Electron attachment reactions, kinetics

Liu, Y. Mayhew, C.A. Peverall, R. A new experimental approach to investigate the kinetics of low energy electron attachment reactions. Int. J. Mass Spectrom. Ion Processes 1996,152, 225-242. 

An interesting approach to measuring rates of electron transfer reactions at electrodes is through the study of surface bound molecules (43-451. Molecules can be attached to electrode surfaces by irreversible adsorption or the formation of chemical bonds (461. Electron transfer kinetics to and from surface bound species is simplified because there is no mass transport and because the electron transfer distance is controlled to some degree. 

In nonpolar liquids, bimolecular electron attachment rate constants, k , are much larger than those for conventional reactions of ions or radicals. This is, in part, related to the high mobility of electrons in these liquids but various other factors, like Vq, the kinetic energy of the electron, and dipole moment of the solute, are important as well. These and other factors are examined below the dependence of on the energy gap, AGr, in representative liquids is also shown and discussed. 

So far, two different mechanisms of single strand break formation based on adiabatically stable anions have been proposed. The first mechanism, suggested by the Leszczynski group, assumes the formation of stable anions of 3 - and 5 -phosphates of thymidine and cytidine in which the cleavage of the C-O bond take place via the SN2-type process. The second reaction sequence, proposed by us, starts from the electron induced BFPT process followed by the second electron attachment to the pyrimidine nucleobase radical, intramolecular proton transfer, and the C-O bond dissociation. In both mechanisms the bottleneck step is associated with very low kinetic barrier which enables the SSB formation to be completed in a time period much shorter than that required for the assay of damage. 

The threshold energy for the dissociative electron attachment process e" + SPg(g) = SF "(g) + F(g) has been measured directly by means of mass spectrometry ( .- ). In addition, mass spectrometric studies (5) have been reported on various electron transfer reactions which provide Independent estimates of the enthalpy for the above process. Presented below is a summary of the results derived from these measurements. Also included in the summary are results obtained from an electron-impact study ( ) of the process e + SFgCl(g) = SFg (g) + Cl(g). We assume that the fragment-ions are formed in each process with no excess or kinetic energies. As a result, the derived electron affinities represent lower limits to the true value. Other reported values of EA(SFg) include >1.465 eV (9), 3.2 eV (5) and 3.66 0.04 eV (10). The first two results are based on charge-transfer studies (5-9) while the latter value represents a direct measurement of EA using the magnetron technique (10). 

To test the ECD hypothesis, E. C. M. Chen measured the temperature dependence of the molar response. This entailed a detailed study of all parameters associated with the pulse sampling method. For these molecules the most important reactions were postulated to be electron generation and collection, electron and ion recombination, electron attachment and detachment. It was discovered that the simple thermodynamic model was not applicable and that a kinetic model was necessary to explain the change in temperature dependence. If we assume a steady state exists, an expression can be obtained that relates the ECD molar response to kinetic rate constants for the above reactions [24, 25],... 

Amazingly, the third-order kinetics of this type of reaction were first measured for li" in 1928 this was the first study of anion reaction chemistry performed using a mass spectrometer. Some electrons are thermalized rather than captured by the precursor. This can lead to electron attachment to form odd-electron ions, while halide ion association forms even-electron ions. 

A general kinetic model has been presented (23) which appears to be consistent with the data presently accumulated oir some 100 compounds. The electron attachment steps and negative ion dissociation reactions are as follows ... 

Physicochemical properties of cyanogen (NC—CN, 1) were the subject of thorough investigations. Papers published deal with Raman spectra, with proton affinity, reactions with low-energy electrons " and cyanide formation by electron attachments. Kinetics of basic hydrolysis of cyanogen were investigated in detail. Photolysis at 193 nm was studied... 

In the above sections, we have presented the electrode kinetics of electron-transfer reaction and reactant transport on planar electrode. However, for practical application, the electrode is normally the porous electrode matrix layer rather thtin a planner electrode siuface because of the inherent advantage of large interfacial area per unit volume. For example, the fuel cell catalyst layers are composed of conductive carbon particles on which the catalyst particles with several nanometers of diameter are attached. On the catalyst particles, some proton or hydroxide ion-conductive ionomer are attached to form a solid electrolyte, which is uniformly distributed within the whole matrix layer. Due to the electrode layer being immersed into the electrolyte solution, this kind of electrode layer is called the flooded electrode layer . 

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