首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1703篇
  免费   159篇
  国内免费   97篇
  2024年   4篇
  2023年   30篇
  2022年   25篇
  2021年   38篇
  2020年   64篇
  2019年   59篇
  2018年   54篇
  2017年   61篇
  2016年   59篇
  2015年   43篇
  2014年   68篇
  2013年   124篇
  2012年   57篇
  2011年   76篇
  2010年   53篇
  2009年   104篇
  2008年   96篇
  2007年   100篇
  2006年   98篇
  2005年   77篇
  2004年   89篇
  2003年   70篇
  2002年   81篇
  2001年   55篇
  2000年   36篇
  1999年   32篇
  1998年   28篇
  1997年   29篇
  1996年   14篇
  1995年   25篇
  1994年   24篇
  1993年   24篇
  1992年   24篇
  1991年   17篇
  1990年   8篇
  1989年   18篇
  1988年   7篇
  1987年   13篇
  1986年   12篇
  1985年   14篇
  1984年   11篇
  1983年   6篇
  1982年   9篇
  1981年   7篇
  1980年   2篇
  1979年   4篇
  1978年   2篇
  1976年   3篇
  1972年   1篇
  1971年   1篇
排序方式: 共有1959条查询结果,搜索用时 16 毫秒
141.
142.
The Neolithic period of the southern Levant was an era of tremendous change. Over the course of the Neolithic, the gradual transition from foraging to agriculture involved not merely economic innovations, but also profound shifts in population size, social organization, and technology. This represents possibly the earliest, and certainly one of the earliest, instances of the transition to agriculture. The advent of farming altered human demography, health, and diversity; it shaped the spread of the world's dominant cultures, genes, and languages. 1 , 2 It recast humans' relationship with the natural world, increasing modification of the environment. It both enabled and required the development of new social structures as humans learned how to live in increasingly large and densely packed groups. These transformations were not easily achieved. This paper traces the development of the southern Levantine Neolithic through two thousand years of socioeconomic elaboration and expansion; a major social recalibration in the middle Neolithic; and a final millennium‐and‐a‐half of smaller‐scale and materially simpler adaptations.  相似文献   
143.
Aminolytic kinetic resolution (AKR) of trans-stilbene oxide and trans-beta-methyl styrene oxide proceeded smoothly under microwave irradiation using chiral dimeric and polymeric Cr(III) salen complexes as efficient catalysts, giving regio-, diastereo-, and enantioselective anti-beta-amino alcohols in high yields (49%) and chiral purity (ee up to 94%) in case of 4-methylaniline within 2 min. The kinetic resolution system is approximately five times faster than traditional oil bath heating at 70 degrees C and 420 times faster than the reaction conducted at room temperature with concomitant recovery of respective chirally enriched epoxides (ee, 92%) in excellent yields (up to 48%). The catalyst 1 worked well in terms of enantioselectivity than the catalyst 2, but both the catalysts were easily recovered and reused five times with the retention of its efficiency.  相似文献   
144.
The aims of this study were to test the influence of grazing intensity, effects of local and landscape parameters, and regional effects on orthopteran assemblages. We made our investigations on extensively and intensively grazed cattle pastures in three regions of the Hungarian Great Plain. The regions differed in landscape complexity; one region was situated in a structurally simple landscape with large landscape units, one in a structurally complex landscape with marshy patches and trees in the grasslands and one in a landscape with intermediate structural complexity. In each region we had seven pairs of differently managed grasslands, which differed in grazing intensity. Grasshoppers were recorded once in July 2003 using sweepnet catches and visual and acoustic observations in two 95 m long transects at each site (84 transects in total). Botanical surveys and measurements of other local factors were also made for each transect. After samplings, we digitised the most important land-use types using aerial photographs to produce landscape scale parameters within 100 and 500 m circles around every site. Analysing the management, regional, landscape and local effects on species richness with linear mixed models, we showed only strong significant regional differences. Linear mixed models for Orthoptera abundance yielded significant regional effects and marginal management effects. However, after including local and landscape parameters in a separate model a marginal local effect was found instead of a management effect in addition to the significant regional effect. Logistic regression models of 15 species also revealed the importance of local factors, particularly the importance of grass height, which is highly dependent on grazing intensity. We conclude that management intensity has indirect effects on Orthoptera species richness and abundance. Landscape scale parameters are also important, at least for some species.  相似文献   
145.
General statistical patterns in community ecology have attracted considerable recent debate. Difficulties in discriminating among mathematical models and the ecological mechanisms underlying them are likely related to a phenomenon first described by Frank Preston. He noted that the frequency distribution of abundances among species was uncannily similar to the Boltzmann distribution of kinetic energies among gas molecules and the Pareto distribution of incomes among wage earners. We provide additional examples to show that four different 'distributions of wealth' (species abundance distributions, species–area and species–time relations, and distance decay of compositional similarity) are not unique to ecology, but have analogues in other physical, geological, economic and cultural systems. Because these appear to be general statistical patterns characteristic of many complex dynamical systems they are likely not generated by uniquely ecological mechanistic processes.  相似文献   
146.
The racemization mechanism of tert-butylphenylphosphido-borane is investigated experimentally and theoretically. Based on this converging approach, it is shown, first, that several phosphido-borane molecular species coexist at the time of the reaction and, second, that one particular of both initially assumed reactive routes most significantly contribute to the overall racemization process. From our converging modeling and experimental measurement, it comes out that the most probable species to be here encountered is a phosphido-borane-Li (THF)2 neutral solvate, whose P-stereogenic center monomolecular inversion through a Y-shaped transition structure (Δr: 81 kJ mol?1) brings the largest contribution to the racemization process.  相似文献   
147.
The stochasticity of molecular motion results in the existence of multiple kinetically relevant pathways in many biomolecular mechanisms. Because it is highly demanding to characterize them for complex systems, mechanisms are often described with a single-pathway perspective. However, kinetic network analysis and sub-ensemble experimental insight are increasingly demonstrating not only the existence of competing pathways but also the importance of kinetic selection in biology. This review focuses on advances in multiscale kinetic analysis of proteins, which connects molecular level information from simulations to macroscopic data to characterize mechanistic reaction networks and the reactive flux through them. We describe a range of methods used and highlight several examples where kinetic modeling has revealed functional importance of pathway heterogeneity.  相似文献   
148.
This article argues that the notion of ‘superdiversity’ implies an investigation of diversity that goes well beyond the nature of migration origins and trajectories. To probe the academic value of superdiversity, I situate it within broader academic debates, suggesting that it is necessary to distinguish between superdiversity as a malleable social science concept – a set of variables that researchers conjunctively investigate – and superdiversity as a context in which these variables play out in complex social patterns. I argue that complexity is an integral aspect of superdiversity, before explaining how innovative research methods were used to investigate superdiversity in a dual-site project in London and Toronto in order to explore its relevance for global comparative research.  相似文献   
149.
In this study, R(+)‐α‐methylbenzylamine‐modified magnetic chiral sorbent was synthesized and assessed as a new enantioselective solid phase sorbent for separation of mandelic acid enantiomers from aqueous solutions. The chemical structures and magnetic properties of the new sorbent were characterized by vibrating sample magnetometry, transmission electron microscopy, Fourier transform infrared spectroscopy, and dynamic light scattering. The effects of different variables such as the initial concentration of racemic mandelic acid, dosage of sorbent, and contact time upon sorption characteristics of mandelic acid enantiomers on magnetic chiral sorbent were investigated. The sorption of mandelic acid enantiomers followed a pseudo‐second‐order reaction and equilibrium experiments were well fitted to a Langmuir isotherm model. The maximum adsorption capacity of racemic mandelic acid on to the magnetic chiral sorbent was found to be 405 mg g?1. The magnetic chiral sorbent has a greater affinity for (S)‐(+)‐mandelic acid compared to (R)‐(?)‐mandelic acid. The optimum resolution was achieved with 10 mL 30 mM of racemic mandelic acid and 110 mg of magnetic chiral sorbent. The best percent enantiomeric excess values (up to 64%) were obtained by use of a chiralpak AD‐H column. Chirality 27:835–842, 2015. © 2015 Wiley Periodicals, Inc.  相似文献   
150.
The RAS/RAF/MEK/ERK signal transduction cascade plays an important role in the regulation of critical cellular processes such as cell proliferation, migration, and differentiation. The up-regulation of this pathway can negatively affect cell homeostasis and is responsible for the development of various forms of cancer and inflammation processes. Therefore, there is a strong interest in pursuing drug programs targeting some of the enzymes involved in this pathway. In addition to the determination of Ki, Kd, IC50, and/or EC50, a more thorough kinetic analysis can provide useful information for the selection of the best lead series during the early stage of the drug discovery process. This study describes a medium-throughput fluorescent probe displacement assay for the rapid determination of the koff constant, residence time, and kinetic efficiency for ERK (extracellular signal-regulated kinase) inhibitors. Using this method, we have identified several inhibitors that we have subjected to further kinetic analysis by comparing koff constants determined for these time-dependent inhibitors using either the active or inactive form of ERK2.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号