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131.
D. de Martino L. B. F. M. Waters G. M. H. J. Habets A. R. Taylor 《Il Nuovo Cimento C》1990,13(2):471-479
Summary The X-ray variability of transient X-ray binaries is analyzed in terms of the orbital motion of the neutron star, centrifugal
inhibition of accretion and of the variable moss loss rate due to activity of the optical counterpart. The systems A0535+26
and V0332+53 are studied in this respect.
To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee. 相似文献
132.
In a paper of the same title, Carlson and Hill [2] established results in inertia theory and controllability for a large class of linear transformations on the space of n × n hermitian matrices. Extensions of some of those results and some generalizations of theorems of Wimmer [12] and of Carlson and Loewy [3] are given here. 相似文献
133.
J. A. Waters George A. Mortimer 《Journal of polymer science. Part A, Polymer chemistry》1972,10(3):895-907
Soluble ethylene polymerization catalysts derived from (π-C5H5)2Ti(R)Cl and R′AlCl2 where R is ethyl or higher alkyl and R′ may be methyl or ethyl, were studied both by polymerization kinetics at 0°C and by diagnostic experiments. An early acceleration in rate occurred at this temperature in toluene solvent which was due to a solvent dependent increase in catalyst activity, not to a gradual formation of catalyst. No such solvent effect was found in nonaromatic solvents. The subsequent decay in rate, at least at low temperatures, did not depend upon valence reduction. The effect of Al/Ti ratio was studied, and certain discrepancies in the literature were shown to be due to the method of making kinetic measurements. Oxygen, which has previously been reported to affect the polymerization rate with these catalysts, was also found to eliminate the acceleration period in toluene when present in the amount of 1% of the catalyst. These catalysts gave polymers at low temperature for which the active site had long life and which did not undergo chain transfer. Therefore, they approximate many of the characteristics of living polymers. 相似文献
134.
The concept of achieving low-resolution separations in internally heated capillary membranes is discussed in terms of controlling the diffusion coefficients of volatile organic compounds in poly(dimethylsilicone) membranes in space and time. The behaviour of 1,1,1-trichloroethane in polydimethylsilicone was used in conjunction with a mixed-physics finite element model, incorporating second order partial differential equations, to describe time and spatial variations of mass-flux, membrane temperature and diffusion coefficients. The model, coded with Femlab, predicted highly non-linear diffusion coefficient profiles resulting from temperature programming a 500 [micro sign]m thick membrane, with an increase in the diffusion coefficient of approximately 30% in the last 30% of the membrane thickness. Simulations of sampling hypothetical analytes, with disparate temperature dependent diffusion coefficient relationships, predicted distinct thermal desorption profiles with selectivities that reflected the extent of diffusion through the membrane. The predicted desorption profiles of these analytes also indicated that low resolution separations were possible. An internally heated poly(dimethylsilicone) capillary membrane was constructed from a 10 cm long, 1.5 mm od capillary with 0.5 mm thick walls. Thirteen aqueous standards of volatile organic compounds of environmental significance were studied, and low-resolution separations were indicated, with temperature programming of the membrane enabling desorption profiles to be differentiated. Further, analytically useful relationships in the [micro sign]g cm(-3) concentration range were demonstrated with correlation coefficients >0.96 observed for linear regressions of desorption profile intensities to analyte concentrations. 相似文献
135.
Feyel S Waters T O'Hair RA Wedd AG 《Dalton transactions (Cambridge, England : 2003)》2004,(23):4010-4016
The anions [M(VI)O(O(2))(2)(OR)](-) and [M(VI)O(3)(OR)](-)(M = Cr, Mo, W; R = H, Me, Et, (n)Pr, (i)Pr) were transferred to the gas phase by the electrospray process. Their decomposition was examined by multistage mass spectrometry and collisional activation experiments. The molybdate and tungstate anions [M(VI)O(O(2))(2)(OR)](-) underwent parallel elimination of aldehyde (ketone) and dioxygen while the equivalent chromate underwent loss of dioxygen only. The peroxo ligands were the source of oxidising equivalents in both reactions. For each alkoxo ligand, the total yield of aldehyde for the tungstate system exceeded that for the molybdate system. Collisional activation of [M(VI)O(3)(OMe)](-) led to clean elimination of formaldehyde with the metal centre supplying the oxidising equivalents. For larger alkoxo ligands, only the chromate centre eliminated aldehyde, while the molybdate and tungstate centres underwent clean loss of alkene. Threshold activation voltages indicated that the peroxo ligands of [W(VI)O(O(2))(2)(OMe)](-) are more oxidising than the tungstate centre of [W(VI)O(3)(OMe)](-). (2)H and (18)O isotope tracing experiments were consistent with a formal hydride transfer mechanism operating for oxidation of alkoxo ligand in each system. In the solid state, anions [M(VI)O(O(2))(2)(OR)](-) are typically pentagonal pyramidal (oxo in apical site) while [M(VI)O(3)(OR)](-) are tetrahedral. The data indicate that an equatorial ligand position is the site of alkoxo oxidation in [M(VI)O(O(2))(2)(OR)](-) anions. Comparisons of the gas phase data with those for a solution phase system are made. 相似文献
136.
Christopher M. Timperley Stephen N. MarriottMatthew J. Waters 《Journal of fluorine chemistry》2002,113(1):111-122
Forty bis(fluoroalkyl) phosphoramidates (RFO)2P(O)R were prepared in 10-91% yield by treating phosphorochloridates (RFO)2P(O)Cl where RF was HCF2CH2, HCF2CF2CH2, HCF2CF2CF2CF2CH2, CF3CH2, C2F5CH2, C3F7CH2, (CF3)2CH, (FCH2)2CH and (CH3)2CF3C with nucleophiles HR, where R was NH2, NHMe, NMe2, NHEt and NEt2 in diethyl ether at 0-5 °C. The bulky chloridate [(CH3)2CF3CO]2P(O)Cl reacted with ammonia, methylamine, dimethylamine and ethylamine, but not with diethylamine—even on heating in the presence of 4-dimethylaminopyridine—due to steric hindrance at phosphorus. Fluorinated phosphoramidates have lower basicity and nucleophilicity than their unfluorinated counterparts: (EtO)2P(O)NH2 is more easily hydrolysed by HCl than (CF3CH2O)2P(O)NH2 and whereas, (EtO)2P(O)NH2 is known to react with oxalyl chloride and thionyl chloride to give (EtO)2P(O)NCO and (EtO)2P(O)NSO respectively, (CF3CH2O)2P(O)NH2 reacted only with oxalyl chloride to give (CF3CH2O)2P(O)NCO in 10% yield. Two other new fluorinated species, (CF3CH2O)2P(O)NHOMe and (CF3CH2O)2P(O)N3, were prepared by nucleophilic substitution of bis(trifluoroethyl) phosphorochloridate with methoxyamine and azide ion. 相似文献
137.
The methylation of lysine in histone tails is a common posttranslational modification that functions in histone-regulated chromatin condensation, with binding of methylated lysine occurring in aromatic pockets on chromodomain proteins. We have synthesized a highly stable 12-residue beta-hairpin peptide that exploits the histone-related cation-pi interaction between a methylated lysine residue and a tryptophan residue. Thermodynamic analysis reveals significant entropic stabilization of the peptide due to methylation of the lysine residue. Chemical denaturation of the peptide demonstrates two-state behavior. In comparison to other reported, highly stable designed beta-hairpins, this peptide is the most thermally stable beta-hairpin reported to date. This study provides insight into the role of Lys methylation in histone proteins and more generally in mediating protein-protein interactions. 相似文献
138.
Homologous anionic surfactants may be separated from each other by solvent extraction of their ion-association compounds with iron(II) chelates. Separations of components from a number of surfactant mixtures were investigated, and the effect of solution variables on extraction and separation was studied. A method is proposed for the determination of surfactants of various chain lengths. Applications of this method to synthetic mixtures and to biodegradation experiments are described. 相似文献
139.
We have measured the rotational barriers of meta- and para-substituted N-benzyl-2-(2-fluorophenyl)pyridinium bromides in aqueous solution by dynamic NMR as a model system for offset-stacking interactions in proteins. Because the benzyl ring can stack with the 2-fluorophenyl ring in the offset conformation in the ground state, but not the transition state, the rotational barrier reflects the magnitude of the stacking interaction. Only a small (0.1 kcal/mol) change in rotational barrier was found for para substituents relative to hydrogen. A much larger energy difference was found for electronegative meta substituents (up to 0.66 kcal/mol for CF3). Evidence suggests that this is due at least in part to an electrostatic interaction between electron-poor hydrogens on one ring with the electronegative substituents on the other ring. 相似文献
140.
The anionic heterocumulene SCCCN(-) was generated in the gas phase by collisional activation of the radical anion of 1,2-dicyanoethylenedithiolate. The mechanism of this reaction, as well as the structures of neutral and anionic products, was investigated by hybrid density functional theory (DFT) calculations. Dissociation to form SCCCN(-) and SCN is proposed to occur by a radical directed cyano migration reaction, with calculations suggesting this is the lowest energy fragmentation pathway available to the precursor anion. In contrast, the even-electron protonated 1,2-dicyanoethylenedithiolate anion fragmented by loss of HCN. 相似文献