摘要:

AbstractThis article is concerned with the structure and dynamic properties of thin films that have been chemically stabilized by polymerization and cross‐linking reactions. By using different types of monomers it is possible to construct networks with rubber‐like elasticity or with glassy properties, or temporary networks, in the interfacial region between oil and water. The systematic study of these ultrathin membrances offers new insights into aspects of current research, and opens up a wide range of potential new applicati

ISSN:0570-0833    

DOI:10.1002/anie.199004393

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractBoron‐phosphorus compounds have not been as thoroughly studied as their boron‐nitrogen counterparts. Until recently many classes of B‐P compounds that had been well established in B‐N chemistry were either unknown or poorly characterized. This statement is particularly true for compounds involving possible multiple bonding between boron and phosphorus. For example, detailed structural information on simple monomeric phosphino‐boranes, R2BPR′2, did not become available until 1986 even though the isoelectronic SiC double bonded species, the silenes, had already been reported. However, new work has shown that it is possible to prepare and characterize several novel types of boron‐phosphorus compounds with varying degrees of multiple B—P bonding. These include not only monomeric phosphinoboranes but also phosphanediylborates (borylphosphides), three‐ and four‐membered rings (diphosphadi‐boretanes), boron phosphorus analogues of borazine, B‐P skeletal analogues of allyl cations and anions, butadiene and cage compounds. Structural, spectroscopic (mainly NMR) and theoretical studies reveal some important differences between B‐P and B‐N compounds which in many cases can be traced to the presence of a high inversion barrier at phosphorus that reduces the π interaction. This usually causes compounds such as R2BPR′2to associate through σ bonding between B and P. Supporting evidence for this view comes from species that involve phosphorus and nitrogen in competitive π bonding with a boron p orbital in which the dative interaction between B and N is dominant and the phosphorus center remains pyramidal. Recently published work has shown that steric and electronic factors can be used to favor π bonding and give an approximately planar system. Furthermore, theoretical studies reveal that pp π overlap in a planar B‐P system is of similar efficiency to its B‐N analogue. Good examples are seen in the phosphanediyl borates, the boron‐phosphorus analogues of borazine and the π‐allyl cations, whose molecular configurations and B—P bond le

ISSN:0570-0833    

DOI:10.1002/anie.199004491

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractVision is initiated in the retina by the photochemical isomerization of an 11‐cis‐retinal Schiff base to its all‐transcounterpart in the photosensitive protein rhodopsin. All‐trans‐retinal is released, reduced to form all‐trans‐retinol (vitamin A), and finally esterified. In order for vision to proceed an 11‐cis‐retinoid must be regenerated in the eye.Kühneobserved in the 1870s that the pigment epithelium must be in place behind the retina in order for rhodopsin to form. And indeed it was in the pigment epithelium that an isomerase system was recently discovered that can produce 11‐cis‐retinol from added vitamin A.Paulingpredicted in the 1930s that certaincisdouble bonds in carotenoids and retinoids, including 11‐cis‐retinoids, should be quite unstable with respect to their all‐transcongeners. This prediction is borne out by experiment, but leaves open the crucial question as to where the energy comes from to drive the isomerization process in the eye. This question was answered by the demonstration that the energy is found in the membrane phospholipids. The fact that membranes may serve as an energy source reveals a previously unsuspected biological function of membranes. The chemistry of the novel energy‐linked isomerization proc

ISSN:0570-0833    

DOI:10.1002/anie.199004611

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractThe discovery of nuclear fission is one of the most outstanding episodes in the history of chemistry: It starts in the spring of 1934 whenEnrico Fermiand his group irradiate uranium with neutrons and seem to succeed in going beyond uranium, the then heaviest known element, reaching the first transuranic element; it continues withOtto Hahn, Lise Meitner and Fritz Strassmannwho believe to have found additional transuranic elements; withIrène CurieandPaul Savitchwho observe an activity which somehow does not fit into that scheme; again withOtto Hahn and Fritz Strassmannwho first identify this activity as radium but then on the 17th of December 1938 after rigorous chemical tests realize that the activity is instead barium, thus discovering the fission of the uranium atom into two lighter nuclei; and withLise MeitnerandOtto Robert Frischwho explain nuclear fission on the basis of an already known nuclear model;Otto Robert Frischfinally performs a physical experiment on the 13th of January 1939 which corroborates the fission of uranium. This discovery of nuclear fission is not only an event of historic dimensions, it is also an excellent example of how science evolves, not by successive logical steps but rather through strange detours

ISSN:0570-0833    

DOI:10.1002/anie.199004811

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199005091

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199005101

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199005121

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199005141

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199005161

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199005171

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY