Proton transfer recombination

In most (but certainly not all ) experiments involving ion-molecule reactions, the structure of the product ions is not determined. As the number of atoms in the product ions increases, the multiplicity of possible isomers becomes greater. Knowledge of the structure of ions is critical in determining what neutral products result from dissociative recombination. Although some classes of ion-molecule reactions, such as proton transfer reactions, lead to products with relatively well-characterized structures, the problem can be more severe with other classes of reactions. 

We discuss acid solutions in greater detail. Two different mechanisms have been established. The first is the Volmer- Tafel mechanism, which consists of a proton-transfer step followed by a chemical recombination reaction ... 

The mechanism for the photoreaction between 133 and cyclohexene can be summarized as in Scheme 8. The initiating electron transfer fluorescence quenching of 133 by cyclohexene resulted in the formation of an w-amino radical-radical cation pair 136. Proton transfer from the 2-position of the cyclohexene radical cation to the nitrogen atom of the a-amino radical leads to another radical cation-radical pair 137. Recombination of 137 at the radical site affords the adduct 134, while nucleophilic attack at the cation radical of 136 leads to another radical pair 138 which is the precursor for the adduct 135. 

Whereas in acetonitrile the rate limiting step is an opening of the solvent shell of a reactant, in benzonitrile the back reaction of (5) between the protonated acridine orange cation (BH ) and the 3-methyl-4-nitrophenolate ion (A ) to form the ion pair is diffusion controlled (although the overall reaction to the neutral molecules is an endothermic process). Because of its lower dielectric constant than acetonitrile, the electrostatic interactions between reactants in benzonitrile outweigh specific solvent effects. In other words, in benzonitrile a rate limiting coupling of proton transfer to the reorientation of solvent dipoles does not occur and the measured rates are very fast. The ion recombination (I) + (II) in benzonitrile has a diffusion controlled specific rate (theoretical) k = 9 -1 -1... 

Another situation in which an already well-studied proton transfer reaction serves as a probe of a physical phenomenon has been suggested by Knight, Goodall and Greenhow (43, 44). They ionized water with single photons of Nd glass laser infrared radiation and measured an ion recombination rate constant for the reaction... 

Molecular-level studies of mechanisms of proton and water transport in PEMs require quantum mechanical calculations these mechanisms determine the conductance of water-filled nanosized pathways in PEMs. Also at molecular to nanoscopic scale, elementary steps of molecular adsorption, surface diffusion, charge transfer, recombination, and desorption proceed on the surfaces of nanoscale catalyst particles these fundamental processes control the electrocatalytic activity of the accessible catalyst surface. Studies of stable conformations of supported nanoparticles as well as of the processes on their surface require density functional theory (DFT) calculations, molecular... 

Recently, Jacobs (48) proposed a unified mechanism incorporating this dissociative sublimation step. He imagines the step towards the adsorbed state to be accomplished by proton transfer. At low temperatures, both NH3 and HCIO4 desorb but recombine in the gas phase to form gaseous NH4CIO4—i.e., pure sublimation occurs. At low temperatures and increased pressures, NH3 does not desorb but remains physically... 

Coherent dissociation Geminate recombination Dephasing Proton transfer Electron transfer Vibrational relaxation 8arrierless reactions Bimolecular reactions Ionic reactions Solvation dynamics Friction dynamics Polarization (kerr)... 

During the y-radiolysis of vitreous solutions containing only biphenyl (0.1 M) or only pyrene (0.02 M), the yield of Ph2 and Py- at 77K is high enough for them to be recorded at an irradiation dose of 1019 eV cm-3. At 77 K these particles have been observed to decay spontaneously (Fig. 5), evidently, due to proton transfer from alcohol molecules (the most probable process in the case of Ph2 anion radicals [14]) or to recombination with counterions formed during radiolysis. Naphthalene and pyrene additives to solutions of Ph2 essentially accelerate the decay of the Ph2 anion radical at 77 K which is naturally accounted for by electron transfer from Ph2 to Nh and Py. In agreement with this conclusion the decay of Py in the presence of Ph2 is slower than its spontaneous decay in the absence of Ph2. ... 

Deprotonation is essential in some cation radical reactions the corresponding examples will be described in Chapter 6. Scheme 1-43 depicts a photoreaction between phenan-threne and triethylamine. This reaction includes photoinduced sequential electron-transfer, proton-transfer, and radical-recombination processes (Lawson et al. 1999). 

The theory of geminate recombination experienced a similar evolution from primitive exponential model and contact approximation [19,20], to distant recombination carried out by backward electron transfer [21], However, all these theories have an arbitrary parameter initial separation of reactants in a pair, / o. This uncertainty was eliminated by unified theory (UT) proposed in two articles published almost simultaneously [22,23], UT considers jointly the forward bimolecular electron transfer and subsequent geminate recombination of charged products carried out by backward electron or proton transfer. The forward transfer creates the initial condition for the backward one. This is the distribution of initial separations in the geminate ion pair/(ro), closely analyzed theoretically [24,25] and inspected experimentally [26,27], It was used to specify the geminate recombination kinetics accompanied by spin conversion and exciplex formation [28-31], These and other applications of UT have been covered in a review published in 2000 [32],... 

It does not always happen that the ion recombination proceeds through the backward electron transfer. Sometimes the ions are discharged because of proton transfer between them, resulting in their transformation to free radicals D and A ... 

Early in 1961, Libby [30] suggested the proton transfer from the radical cation and subsequent charge recombination between the protonated site of polyethylene chain and an electron for the alkyl radical formation ... 

The time evolution of the O—D and D—Ow distances in the ionized clusters is also shown in Figure 5-6 (A-H right panels). In contrast with the neutral system, significant changes of the distances can be observed. These variations describe the occurrence of proton transfer from the phenolic moiety to water and also recombination of the transferred proton with the phenoxy moiety. As expected, the PT dynamics is dependent on the initial configuration. 

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