Polar solvents, proton transfer reactions

Analysis of the data in Table XVIII suggests that silene formation is kinetically the most favorable process. However, according to experiment, metallated silenes are formed. This is related to the fact that in polar solvents proton transfer from the carbon atom to silicon is intermolecular, which leads to a considerable decrease in the reaction barrier. We believe that when the migration of substituents from the carbon atom to silicon is suppressed, for example, by the introduction of two alkyl radicals, the elimination of phosphines resulting in silene formation becomes the most probable process. 

A Warshel. Dynamics of reactions m polar solvents. Semiclassical trajectory studies of electron-transfer and proton-transfer reactions. J Phys Chem 86 2218-2224, 1982. 

A large red shift observed in polar solvents was indicative of the intramolecular charge transfer character of the triplet state. The change of dipole moment accompanying the transition Tj - Tn, as well as rate constants for electron and proton transfer reactions involving the T state of a-nitronaphthalene, were determined. The lower reactivity in polar solvents was attributed to a reduced n-n and increased charge transfer character of the triplet state... 

This type of charge reduction by charge transfer to the solvent molecule occurs in general when SI are polar solvent molecules of aprotic character such as dimethyl-sulfoxide, dimethyl formamide, and acetonitrile. Protic solvents such as water lead to charge reduction which involves an intracluster proton transfer reaction ... 

Chou PT, Yu WS, Cheng YM et al (2004) Solvent-polarity tuning excited-state charge coupled proton-transfer reaction in p-N, N-ditolylaminosalicylaldehydes. J Phys Chem A 108 6487-6498... 

Atmospheric pressure chemical ionization APCI is a method closely related to electrospray ionization. It uses ion-molecule reactions to produce ions from analyte molecules. The sample is electrohydrodynamically sprayed into the source (Figure 14.3). The evaporation of the solvent is often supported by a heated gas at temperatures between 80 and 400°C. Within the source, a plasma is created using a corona discharge needle that is placed close to the end of the metal capillary. In this plasma, proton transfer reactions occur, leading to the ionization of the analyte, mainly by the formation of [M+H]+ ions. Compared to ESI MS, APCI MS is very well suited for the analysis of less-polar components and can therefore... 

Similar reactions have been reported for a large number of substrates [43], In polar media, proton transfer does not necessarily involve the radical anion, a relatively weak base, but rather the solvent (Scheme 13), particularly when... 

For these systems, direct hydrogen abstraction by benzophenone triplets was observed in benzene whereas in a polar solvent electron transfer and hydrogen-atom abstraction were observed. Electron transfer followed by an intramolecular proton transfer was observed in these systems although such proton-transfer reactions are not observed in unlinked systems of primary and secondary amines. The observed differences between the linked and unlinked systems have been attributed to the dependence of electron transfer, proton transfer, and hydrogen transfer on mutual distance and orientation. In the unlinked systems, rotational and translational motion of two reacting molecules are usually much faster than those in linked systems. 

Detailed analyses of intramolecular structures are possible. Comparison of NMR and fluorescence data shows meso- and racemic diastereoisomers are found from 2,4-di(2-pyrenyl)pentane 24 jhe polarization of monomer and excimer of 4,9, disubstituted pyrenes have been measured in nematic liquid crystals 25 Quenching of pyrene fluorescence by alcohols in cyclodextrin inclusion complexes has also been studied in detail 26 Solvent effects on the photophysical properties of pyrene-3-carboxylic acid has been used to measure the pJJ, in different solvents 27 Geminate recombination in excited state proton transfer reactions has been studied with... 

Electron Transfer. Neta and coworkers have worked extensively with halogen-substituted methyl peroxyl radicals (X H COO , where X = Cl, Br or F) in aqueous and non-aqueous media, using combinations of solvents in different ratios to change the polarity of the mixture. They describe the mechanism for the reaction of the water-soluble antioxidant Trolox with their peroxyl radicals as H-mediated electron transfer , having determined that the rate of the reaction increases with an increase in solvent polarity. They examined solvent polarity in terms of the dielectric constant of the solvent, e, and solvent basicity, reported as either the coordinate covalency parameter, f, which is a measure of solvent proton-transfer basicity, or the value, which is a measure of solvent hydrogen bond basicity . 

A very convincing support for the existence of solvent controlled proton dissociation reactions in aqueous solutions has risen from the theoretical studies of Ando and Hynes [105-108] who have studied the proton dissociation of simple mineral acids HCl and HF in aqueous solutions. The two acids seem to follow a solvent-controlled proton transfer mechanism with a Marcus-like dependence of the activation energy on the acid strength. Recently, a free energy relationship for proton transfer reactions in a polar environment in which the proton is treated quantum mechanically was found by Kiefer and Hynes [109, 110]. Despite the quite different conceptual basis of the treatment the findings bear similarity to those resulting from the Marcus equation Eq. (12.19) which has been used to correlate the proton transfer rates of photoacids with their piG [ 101,102 ]... 

Warshel, a., Dynamics of Reactions in Polar Solvents. Semidassical Trajectory Studies of Electron-Transfer and Proton-Transfer Reactions, ]. Phys. Chem. 1982, 86, 2218-2224. 

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