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31.
On the asymmetric autocatalysis of aldol reactions: The case of 4‐nitrobenzaldehyde and acetone. A critical appraisal with a focus on theory
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Under neutral conditions, spontaneous mirror symmetry breaking has been occasionally reported for aldol reactions starting from achiral reagents and conditions. Chiral induction might be interpreted in terms of autocatalysis exerted by chiral mono‐aldol or bis‐aldol products as source of initial enantiomeric excesses, which may account for such experimental observations. We describe here a thorough Density Functional Theory (DFT) study on this complex and otherwise difficult problem, which provides some insights into this phenomenon. The picture adds further rationale to an in‐depth analysis by Moyano et al, who showed the isolation and characterization of bis‐aldol adducts and their participation in a complex network of reversible steps. However, the lack of enantiodiscrimination (ees vanish rapidly in solution) suggests, according to the present results, a weak association in complexes formed by the catalysts and substrates. The latter would also be consistent with almost flat transition states having similar heights for competitive catalyst‐bound transition structures (actually, we were unable to locate them at the level explored). Overall, neither autocatalysis as once conjectured nor mutual inhibition of enantiomers appears to be operating mechanisms. Asymmetric amplification in early stages harnessing unavoidable enantiomeric imbalances in reaction mixtures of chiral products represents a plausible interpretation. 相似文献
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In a 2 × 2 crossover bioavailability study, the sets of estimates of the pharmacokinetic parameters quite often have a symmetric covariance structure between the two treatments. For testing the equality of the intra‐subject covariance matrices for the two treatments in such studies, we suggest in this paper some statistical tests. When the response vectors are bivariate, we propose an exact test. Since the statistical procedures depend on the assumption of a symmetric covariance structure between the two treatments, we put forth some statistical tests for this assumption. We then apply the discussed tests to real data from a crossover bioavailability trial. 相似文献
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T. David Westmoreland 《Inorganica chimica acta》2008,361(4):1187-1191
A symmetry analysis of the Berry pseudorotation of trigonal bipyramidal transition metal complexes is presented. Predictions are made regarding which electronic configurations are allowed to undergo pseudorotation and which configurations are symmetry forbidden. A comparison to reported fluxional five-coordinate complexes shows that the predictions are consistent with experimental observations. 相似文献
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Matthieu Gilson Anthony N. Burkitt David B. Grayden Doreen A. Thomas J. Leo van Hemmen 《Biological cybernetics》2009,101(2):103-114
Spike-timing-dependent plasticity (STDP) is believed to structure neuronal networks by slowly changing the strengths (or weights)
of the synaptic connections between neurons depending upon their spiking activity, which in turn modifies the neuronal firing
dynamics. In this paper, we investigate the change in synaptic weights induced by STDP in a recurrently connected network
in which the input weights are plastic but the recurrent weights are fixed. The inputs are divided into two pools with identical
constant firing rates and equal within-pool spike-time correlations, but with no between-pool correlations. Our analysis uses
the Poisson neuron model in order to predict the evolution of the input synaptic weights and focuses on the asymptotic weight
distribution that emerges due to STDP. The learning dynamics induces a symmetry breaking for the individual neurons, namely
for sufficiently strong within-pool spike-time correlation each neuron specializes to one of the input pools. We show that
the presence of fixed excitatory recurrent connections between neurons induces a group symmetry-breaking effect, in which
neurons tend to specialize to the same input pool. Consequently STDP generates a functional structure on the input connections
of the network. 相似文献
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对银沙槐种子破除休眠的适宜方法及其幼苗生长进行研究。结果表明:破口和沙磨处理休眠破除效果最好,但容易引起种子腐烂死亡;分别用65、80和95℃热水处理种子,以95℃1 min破除种子休眠效果最好,且幼苗长势好,65℃效果最差,80℃可破除部分种子休眠,除80℃外延长热水处理时间可导致死种子数增加,95℃则引致大部分种子死亡;液氮浸种处理效果均显著,其中以液氮处理10 min效果最好,但幼苗多为不正常苗而致死;硫酸处理以浸种120 min效果最好,但处理时间过长可导致死种子数增多,且造成少数幼苗根部腐蚀死亡。综上所述,打破银沙槐种子休眠的较优处理方法是硫酸120 min浸种处理和95℃1 min热水处理。 相似文献
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GA3和变温层积对天女木兰种子萌发及内源激素的影响 总被引:2,自引:0,他引:2
用不同质量浓度GA3浸泡天女木兰种子并结合变温层积处理,应用高效液相色谱法对不同时期种子中4种激素GA3、IAA、ABA、ZR含量进行测定,并测量种胚长和萌发率,以探讨天女木兰种胚发育,内源激素含量变化与种子休眠萌发之间的调控关系,为进一步研究种子休眠机理提供理论基础。结果表明:(1)天女木兰成熟种子胚发育不完全,胚乳内高浓度ABA和低浓度GA3是其休眠的主要原因。(2)GA3处理能促使天女木兰种子提前30d完成形态后熟,并以1 500mg·L-1 GA3处理效果最佳。(3)在变温层积过程,天女木兰种胚发育可分三个阶段:阶段Ⅰ(0~70d)完成种胚进一步分化;阶段Ⅱ(70~120d)种胚快速生长时期;阶段Ⅲ(120~150d)休眠完全解除,种子具备发芽能力。种子能否打破休眠主要取决于阶段Ⅰ和Ⅱ的状况。(4)GA3/ABA、IAA/ABA和ZR/ABA在种子后熟期间的变化同胚生长发育存在一致性,认为内源激素的相对水平对种子休眠具有重要的调控作用。 相似文献
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