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1.
The origin of the alpha-effect has been probed through a combination of calorimetric and kinetic studies involving butane-2,3-dione monoximate as alpha-nucleophile and p-chlorophenoxide as normal nucleophile in the reaction with p-nitrophenyl acetate in DMSO-H(2)O mixtures, which has been shown to exhibit a bell-shaped profile in the alpha-effect with solvent composition. The study, involving determination of enthalpies of solution and activation parameters, has allowed a dissection of contributions to the alpha-effect of ground-state destabilization and transition-state stabilization in these DMSO-H(2)O solvent media. It has been found that over the solvent composition 0-50 mol % DMSO desolvation of the alpha-nucleophile is the main driving factor to the increasing alpha-effect. However, in solvent mixtures covering 50-90 mol % DMSO the thermodynamic activation parameters suggest an interplay of factors that result in the bell-shaped alpha-effect profile. Discussion is presented that includes possible medium-dependent nonsynchronicity of nucleophile desolvation and bond formation for the alpha-nucleophile. 相似文献
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During radiotherapy of cancer, neighboring normal cells may receive sub-lethal doses of radiation. To investigate whether such low levels of radiation modulate normal cell responses to death stimuli, primary cultured human fibroblasts were exposed to various doses of gamma-rays. Analysis of cell viability using an exclusion dye propidium iodide revealed that the irradiation up to 10 Gy killed the fibroblasts only to a minimal extent. In contrast, the cells efficiently lost their viability when exposed to 0.5-0.65 mM H(2)O(2). This type of cell death was accompanied by JNK activation, and was reversed by the use of a JNK-specific inhibitor SP600125. Interestingly, H(2)O(2) failed to kill the fibroblasts when these cells were pre-irradiated, 24 h before H(2)O(2) treatment, with 0.25-0.5 Gy of gamma-rays. These cytoprotective doses of gamma-rays did not enhance cellular capacity to degrade H(2)O(2), but elevated cellular levels of p21(Cip/WAF1), a p53 target that can suppress H(2)O(2)-induced cell death by blocking JNK activation. Consistently, H(2)O(2)-induced JNK activation was dramatically suppressed in the pre-irradiated cells. The overall data suggests that ionizing radiation can impart normal fibroblasts with a survival advantage against oxidative stress by blocking the process leading to JNK activation. 相似文献
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Second-order rate constants have been measured spectrophotometrically for the reactions of O-2,4-dinitrophenyl thionobenzoate (1) and 2,4-dinitrophenyl benzoate (2) with a series of substituted pyridines in 80 mol % H(2)O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The Br?nsted-type plots obtained are nonlinear with beta(1) = 0.26, beta(2) = 1.07, and pK(a) degrees = 7.5 for the reactions of 1 and beta(1) = 0.40, beta(2) = 0.90, and pK(a) degrees = 9.5 for the reactions of 2, suggesting that the pyridinolyses of 1 and 2 proceed through a zwiterionic tetrahedral intermediate T(+/-) with a change in the rate-determining step at pK(a) degrees = 7.5 and 9.5, respectively. The thiono ester 1 is more reactive than its oxygen analogue 2 except for the reaction with the strongest basic pyridine studied (pK(a) = 11.30). The k(1) value is larger for the reactions of 1 than for those of 2 in the low pK(a) region, but the difference in the k(1) value becomes negligible with increasing the basicity of pyridines. On the other hand, 1 exhibits slightly larger k(2)/k(-1) ratio than 2 in the low pK(a) region but the difference in the k(2)/k(-1) ratio becomes more significant with increasing the basicity of pyridines. Pyridines are more reactive than alicyclic secondary amines of similar basicity toward 2 in the pK(a) above ca. 7.2 but less reactive in the pK(a) below ca. 7.2. The k(1) value is slightly larger, but the k(2)/k(-1) ratio is much smaller for the reactions of 2 with pyridines than with isobasic secondary amines in the low pK(a) region, which is responsible for the fact that the weakly basic pyridines are less reactive than isobasic secondary amines. 相似文献
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We examine the magnetoresistance (MR) of conducting polymers with interest paid on the role of structural flexibility. Through
Monte Carlo simulation and Green function method, we evaluate the electric transmission for a variety of polymer configurations.
It is found that for a single polymer the transmission displays a complex oscillation and also a parity-dependent periodicity.
For an ensemble of polymers the averaged transmission yields the nonlinear behavior of MR under varying magnetic fields. Interestingly,
more flexible polymers are shown to achieve higher MR, depending on the population and the size of the loops. 相似文献
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