Geminate recombination proton transfer

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

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],... 

In radiolysis, one of the most important reactions of solvated electrons is recombination with positive ions and radicals that are simultaneously produced in close proximity inside small volumes called spurs. These spurs are formed through further ionization and excitation of the solvent molecules. Thus, in competition with diffusion into the bulk, leading to a homogeneous solution, the solvated electron may react within the spurs. Geminate recombinations and spur reactions have been widely studied in water, both experimentally and theoretically, ° and also in a few other solvents. " Typically, recombinations occur on a timescale of tens to hundreds of picoseconds. In general, the primary cation undergoes a fast proton transfer reaction with a solvent molecule to produce the stable solvated proton and the free radical. Consequently, the... 

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... 

Proton transfer processes are specially important excited state properties, and several detailed time resolved studies have been reported. Time resolved fluorescence studies of excited l-naphthol-3,6-disulphonate shows there is geminate recombination by proton transfer. Another detailed study is the examination of proton transfer and solvent polarization dynamics in 3-hydroxyflavone . The dynamics of proton transfer using a geminate dissociation and recombination model has also been investigated with 8-hydroxypyrene-l,3,6-trisulphonate 5 and also with... 

More direct evidence for the inherent microscopic reversibility of an excited-state proton transfer reaction was found in ps-time-resolved measurements of a strongly reactive photoacid, namely HPTS (Fig. 12.2). With its conjugated-base, fourfold charged, the observation of the back (geminate) recombination of the pro-... 

Pines, E., Geminate Recombination in Excited State Proton Transfer, Ph.D. Thesis, Tel-Aviv University, 1989. 

Acids are in equilibrium with their conjugate bases in protic solvents, where the relative concentrations depend on the pK value. The observed dynamics of an electronically excited photoacid, typically interpreted as the proton transfer rate to the (protic) solvent [77, 78], is thus governed by the equilibration dynamics to the new configuration - as long as the photoacid and conjugate photobase remain in the electronically excited state - as dictated by the new excited state pJ a value. Depending on the pFI of the solvent one can observe the reversible time-depen-dent geminate recombination of the photobase with the released proton [79-83], or even the reaction of the photobase with other protons present in solution. 

With the intensive development of ultrafast spectroscopic methods, reaction dynamics can be investigated at the subpicosecond time scale. Femtosecond spectroscopy of liquids and solutions allows the study of sol-vent-cage effects on elementary charge-transfer processes. Recent work on ultrafast electron-transfer channels in aqueous ionic solutions is presented (electron-atom or electron-ion radical pairs, early geminate recombination, and concerted electron-proton transfer) and discussed in the framework of quantum theories on nonequilibrium electronic states. These advances permit us to understand how the statistical density fluctuations of a molecular solvent can assist or impede elementary electron-transfer processes in liquids and solutions. 

The aim of this lecture is to provide a qualitative description of reversible proton transfer reactions in the excited-state, using the extended theory of diffusion influenced reactions. The complete equations and numerical procedures may be found in the literature [10-14]. Major results include (i) the asymptotic power-law decay and the evidence for diffusive kinetics [10] (ii) The salt effect [11] and the Naive Approximation for the screening function [17, 11] and (iii) an extension [18] of the theory for approximating the effect of competing geminate and homogeneous proton recombination expected atdow pH values. 

Pines E, Huppert D, Agmon N (1988) Geminate recombination in excited-stale proton-transfer reactions—numerical solution of the debye-smoluchowski equation with backreaction and comparison with experimental results. J Chem Phys 88 5620... 

Ultrafast proton transfer including geminate proton recombination in the excited states... 

Ionised water molecules (which avoid immediate geminate recombination with an electron) can also undergo proton transfer with other neighbouring water molecules in the picosecond timescale to form an oxonium ion and hydroxyl radical [43]... 

Electrons escaping their positive ions are gradually thermalized and solvated whereas electrons that recombine with their geminate (i.e. original ion pair) radical cations form excited molecules. The radical cations may react with solvit molecules. The types of reactions to be expected are e.g. proton, hydrogen atom and hydride ion transfer. Excited molecules are thus formed directly and by geminate ion recombination... 

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