全文获取类型
收费全文 | 121篇 |
免费 | 7篇 |
国内免费 | 1篇 |
专业分类
化学 | 101篇 |
力学 | 1篇 |
数学 | 19篇 |
物理学 | 8篇 |
出版年
2023年 | 3篇 |
2022年 | 3篇 |
2021年 | 1篇 |
2020年 | 3篇 |
2019年 | 1篇 |
2018年 | 3篇 |
2017年 | 2篇 |
2016年 | 4篇 |
2015年 | 6篇 |
2014年 | 3篇 |
2013年 | 9篇 |
2012年 | 8篇 |
2011年 | 12篇 |
2010年 | 4篇 |
2009年 | 9篇 |
2008年 | 7篇 |
2007年 | 7篇 |
2006年 | 7篇 |
2005年 | 7篇 |
2004年 | 6篇 |
2003年 | 2篇 |
2002年 | 7篇 |
2001年 | 2篇 |
2000年 | 3篇 |
1999年 | 3篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1993年 | 1篇 |
1991年 | 2篇 |
1985年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有129条查询结果,搜索用时 375 毫秒
51.
We study the class of 1‐perfectly orientable graphs, that is, graphs having an orientation in which every out‐neighborhood induces a tournament. 1‐perfectly orientable graphs form a common generalization of chordal graphs and circular arc graphs. Even though they can be recognized in polynomial time, little is known about their structure. In this article, we develop several results on 1‐perfectly orientable graphs. In particular, we (i) give a characterization of 1‐perfectly orientable graphs in terms of edge clique covers, (ii) identify several graph transformations preserving the class of 1‐perfectly orientable graphs, (iii) exhibit an infinite family of minimal forbidden induced minors for the class of 1‐perfectly orientable graphs, and (iv) characterize the class of 1‐perfectly orientable graphs within the classes of cographs and of cobipartite graphs. The class of 1‐perfectly orientable cobipartite graphs coincides with the class of cobipartite circular arc graphs. 相似文献
52.
Denofrio MP Lorente C Breitenbach T Hatz S Cabrerizo FM Thomas AH Ogilby PR 《Photochemistry and photobiology》2011,87(4):862-866
Pterins, heterocyclic compounds widespread in biological systems, participate in relevant biological processes and are able to act as photosensitizers. In the present study, we ascertained that 2-aminopteridin-4(3H)-one, abbreviated as Ptr, is readily incorporated into and/or onto cervical cancer cells (HeLa) and that these cells die upon UV-A irradiation of Ptr. Cell death was assessed using two tests: (1) the Rhodamine 123 fluorescence assay for mitochondrial viability and (2) the Trypan Blue assay for membrane integrity. The data suggest that, for Ptr-dependent photoinitiated cell death, events related to mitochondrial failure precede those associated with the failure of the cell membrane. 相似文献
53.
Feroci M Chiarotto I Orsini M Pelagalli R Inesi A 《Chemical communications (Cambridge, England)》2012,48(43):5361-5363
The umpolung reaction of α,β-unsaturated aldehydes to saturated esters has been carried out in an ionic liquid by organocatalysis of electrogenerated NHC. The roles of solvent, precatalyst and proton donor of the ionic liquid have been verified and good to high yields of esters have been obtained using a "green" and mild methodology. 相似文献
54.
Mariana Vignoni Franco M. Cabrerizo Carolina Lorente Andrés H. Thomas 《Photochemistry and photobiology》2009,85(1):365-373
New photochemical studies of the reactivity of biopterin (BPT) and neopterin (NPT) in acidic (pH = 5.5) and alkaline (pH = 10.5) aqueous solutions at 350 nm and room temperature were performed. The photochemical properties of BPT are of particular interest because the photolysis of this compound takes place in the white skin patches of patients affected by vitiligo. The photochemical reactions were followed by UV/VIS spectrophotometry, HPLC, electrochemical measurement of dissolved O2 and enzymatic methods for hydrogen peroxide (H2 O2 ) and superoxide anion (O2 •− ) determinations. When BPT or NPT are exposed to UVA radiation, a red intermediate, very likely 6-formyl-5,8-dihydropterin, is generated in an O2 -independent process. That product is rapidly oxidized on admission of O2 to yield 6-formylpterin and H2 O2 . When the photolysis takes place in aerobic conditions, no additional pathways exist. On the other hand, in the absence of O2 , the intermediate generated is not stable and leads to the formation of many products. O2 •− is also generated during photo-oxidation of BPT and NPT. The quantum yields of reactant consumption depends on the O2 concentration: the higher the O2 concentration, the lower the quantum yields. This behavior is discussed in connection with the excited state of the pterins. 相似文献
55.
Sima Mehdizadeh Morteza Jabbari Romina Shakibazadeh Farrokh Gharib 《Heteroatom Chemistry》2008,19(7):654-660
The hydrolysis of [(Ph)2MeSn(IV)]+ has been studied spectrophotometrically at 25ˆC and constant ionic strength of 0.1 mol dm−3 sodium perchlorate. Over a wide pH range, 1–11, the investigation has been performed in different aqueous solutions of ethanol. The species formed together with their formation constants have been determined using the computer program Squad. The hydrolysis constants at different media were analyzed in terms of Kamlet and Taft's parameters. A single‐parameter correlation of the formation constants, K1‐1 and K1‐2, versus α (hydrogen‐bond donor acidity), β (hydrogen‐bond acceptor basicity), and π* (dipolarity/polarizability) for both cases are relatively poor in all solutions, but multiparameter correlation represents significant improvement with regard to the single‐parameter models. In this work, we have also used the normalized polarity parameter, E, alone and in combination with the Kamlet–Taft's parameters to find a better correlation of the formation constants in different aqueous solutions of ethanol. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:654–660, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20484 相似文献
56.
57.
58.
Photochemical behavior of 6-methylpterin in aqueous solutions: generation of reactive oxygen species
Cabrerizo FM Lorente C Vignoni M Cabrerizo R Thomas AH Capparelli AL 《Photochemistry and photobiology》2005,81(4):793-801
Pterins are a family of heterocyclic compounds present in a wide range of living systems that participate in relevant biological functions and are involved in different photobiological processes. 6-Methylpterin (MPT) was investigated for its efficiency of singlet-oxygen (1O2) production and quenching in aqueous solution. The quantum yields of 1O2 production (phi(delta)) was determined by measurements of the 1O2 luminescence in the near-infrared upon continuous excitation of the sensitizer. Values of phi(delta) were found to be 0.10 +/- 0.02 and 0.14 +/- 0.02 in acidic and alkaline media, respectively. Studies of the photooxidation of MPT in acidic (pH = 5.0-6.0) and alkaline (pH = 10.2-10.8) aqueous solutions at 350 nm and room temperature have been performed. The photochemical reactions were followed by UV-visible spectrophotometry, high-performance liquid chromatography and an enzymatic method for H2O2 determination. MPT is not light sensitive in the absence of oxygen, but it undergoes a photooxidation reaction in the presence of oxygen, yielding several nonpteridinic products. The quantum yields of MPT disappearance were determined and values of 2.4 (+/-0.5) x 10(-4) and 8.1 (+/-0.8) x 10(-4) were obtained in acidic and alkaline media, respectively. H2O2 was detected and quantified in irradiated solutions of MPT. The rate constant of the chemical reaction between 1O2 and MPT (k(r)) was determined to be 4.9 x 10(6) M(-1) s(-1) in alkaline medium and the role of 1O2 in the photooxidation of MPT is discussed. 相似文献
59.
The enantiospecific synthesis of (-)cinatrin C1 (3) and (+)-cinatrin C3 (5) from the D-arabinose derivative 9 is described. The stereochemistry at C2 was introduced via a chelation-controlled addition of a carbanion to alpha-hydroxy ketone 8. The best selectivity was achieved by use of the Grignard reagent derived from trimethylsilylacetylene. Transformation of the terminal alkyne into methyl ester 17 followed by acetal hydrolysis and selective lactol oxidation gave cinatrin C1 dimethyl ester (7). Base hydrolysis and acid induced relactonization then gave a 1:1 mixture of cinatrins C1 (3) and C3 (5). 相似文献
60.
Abramo MC Caccamo C Costa D Ruberto R 《The journal of physical chemistry. B》2005,109(50):24077-24084
Extensive molecular dynamics simulations show that a short-range central potential, suited to model C60, undergoes a high temperature transition to a glassy phase characterized by the positional disorder of the constituent particles. Crystallization, melting, and sublimation, which also take place during the simulation runs, are illustrated in detail. It turns out that vitrification and the mentioned phase transitions occur when the packing fraction of the system-defined in terms of an effective hard-core diameter-equals that of hard spheres at their own glass and melting transition, respectively. A close analogy also emerges between our findings and recent mode coupling theory calculations of structural arrest lines in a similar model of protein solutions. We argue that the conclusions of the present study might hold for a wide class of potentials currently employed to mimic interactions in complex fluids (some of which are of biological interest), suggesting how to achieve at least qualitative predictions of vitrification and crystallization in those systems. 相似文献