Free electron transfer

Phenol radical cations exist only in strong acidic solutions (pKa -1) [1, 2]. However, in non-polar media phenol radical cations with lifetimes up to some hundred nanoseconds were obtained by pulse radiolysis [3], The free electron transfer from phenols (ArOH) to primary parent solvent radical cations (RX +) (1) resulted in the parallel and synchroneous generation of phenol radical cations as well as phenoxyl radicals in equal amounts, caused by an extremely rapid electron jump in the time scale of molecule oscillations since the rotation of the hydroxyl groups around the C - OH is strongly connected with pulsations in the electron distribution of the highest molecular orbitals [4-6]. 

Femtosecond Events in Bimolecular Free Electron Transfer... 

In recent years, much work has been done on the electron transfer phenomenon called free electron transfer (FET). FET stands for an electron transfer process where the molecule oscillations of the donor are reflected in a bimolecular reaction. This is reasoned by an unhindered electron jump proceeding in the first encounter of the reactants. So free means unhindered and concerns the transfer, not the electron. 

Here, this special kinetic behavior named free electron transfer (FET) should be discussed in more detail in this chapter. 

As already mentioned, the electron transfer [Eq. (3)] shows an unusual product pattern. Therefore from here we are using the term free electron transfer (FET) indicating peculiarities of this reaction type elucidated later on. Taking phenols (ArOH) as electron donors, a 1 1 product ratio of radical cations ArOH + and phenoxyl radicals ArO has been observed. These species were produced in a parallel manner as shown in the reaction sequence given in Eqs. (7). 

The concept of the free electron transfer is based on the hypotheses that molecule-deformation motions happening in the time range >100 femtoseconds are directly connected with electron shifts in the HOMO-tt- and -orbitals of the donor molecules. Hence, free and non-hindered electron jumps visualize this dynamic situation by formation of different primary product radical cations, which can be experimentally distinguished if one of the product cations is stable and the other one is dissociative, forming a radical and a solvent-stabilized proton. 

Femtosecond Events in BimolecMlar Free Electron Transfer 421... 

Figure 7 gives an idea of the time regimes of the physical and chemical steps of an electron transfer reaction. Above the time axis common knowledge about the time range of some principal processes is given. More specifically, below the axis you will find the processes which are in particular involved in the free electron transfer. It can be seen... 

Fig. 7. Time range of processes relevant to the free electron transfer. Rotation (torsion) stands also for other similar motions such as necking, wagging, bending, etc.

The unrestricted and free electron transfer (FET) from donor molecules to solvent radical cations of alkanes and alkyl chlorides has been studied by electron pulse radiolysis in the nanosecond time range. In the presence of arenes with hetero-atom-centered substituents, such as phenols, aromatic amines, benzylsilanes, and aromatic sulfides as electron donors, this electron transfer leads to the practically simultaneous formation of two distinguishable products, namely donor radical cations and fragment radicals, in comparable amounts. 

Mahalaxmi GR, Hermann R, Naumov S, Brede O. (2000) Free electron transfer from several phenols to radical cations of non-polar solvents. Phys Chem Chem Phys 2 4947 955. 

Brede O, Naumov S. (2006) Femtodynamics reflected in nanoseconds Bimolelcular free electron transfer in nonpolar media. J Phys Chem A 110 11906-11918. 

Brede O. (2001) Peculiarities in free electron transfer. Res Chem Intermed 27 709-715. 

Brede O, Naumov S, Hermann R. (2003) Monitoring molecule dynamics by free electron transfer. Radiat Phys Chem 67 225-230. 

Brede O, Maroz A, Hermann R, Naumov S. (2005) Ionization of cyclic aromatic amines by free electron transfer Products are governed by molecule flexibility. /Chem A 1009 8081-8087. 

Free electron transfer mirrors rotational conformers of substituted aromatics Reaction of benzyltrimethylsilanes with -butyl chloride radical cations. Phys Chem Chem Phys 6 2267-2275. 

Free electron transfer from xanthenyl and fluorenylsilanes (Me-3 or Ph-3) to parent solvent radical cations Effect of molecule dynamics. J Phys Chem A 109 11679-11686. 

Brede O, Hermann R, Karakostas N, Naumov S. (2004) ionization of the three isomeric hydroxybenzoates by free electron transfer Product distribution depends on the mobility of the phenoxyl group. Phys Chem Chem Phys 6 5184-5188. 

Free electron transfer (FET) from thiophenols (ArSH) to molecular radical cations of some selected nonpolar solvents was studied using the pulse radiolysis technique [reaction (24)] " ... 

Aromatic sulfides analogous to thiophenols constitute a group of molecules that fulfils the structural conditions necessary for the observation of FET (Sec. 2.4), i.e. they exhibit a low barrier to rotation about the Qp2 S bond. Thus, the torsion motions of the substituents can be accompanied by considerable fluctuation of the electrons in the highest molecular orbitals with two extreme examples of conformers, planar and vertical. The presence of two radical cation conformers was deduced as primary products of the bimolecular free electron transfer (FET) from aromatic sulfides PhSCH2Ph, PhSCHPhj, and PhSCPhg to w-butyl chloride radical based on the nanosecond pulse radiolysis experiments. ... 

Brede O, Hermarm R, Naumarm W, Naumov S. (2002) Monitoring of the heterogroup twisting dynamics in phenol type molecules via different characteristic free-electron-transfer products./Pfryr Chem A 106 1398-1405. 

The fundamental aspects of structure-reactivity relationships in radiation-induced oxidation of substituted benzenes, bimolecular free electron transfer on the femtosecond time scale, the chemistry of sulfur-centered radicals and the radiolysis of metalloproteins are discussed in succeeding chapters. The effects of the direct and indirect mechanisms of radiation-induced DNA damage are discussed individually in two complementary chapters. The last chapter highlights the application of radiation chemical techniques to antioxidant research. 

Heat conduction is the transfer of energy between neighbouring molecules in a substance due to a temperature gradient. In metals also the free electrons transfer energy. In solids which do not transmit radiation, heat conduction is the only process for energy transfer. In gases and liquids heat conduction is superimposed by an energy transport due to convection and radiation. 

Futamura, S., Chemical behavior of aromatic radical cations under superoxide-free electron transfer photooxygenation conditions. Bull. Chem. Soc. Jpn., 65,1779,1992. 

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