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   1440篇
  免费   25篇
  国内免费   13篇
化学   833篇
晶体学   6篇
力学   23篇
数学   312篇
物理学   304篇
  2021年   12篇
  2020年   26篇
  2019年   15篇
  2018年   13篇
  2016年   24篇
  2015年   16篇
  2014年   25篇
  2013年   57篇
  2012年   58篇
  2011年   67篇
  2010年   24篇
  2009年   29篇
  2008年   49篇
  2007年   68篇
  2006年   58篇
  2005年   63篇
  2004年   48篇
  2003年   45篇
  2002年   37篇
  2001年   27篇
  2000年   36篇
  1999年   15篇
  1998年   26篇
  1997年   10篇
  1996年   32篇
  1995年   16篇
  1994年   22篇
  1993年   32篇
  1992年   29篇
  1991年   22篇
  1990年   33篇
  1989年   24篇
  1988年   25篇
  1987年   26篇
  1986年   14篇
  1985年   23篇
  1984年   23篇
  1983年   20篇
  1982年   15篇
  1981年   27篇
  1980年   10篇
  1979年   19篇
  1978年   24篇
  1977年   18篇
  1976年   21篇
  1975年   19篇
  1974年   18篇
  1973年   16篇
  1969年   8篇
  1968年   10篇
排序方式: 共有1478条查询结果,搜索用时 15 毫秒
1.
Foundations of Computational Mathematics - This paper develops nonasymptotic growth and concentration bounds for a product of independent random matrices. These results sharpen and generalize...  相似文献   
2.
Stroke is the commonest cause of disability. Novel treatments require an improved understanding of the underlying mechanisms of recovery. Fractal approaches have demonstrated that a single metric can describe the complexity of seemingly random fluctuations of physiological signals. We hypothesize that fractal algorithms applied to electroencephalographic (EEG) signals may track brain impairment after stroke. Sixteen stroke survivors were studied in the hyperacute (<48 h) and in the acute phase (∼1 week after stroke), and 35 stroke survivors during the early subacute phase (from 8 days to 32 days and after ∼2 months after stroke): We compared resting-state EEG fractal changes using fractal measures (i.e., Higuchi Index, Tortuosity) with 11 healthy controls. Both Higuchi index and Tortuosity values were significantly lower after a stroke throughout the acute and early subacute stage compared to healthy subjects, reflecting a brain activity which is significantly less complex. These indices may be promising metrics to track behavioral changes in the very early stage after stroke. Our findings might contribute to the neurorehabilitation quest in identifying reliable biomarkers for a better tailoring of rehabilitation pathways.  相似文献   
3.
By transplanting identity elements into E. coli tRNAfMet, we have engineered an orthogonal initiator tRNA (itRNATy2) that is a substrate for Methanocaldococcus jannaschii TyrRS. We demonstrate that itRNATy2 can initiate translation in vivo with aromatic non-canonical amino acids (ncAAs) bearing diverse sidechains. Although the initial system suffered from low yields, deleting redundant copies of tRNAfMet from the genome afforded an E. coli strain in which the efficiency of non-canonical initiation equals elongation. With this improved system we produced a protein containing two distinct ncAAs at the first and second positions, an initial step towards producing completely unnatural polypeptides in vivo. This work provides a valuable tool to synthetic biology and demonstrates remarkable versatility of the E. coli translational machinery for initiation with ncAAs in vivo.  相似文献   
4.
5.
Journal of Solid State Electrochemistry - In this work, we evaluate the electrochemical properties of MBH4-C60 (M = Li+, Na+) nanocomposites for potential use as a solid-state electrolyte in...  相似文献   
6.
Platinum-based chemotherapy persists to be the only effective therapeutic option against a wide variety of tumours. Nevertheless, the acquisition of platinum resistance is utterly common, ultimately cornering conventional platinum drugs to only palliative in many patients. Thus, encountering alternatives that are both effective and non-cross-resistant is urgent. In this work, we report the synthesis, reduction studies, and luminescent properties of a series of cyclometallated (C,N,N′)PtIV compounds derived from amine–imine ligands, and their remarkable efficacy at the high nanomolar range and complete lack of cross-resistance, as an intrinsic property of the platinacycle, against multiplatinum-resistant colorectal cancer (CRC) and castration-resistant prostate cancer (CRPC) metastatic cell lines generated for this work. We have also determined that the compounds are effective and selective for a broader cancer panel, including breast and lung cancer. Additionally, selected compounds have been further evaluated, finding a shift in their antiproliferative mechanism towards more cytotoxic and less cytostatic than cisplatin against cancer cells, being also able to oxidize cysteine residues and inhibit topoisomerase II, thereby holding great promise as future improved alternatives to conventional platinum drugs.  相似文献   
7.
A crystallographic investigation of a series of host–guest complexes in which small-molecule organic guests occupy the central cavity of an approximately cubic M8L12 coordination cage has revealed some unexpected behaviour. Whilst some guests form 1:1 H⋅G complexes as we have seen before, an extensive family of bicyclic guests—including some substituted coumarins and various saturated analogues—form 1:2 H⋅G2 complexes in the solid state, despite the fact that solution titrations are consistent with 1:1 complex formation, and the combined volume of the pair of guests significantly exceeds the Rebek 55±9 % packing for optimal guest binding, with packing coefficients of up to 87 %. Re-examination of solution titration data for guest binding in two cases showed that, although conventional fluorescence titrations are consistent with 1:1 binding model, alternative forms of analysis—Job plot and an NMR titration—at higher concentrations do provide evidence for 1:2 H⋅G2 complex formation. The observation of guests binding in pairs in some cases opens new possibilities for altered reactivity of bound guests, and also highlights the recently articulated difficulties associated with determining stoichiometry of supramolecular complexes in solution.  相似文献   
8.
The directionality of the hole-transfer processes between DNA backbone and base was investigated by using phosphorodithioate [P(S)=S] components. ESR spectroscopy in homogeneous frozen aqueous solutions and pulse radiolysis in aqueous solution at ambient temperature confirmed initial formation of G.+-P(S)=S. The ionization potential of G-P(S)=S was calculated to be slightly lower than that of guanine in 5′-dGMP. Subsequent thermally activated hole transfer from G.+ to P(S)=S led to dithiyl radical (P-2S.) formation on the μs timescale. In parallel, ESR spectroscopy, pulse radiolysis, and density functional theory (DFT) calculations confirmed P-2S. formation in an abasic phosphorodithioate model compound. ESR investigations at low temperatures and higher G-P(S)=S concentrations showed a bimolecular conversion of P-2S. to the σ2-σ*1-bonded dimer anion radical [-P-2S 2S-P-]G (150 K, DFT)=−7.2 kcal mol−1]. However, [-P-2S 2S-P-] formation was not observed by pulse radiolysis [ΔG° (298 K, DFT)=−1.4 kcal mol−1]. Neither P-2S. nor [-P-2S 2S-P-] oxidized guanine base; only base-to-backbone hole transfer occurs in phosphorodithioate.  相似文献   
9.
Journal of Radioanalytical and Nuclear Chemistry - Polonium is rapidly emerging as an international environmental health concern primarily because of the recent rise in hydraulic fracturing...  相似文献   
10.
Whilst pursuing the synthetic utility of quinoline‐based tin complexes, Me2Sn(Quin‐NO2)2 ( 1 ) and Ph2Sn(Quin‐NO2)2 ( 2 ) (Quin‐NO2 = 5‐nitroquinolino‐8‐oate) were synthesized bearing coordinatively inert nitro groups. Conventional reduction methodologies successfully converted 1 to Me2Sn(Quin‐NH2)2 ( 3 ) and 2 to Ph2Sn(Quin‐NH2)2 ( 4 ) (Quin‐NH2 = 5‐aminoquinolino‐8‐oate). The synthetically useful amine groups proved difficult to exploit in the presence of the central tin atom, however, a complete Schiff base functionalized Sn complex of the dimethyltin pro‐ligand Me2Sn(Quin‐py)2 ( 6 ) was successfully synthesized from 5‐[(pyridin‐2‐ylmethylene)amino]quinolin‐8‐ol (HQuin‐py; 5 ) in good yield via an alternative strategy exploiting the oxophilic tendencies of tin. All species were fully characterized by NMR (including 119Sn NMR spectroscopy), HR‐ESI MS and single‐crystal X‐ray diffraction, and preliminary studies of their supramolecular potential are also discussed.  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