Fragmentation Electron Transfer

Single-electron transfer from a borate anion particle to the excited polymethine cation generates a dye radical and an aLkylphenylbotanyl radical. The aLkylphenylbotanyl radical fragments to form an active alkyl radical. It is the alkyl radical particles that initiate the polymerization reactions (101). 

The possible mechanism of ionization, fragmentation of studied compound as well as their desoi ption by laser radiation is discussed. It is shown that the formation of analyte ions is a result of a multi stage complex process included surface activation by laser irradiation, the adsoi ption of neutral analyte and proton donor molecules, the chemical reaction on the surface with proton or electron transfer, production of charged complexes bonded with the surface and finally laser desoi ption of such preformed molecules. 

Another useful generalization is the principle of maximum hardness. This states that molecular arrangements that maximize hardness are preferred. Electronegativity and hardness detennine the extent of electron transfer between two molecular fragments in a reaction. This can be approximated numerically by the expression... 

The use of direct electrochemical methods (cyclic voltammetry Pig. 17) has enabled us to measure the thermodynamic parameters of isolated water-soluble fragments of the Rieske proteins of various bci complexes (Table XII)). (55, 92). The values determined for the standard reaction entropy, AS°, for both the mitochondrial and the bacterial Rieske fragments are similar to values obtained for water-soluble cytochromes they are more negative than values measured for other electron transfer proteins (93). Large negative values of AS° have been correlated with a less exposed metal site (93). However, this is opposite to what is observed in Rieske proteins, since the cluster appears to be less exposed in Rieske-type ferredoxins that show less negative values of AS° (see Section V,B). 

Another promising way of transmembrane PET includes intramolecular electron transfer along bridge molecule D-PC-A which spans the bilayer and contains PC, D and A fragments linked by covalent bonds [17]. 

Leconte and Basset [161-166] proposed two other possible mechanisms (Scheme 39) the first one implies a 1,2 carbon-carbon activation which invokes the de-insertion of a methylidene fragment from a surface metal-alkyl species, and the second implies a 1,3 carbon-carbon bond activation in which the key steps are the formation of a dimetallacyle by y-H activation from a metal-alkyl followed by carbon-carbon bond cleavage via a concerted electron transfer. 

An important synthetic application of this reaction is in dehalogenation of dichloro- and dibromocyclopropanes. The dihalocyclopropanes are accessible via carbene addition reactions (see Section 10.2.3). Reductive dehalogenation can also be used to introduce deuterium at a specific site. The mechanism of the reaction involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

Anti stereospecificity is associated with a concerted reductive elimination, whereas single-electron transfer fragmentation leads to loss of stereospecificity and formation of the more stable A-stereoisomer. 

In fact, in a precise sense, no molecular fragment is rigorously transferable, although approximate transferability is an exceptionally useful and, if used judiciously, a valid approach within the limitations of the approximation. In particular, it is possible to define non-physical entities, such as fuzzy fragment electron densities, which do not exist as separate objects, yet they show much better transferability properties than actual, physically identifiable subsystems of well-defined, separate identity. This aspect of specially designed, custom- made , artificial subsystems of nearly exact additivity has been used to generate ab initio quality electron densities for proteins and other macromolecules. 

Maslak, P. Fragmentations by Photoinduced Electron Transfer. Fundamentals and Practical Aspects. 168, 1-46 (1993). 

Comparable compounds have recently been described by Weidenbruch et al., starting from cyclotrisilanes as a source for di-organylsilylenes [10]. Whether these complexes are formed by electron donation from the nitrogen lone pairs to silicon with subsequent electron transfer from silicon to the bipyridyl system or by a [4+1]cycloaddition process involving the diazadiene fragment is still uncertain. 

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