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621.
622.
Deuterium NMR and modulated differential scanning calorimetry (MDSC) were used to probe the behavior of ultrathin adsorbed poly(methyl acrylate) (PMA). The spectra for the bulk methyl-labeled PMA-d3 were consistent with the motions of the polymer segments being spatially homogeneous. For the polymers adsorbed on silica, multicomponent line shapes were observed. The segmental mobility of the surface polymers increased with increased adsorbed amounts. In contrast to the behavior of the polymers in bulk, the adsorbed lower-molecular-mass PMA-d3 was less mobile than the adsorbed high-molecular-mass polymer. The presence of a polymer overlayer was sufficient to suppress the enhanced mobility of the more-mobile segments of the adsorbed (inner) polymer. MDSC studies on adsorbed poly(methyl methacrylate) showed that the glass-transition temperature of the thin polymer films increased and broadened compared to the behavior of the polymer in bulk. The presence of a motional gradient with the less-mobile segments near the solid-polymer interface and the more-mobile segments near the polymer-air interface was consistent with the experimental observations. 相似文献
623.
624.
625.
Kitchin SJ Tutoveanu G Steele MR Porter EL Harris KD 《The journal of physical chemistry. B》2005,109(48):22808-22813
Solid state dynamic properties have been investigated for the racemic (DL) and enantiomerically pure (L) crystalline forms of the amino acid serine [HO x CH2 x CH(NH3(+)) x CO2(-)] using 2H NMR line shape analysis and 2H NMR spin-lattice relaxation time measurements for samples of DL-serine and L-serine deuterated in the NH3(+) and OH groups. 2H NMR line shape analysis indicates that, for both L-serine and DL-serine, the ND3(+) group undergoes a 3-site 120 degrees jump motion, with jump frequencies in the intermediate motion regime (10(3) s(-1) to 10(8) s(-1)) in the temperature range 153-313 K. However, at a given temperature, the jump frequency is substantially higher for L-serine (e.g., at 233 K, the jump frequency is 5.0 x 10(6) s(-1) for L-serine and 6.0 x 10(4) s(-1) for DL-serine). The OD group is not dynamic on the 2H NMR time scale within the temperature range studied. The results from both 2H NMR line shape analysis (LA) and 2H NMR spin-lattice relaxation time measurements (SLR) indicate that the activation energy for the 3-site 120 degree jump motion of the ND3(+) group is significantly higher for DL-serine [38.0 +/- 1.0 kJ mol(-1) (LA); 39.7 +/- 0.8 kJ mol(-1) (SLR)] than for L-serine [23.4 +/- 0.8 kJ mol(-1) (LA); 23.8 +/- 0.3 kJ mol(-1) (SLR)]. The difference in activation energies between DL-serine and L-serine is substantially greater than any reported previously for an amino acid in different crystal forms. 相似文献
626.
The chemical synthesis, isolation, and characterization of phenyl viologen (PV) as a dication, radical cation, and neutral species are described. Single-crystal X-ray diffraction of PV(2+)2Cl(-.)2H2O and PV(.+)PF(6)(-).pyridine reveals the expected differences in bond lengths and also a structural change from two coplanar central rings in PV(.+) to a twist of 36 degrees between the two central rings in PV(2+). The phenyl viologen radical cation exhibits behavior characteristic of many radical cations, including weak pi-dimerization in the solid state and reversible pi-dimerization in solution. Electrical conductivity measurements of neutral phenyl viologen, the first such measurements of a neutral viologen, reveal that it is a significantly better conductor than the radical cation. Differences in geometric relaxation during charge transfer offer a possible explanation for the higher conductivity of the neutral viologen. 相似文献
627.
C. J. Porter C. J. Proctor E. A. Larka J. H. Beynon 《Journal of mass spectrometry : JMS》1982,17(7):331-334
Translational energy release measurments on metastable ions are used in the comparison of the structures of isomeric ions. Metastable ions, m2+, formed from m1+ ions as the result of a high energy process in the ion source are compared with isomeric metastable ions formed as daughters from fragmentation of metastable m1+ ions in a field. In the case of o-, m- and p-nitrophenol the structure of the [C5H5O]+ ions formed from [C6H5O]+ ions by these two independent methods is different as verified by comparison of the behaviour of [C5H5O]+ ions formed from several other compounds. 相似文献
628.
John C. Wright D. R. Tallant F. J. Gustafson M. V. Johnston M. P. Miller D. S. Moore L. C. Porter J. R. Akse 《Angewandte Chemie (International ed. in English)》1979,18(10):738-752
Although it has been recognized for many years that the spectra of lanthanoid ions can provide useful information about short range phenomena in the neighborhood of these inos in a solid material, lanthanoid spectroscopy has only infrequently been used for studying complex materials because of the problem of line-sorting the complex spectra that are obtained. The advent of convenient tuneable lasers has eliminated this problem. By selectively exciting probe ion luminescence (SEPIL), it is possible to obtain fluorescence and excitation spectra from a single kind of crystallographic environment. Two applications of this method are dicussed in this paper. The first application is the study of the defect chemistry of fluorite materials (compounds with CaF2 lattice). It is shown how this method can provide unique information about the solid state chemistry, thus clarifying many of the unexplained behaviors of this important class of material. The second application shows application shows how ultra-trace analysis can be carried out by causing an association between an analyte ion and a fluorecent probe ion. The unique crystal field levels of a probe ion associated with a particular analyte can be selectively excited so that traces of the ion to be analyzed can be detected with very high selectivity and with very low detection limits. 相似文献
629.
Biochemical characteristics, metabolism, and antitumor activity of several acetylated hexosamines 总被引:1,自引:0,他引:1
R J Bernacki M Sharma N K Porter Y Rustum B Paul W Korythyk 《Journal of supramolecular structure》1977,7(2):235-250
We have synthesized several potential inhibitors and/or modifiers of the carbohydrate portion of plasma membrane glycoconjugates. These include fluorinated and actylated analogs of D-glucosamine, D-galactosamine, and D-mannosamine. These compounds have been tested to determine their effects on both[14C] glucosamine and [3H] leucine incorporation into glycoconjugate and on cell growth and viability using P-288 murine lymphoma cells maintained in tissue culture. The most cytotoxic agent tested was 2-acetamido-2-deoxy-1,3,4,6-tetra-O-acetyl-beta-D-glucopyranose or simply beta-pentaacetylglucosamine which prevented cell growth at 10(-4)-10(-3) M. beta-Pentaacetylglucosamine cytotoxicity was correlated with its high lipid solubility, having an octanol/water partition coefficient of 0.424 as compared with 0.278 for the alpha-anomer and 0.017 for N-acetylglucosamine. In vitro metabolism studies with [4C]- and/or [3H]-labeled pentaacetylglucosamine have indicated intracellular de-O-acetylation leading to the biosynthesis of UDP-N-acetylglucosamine, followed by the incorporation of this sugar into cellular glycoprotein. Concomitant with the formation of increased amounts of this nucleotide sugar, intracellular UTP and CTP pools fell to one third normal within 3 h after the administration of 1 mM pentaacetylglucosamine. At present it is unclear whether the cytotoxicity of beta-pentaacetylglucosamine or other similar agents is due to alterations in nucleotide and nucleotide-sugar pools causing a decrease in energy charge and polynucleotide biosynthesis or is due to a direct effect on membrane glycoconjugate biosynthesis. 相似文献
630.
Douglas M. Hudgins Asit B. Raksit Richard F. Porter 《Journal of mass spectrometry : JMS》1988,23(5):375-380
A combination of charge-stripping and beam-scattering techniques has been used to study the molecular states formed when a fast beam of [C2H2]+ and [C2H3]+ in several isotopic forms are neutralized by electron transfer from metal target atoms (K, Na, Mg and Zn). For [C2H3]+ the isotopic compositions and relative abundances of product states were found to be insensitive to the method of ion preparation (electron impact and chemical ionization). Ground state neutrals are formed in partial abundance when Mg or Zn is used as a target atom. With low ionization potential targets (K and Na) excitel dissociative states of C2H2 and C2H3 are formed as major beam constituents. For these states decomposition products have been identified and fragmentation energies measured. The excited states of C2H2 and C2H3 lie alout 6.8 eV and 2.9 eV, respectively, above their stable ground states. The discussion focuses on the possible identity of the excited states and their structural relations to the precursor ions. 相似文献