Electron proton transfer processes

Sobolewski AL, Domcke W (2003) Ab initio study of the excited-state coupled electron-proton-transfer process in the 2-aminopyridine dimer. Chem Phys 294 2763... 

A frequently used indirect method involves cyclizable (cf. (7)) or other mechanistic probes which should provide evidence for free radical intermediates and thus for SET [19,37,59]. However, Newcomb and Curran have pointed out the pitfalls of such an approach especially if iodide precursors are used [17]. The supposedly radical-indicative reaction may come about albeit slower by a different, nonradical mechanism or the radical formation may occur via a secondary process which is not directly related to the first reaction step. A similar side-route can be made responsible for the appearance of stable radical compounds which may arise via a comproportionation reaction between non-reduced starting material and the doubly reduced species which can be formed from a hydro form (the normal product, Eq. (5)) and the usually strongly basic organometallic or hydridic reagents (Eq. (9)) [58]. The ability of strong bases to produce reduced radical species via complicated electron/proton transfer processes has been known for some time in the chemistry of quinones and quaternary salts [60,61]. 

In these systems, illumination with a subsequent hydrogen abstraction or electron/proton transfer process results in the production of two radicals, both of... 

Electrochemical and e.s.r. studies of the dibenzonorcaradiene anion radical indicate that neither the thermally allowed nor the photochemically forbidden disrotatory mode of cleavage is involved in the conversion of (247) into (248). In all probability a sequential electron-proton transfer process operates. E.s.r. studies have also shown anion radicals to be involved in the formation of diquinocyclo-propanones (250) employing a sodium-potassium alloy oxidation of (249) with potassium ferricyanide clearly involves monoradicals. ... 

Cationic photoinitiators are usually based on iodonium salts [e.g. Phal ) and sulfonium salts (e.g. PhsS" ") which liberate a proton upon exposure to UV light (after subsequent rearrangement of the primary pairs formed, cleavage of a C-I or C-S bond and/or hydrogen abstraction on a H-donating structure). Photoinitiators of radical polymerization were originally classified as Type I PI (cleavable systems e.g. aryl allq l ketones or phosphine oxides mostly through a Norrish I scission, Scheme 2a) and Type II PI (PI and a co-initiator such as an amine AH or electron/proton transfer process, Scheme 2b PI stands e.g. for benzophenones. 

Sobolewski AL, Domcke W, Hattig C (2005) Tautomeric selectivity of the excited-state lifetime of guanine/cytosine base pairs The role of electron-driven proton-transfer processes. Proc Natl Acad Sci USA 102 17903-17906... 

Sobolewski AL, Domcke W (2006) Role of electron-driven proton-transfer processes in the excited-state deactivation adenine-thymine base pair. J Phys Chem A 110 9031-9038... 

The second group of intermolecular reactions (2) includes [1, 2, 9, 10, 13, 14] electron transfer, exciplex and excimer formations, and proton transfer processes (Table 1). Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. PET is involved in many photochemical reactions and plays... 

More complicated reactions that combine competition between first- and second-order reactions with ECE-DISP processes are treated in detail in Section 6.2.8. The results of these theoretical treatments are used to analyze the mechanism of carbon dioxide reduction (Section 2.5.4) and the question of Fl-atom transfer vs. electron + proton transfer (Section 2.5.5). A treatment very similar to the latter case has also been used to treat the preparative-scale results in electrochemically triggered SrnI substitution reactions (Section 2.5.6). From this large range of treated reaction schemes and experimental illustrations, one may address with little adaptation any type of reaction scheme that associates electrode electron transfers and homogeneous reactions. 

As discussed in Section 2.5.1, aryl radicals are easily reduced at the potential where they are generated. This reduction that can take place at the electrode surface (ECE) or in the solution (DISP) opposes the substitution process. This three-cornered competition between substitution (SUBST) electron + proton transfer (ECE or DISP) depends on two competition parameters that are closely similar to the HAT-ECE-DISP parameters described in the preceding section ... 

These proton transfer processes increase the driving force of the electron transfer reactions, which can thus be considered in terms of a proton-coupled electron transfer process [57-60]. 

In summary, in this section we have discussed the electronic and steric effects of structural moieties on the pKz value of acid and base functions in organic molecules. We have seen how LFERs can be used to quantitatively describe these electronic effects. At this point, it is important to realize that we have used such LFERs to evaluate the relative stability and, hence, the relative energy status of organic species in aqueous solution (e.g., anionic vs. neutral species). It should come as no surprise then that we will find similar relationships when dealing with chemical reactions other than proton transfer processes in Chapter 13. 

Hydrated electrons react with many Bronsted acids. This reaction is not a proton transfer process but an incorporation of the electron into the acid to form an AH - ion radical, which may subsequently undergo decomposition. This decomposition may occasionally yield a hydrogen atom, but in many cases other pathways of dissociation have been observed. 

Because of the high acidity of a-methylstyrene, proton transfer processes play a leading role in its anionic polymerization. This is in distinction to styrene which is polymerized without proton transfers. It reacts with the NH2 ion as an electron-acceptor (acid-like substance) and the amide ion becomes attached in the j3 position with the formation of the carbanion C6H5.CH.CH2.NH2 (Astaf ev et al., 1961). 

The 1977 review of Martynov et al. [12] discusses existing mechanisms of ESPT, excited-state intramolecular proton transfer (ESIPT) and excited-state double-proton transfer (ESDPT). Various models that have been proposed to account for the kinetics of proton-transfer reactions in general. They include that of association-proton-transfer-dissociation model of Eigen [13], Marcus adaptation of electron-transfer theory [14], and the intersecting state model by Varandas and Formosinho [15,16], Gutman and Nachliel s [17] review in 1990 offers a framework of general conclusions about the mechanism and dynamics of proton-transfer processes. 

The ET processes under discussion here correspond by definition to pure ET, in which molecular or medium coordinates may shift (the polaron response) [17], but no overall bonding rearrangements occur. More complex ET processes accompanied by such rearrangements (e.g., coupled electron/proton transfer and dissociative ET) are of great current interest, and many theoretical approaches have been formulated to deal with them, including quantum mechanical methods based on DC treatment of solvent [31,32],... 

Electron-driven Proton Transfer Processes in the Solvation of Excited States... 

With adequate hydronium-ion fluxes, HOO- is transformed to HOOH via a second electron-proton transfer (HOO1 + H30+ + e — - HOOH E°, +0.8 V vs. NHE). Achievement of a full two-electron peak height for the process of Eq. (9.33) requires a ratio of at least four (DMF)H30+ ions per 02 molecule in DMF. This results because (H30)C104 forms (DMF)H30+, and the diffusion coefficient for the latter is much smaller than that for 02. Furthermore, the flux of (DMF)H30+ to the electrode must be twice that for 02 to achieve the second cycle of Eq. (9.33) with the products of Eq. (9.34). 

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