 0.66, corresponding to the proportion
of MAPTAC units. Dynamic light scattering (DLS) and rheometry (static low-shear and capillary viscometry) have been performed
on the systems at different charge ratios, i.e., SDS/MAPTAC molar ratios, r. Regarding the phase behavior, the maximum binding ratio prior to precipitation differs between the copolymers. A 1.0% w/v
solution of SDS/31% MAPTAC-AM is soluble at r = 0.4 , while an SDS/66% MAPTAC-AM solution of 1.0% w/v shows phase separation at this ratio. With excess surfactant, the
complex in the former system is resolubilized at r=2.0, whereas the latter system is still phase-separated at r=5.0. DLS results show that, for both copolymers, the hydrodynamic radius, R
h, of the single-chain copolymer-surfactant complex decreases as a function of r, but then increases slightly prior to phase separation. The corresponding hydrodynamic virial coefficient, k
D, changes in the same manner as R
h. The light-scattering data also show that the formation of larger structures is promoted as the polymer concentration is
increased from 0.2 to 1.0% w/v. This is shown by the increase in the relative aggregate-to-single coil peak areas in the relaxation
time distributions. Both systems have this common trend. The results from rheological measurements support the results from
DLS. A reduction in intrinsic viscosity, [η], is observed on increasing r up to phase separation. The major part of the static low-shear measurements showed Newtonian behavior for both systems at
different copolymer concentrations (27.6–138 mM), and at different r. These systems, partially ionic polymer/oppositely charged surfactant, present very interesting rheological behavior at relatively
high polymer concentrations and at low r values. Their behavior is similar to those of hydrophobically modified polyelectrolytes.
Received: 17 June 1998 Accepted: 12 August 1998 相似文献
966.
The acyl endoperoxide arteannuin H, recently reported as a novel natural product from Artemtsia annua, has been obtained in two steps from the photooxidation of dihydroartemisinic acid, thereby confirming biogenetic speculation regarding its derivation from a secondary allylic hydroperoxide. The little studied 3,2-rearrangement reaction of such allylic hydroperoxides is also discussed. 相似文献
967.
Photooxidation of 4-amorphen-11-ol (1), recently reported as one of the major sesquiterpene natural products from the medicinal plant Fabiana imbricata, results in three allylic hydroperoxides 6, 9 and 10, which are expected from the “ene-type” reaction of molecular oxygen with the tri-substituted double bond in 1. The tertiary allylic hydroperoxide 6 undergoes carbon-carbon bond cleavage and a second autoxidation reaction to yield the more highly oxygenated seco-amorphane 11 under very mild conditions. In acid, this compound may then undergo either a second carbon-carbon bond cleavage reaction to yield nor-sesquiterpenes 2 and 3 (reported as bona fide natural products from F. imbricata, or cyclize to the sesquiterpene peroxofabianane (5), which is a presumed precursor to the natural product fabianane (4). Some mechanistic investigations concerning the two chemical processes: viz:- carbon-carbon bond cleavage and autoxidation which would account for the formation of natural products 2, 3 and 4 from 1 are reported. Tertiary allylic hydroperoxide 32, which lacks the 11-hydroxyl functional group present in 1 undergoes only C-4/C-5 carbon-carbon bond cleavage under more forcing conditions, suggesting a role for this functional group in assisting the autoxidation reactions of 4-amorphen-11-ol. 相似文献
968.
Multiplexed piezoresistive sensor arrays hold great potential for measuring contact stress distributions for orthopedic research
applications. However, their acceptance has been handicapped by output drift and the fact that their dynamic response has
not been well characterized. In this report, the static and dynamic responses of one device of this class (the K-Scan piezoresistive
contact stress sensor) are formally characterized using a specially made pressure vessel that provides spatially homogeneous
contact stress. Drift was predominant early in static loading, reaching relationtive errors of approximately 30 percent over
a 10-min period. During loading, first-order dynamic analysis showed that the time constant (and time lag) was nearly zero
and there was little attenuation of the output up to 20 Hz. A deconvolution algorithm proved capable of compensating for the
great majority of static drift. 相似文献
969.
Laurie A. Christianson Melissa J. Lucero Daniel H. Appella Daniel A. Klein Samuel H. Gellman 《Journal of computational chemistry》2000,21(9):763-773
We added parameters to the AMBER* force field to model cyclic β‐amino acid derivatives more accurately within the commonly used MacroModel program. In an effort to generate an improved treatment of cyclohexane and cyclopentane conformational preferences, carbon–carbon torsional parameters were modified and incorporated into a force field we call AMBER*C. Simulation of trans‐2‐aminocyclohexanecarboxylic acid (trans‐ACHC) and trans‐2‐aminocyclopentanecarboxylic acid (trans‐ACPC) derivatives using AMBER*C produces more realistic energy differences between (pseudo)diaxial and (pseudo)diequatorial conformations than does simulation using AMBER*. AMBER*C molecular dynamics simulations more accurately reproduce the experimental hydrogen‐bonding tendencies of simple diamide derivatives of trans‐ACHC and trans‐ACPC than do simulations using the AMBER* force field. More importantly, this modified force field allows accurate qualitative prediction of the helical secondary structures adopted by β‐amino acid homo‐oligomers. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 763–773, 2000 相似文献
970.