Gas-phase electron transfer

On the other hand, if one looks at gas phase electron transfer we find that it s dominated by small molecules and nonmethyl molecule and the system is quite remote from what we see in solution (H ions, and so forth). And I do wonder whether people will turn their attention to using, for example, organo-metallic complexes, many of which are volatile and unstable in the gas phase. 

This chapter reviews several gas-phase studies involving atoms, simple molecules, van der Waals complexes and clusters. Electron-transfer reactions are central processes in a variety of scientific disciplines as outlined in a recent review by Bixon and Jortner [1]. We highlight here the current understanding in the dynamics of the gas-phase electron-transfer reactions, and end the chapter by presenting work which intends to bridge the gap between the standard knowledge of electron-transfer reactions in the gas phase and in condensed phases. 

An important class of the gas-phase electron-transfer reactions is well described, at least qualitatively, by the model named pictorially the harpoon model , which was proposed originally by Michael Polanyi [8] to account for exceptionally large cross-sections in the oxidation reaction of alkali metal atoms by halogen molecules. It is striking to observe that such a simple model is still a useful tool to rationalize these reactions. It is illustrated in Figure 2. 

Gas-phase electron-transfer reactions have been studied using all the experimental methods which are of common use in the reaction dynamics community. These methods have been extensively described in the literature, and it is not necessary to describe them again in detail. We just need to recall a few points, which are necessary to understand the origin of the results discussed in the remainder of the chapter, and only the most recent and comprehensive reviews and a few original papers describing the principal techniques are referred to hereafter. It must be mentioned that the techniques used to study reactive collisions are in very wide use and have also been used to investigate non-reactive collisions, both inelastic and elastic. 

Figure 3. Center-of-mass angular distributions of an alkali metal monohalide compound in five gas-phase electron-transfer reactions as labeled. Forward scattering is to be found around 0° and backward scattering at 180°. Adapted from Ref. [63].

Few experiments allow one to bridge gas-phase electron transfer mechanism to liquid (or condensed)-phase electron transfer reactions. The major problem is to model the so-called solvent coordinates in the gas phase. Of course, clusters seems to be the ideal medium to build solvent effects in a stepwise manner. However, clusters are much colder than liquids, with the consequence that only a limited number of isomers are explored, as compared with the room temperature configurations involved in liquid processes. Discrepancies are observed in the case of cluster solvation of ionic molecules in clusters Nal remains at the surface of water clusters whereas it dissolves in bulk water [275]. Clusters thus do not allow one to explore in a single step all the aspects of a liquid-phase electron-transfer reaction. Their main advantage arises from this limitation since they allow one to study separate aspects of the solution processes. 

Chowdhuty, S. Grimsnid, E. P. Kebarle, P. "Entropy Changes and Electron Affinities from Gas-Phase Electron-Transfer Equilibria A <- B = A -1- B, "/ Phys. Chem. 1986,90,2747-2752. 

Gas-phase electron-transfer processes for the 1,2,4,6-thiatriazinyl radicals 13 were investigated using the Hartree-Fock (HF) and B3LYP methods with the 6-31G(d) basis set augmented with diffuse functions. The calculated... 

Herron, W.J. Goeringer, D.E. McLuckey, S.A. Gas-phase electron-transfer reactions from multiply-charged anions to rare-gas cations. J. Am. Chem. Soc. 1995,117,11555-11562. 

A short review on electron tunnelling contains references to the development of electron-transfer theory, and to some more recent experimental work. A review on inelastic atom-atom collisions includes the theory of gas-phase electron-transfer processes. Developments within the Marcus adiabatic theory are considered under the first two headings. 

Boltalina OV, Dashkova EV, Sidorov LN (1996) Gibbs energies of gas-phase electron transfer reactions involving the larger fuUerene anions. Chem Phys Letters 256 253-260... 

Stephenson, J. L., Jr. McLuckey, S. A. Charge reduction of oligonucleotide anions via gas-phase electron transfer to xenon cations. Rapid Commun. Mass Spectrom 1997, 77(8), 875-880. 

The limiting case to assess the role of solvent reorganisation for an electron-transfer reaction is the absence of bulk solvent, that is, the gas phase. The study of gas-phase electron transfers has the same simpUlying and complicating features as the study of 8 2 reactions in the gas phase, discussed in detail in Chapter 11. The reaction coordinate can be defined more precisely, but, on the other hand, the formation of precursor and successor complexes makes the interpretation of the kinetics more elaborate. For very negative... 

Intense sodium D-line emission results from excited sodium atoms produced in a highly exothermic step (175). Many gas-phase reactions of the alkafl metals are chemiluminescent, in part because their low ioni2ation potentials favor electron transfer to produce intermediate charge-transfer complexes such as [Ck Na 2] (1 )- There appears to be an analogy with solution-phase electron-transfer chemiluminescence in such reactions. 

Electron transfer is a fast reaction ( 10-12s) and obeys the Franck-Condon Principle of energy conservation. To describe the transfer of electron between an electrolyte in solution and a semiconductor electrode, the energy levels of both the systems at electrode-electrolyte interface must be described in terms of a common energy scale. The absolute scale of redox potential is defined with reference to free electron in vacuum where E=0. The energy levels of an electron donor and an electron acceptor are directly related to the gas phase electronic work function of the donor and to the electron affinity of the acceptor respectively. In solution, the energetics of donor-acceptor property can be described as in Figure 9.6. 

An isolated diorganylmagnesium molecule in the gas phase usually adopts a linear structure with a central i p-hybridized magnesium atom and two negatively charged ligands at opposite positions. In practice, this situation is very difficult to achieve only a few organomagnesium compounds can be transferred into the gas phase as thermal decomposition normally precedes vaporization. In a gas-phase electron diffraction study of dineo-pentylmagnesium (Section II), a linear molecule was indeed found. 

The feasibility of electron transfer from alkoxides to acceptor species, attractive as it is for its simplicity, has been prospected as a real possibility only in a few instances (44, 45a) and, in some cases, questioned (9, 45b, 46a). Buncel and Menon (45b) estimated that electron transfer from t-BuO- to 4-nitrotoluene in f-BuOH is energetically unfavorable to the extent of about 3 eV. The estimate is based on calculations involving gas-phase electron affinity data for t-BuO (1.93 eV) instead of its oxidation potential in solution, which, as is the case for other alkoxides, is not known. The approximation is necessarily crude, because solvation effects could be of significant magnitude nevertheless, the estimated value has met wide acceptance. 

A bridging ligand reduction model vs. the outer-sphere mechanism for electron transfer has been tested using rate constants from Cr(II) systems. A correlation between the rate constant and the gas-phase electron affinity of the bridging group implies an inner-sphere mechanism. If such a correlation is absent an outer-sphere mechanism is assumed. ... 

Attachment of thermal electrons to perfluoro-cyclopentene and -cyclohexene has been studied. The rates of proton transfer from CHs+ and H3+ to cis- and trans-1,2-difluoroethylene have been measured in a study of gas-phase ion-dipole inter-actions.42 In the reaction of the base CDsO" with fluoroethanes in the gas phase, proton transfer and HF elimination, e.g.. Scheme 2, occur at comparable rates, except for the ethane CFs CHa, where elimination predominates and for CF3 -CFaH, where proton transfer occurs exclusively. ... 

In addition to direct emission in combustion processes, most secondary formation pathways in the gas and aqueous phases depends on ozone. By contrast, there is only one way that is light independent (ozonolysis) and another way that is independent of oxidant precursors (aqueous phase electron transfer onto oxygen Eqs. 5.96 and 5.102). 

This type of expression is often adopted to express the change in material conductivity according to oxygen pressure. In the simple cases where the gas adsorption reaction is controlled by the adsorbed-phase electron transfer, the value of n depends on whether or not the adsorption process is dissociative. Generally, in the case of oxygen adsorption, n assumes the value 1 if adsorption is not dissociative, and assumes the value 2 if it is. 

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