Electron Transfer in the Gas Phase

The subject of drift velocities, particularly as it pertains to inert gas systems, was discussed by Hornbeck (5S) (experimental results) and Holstein (57) (theoretical) as part of the program for a symposium on Electron Transfer Processes in general, held at Notre Dame in 1952. The significant observation is that the drift velocity of an ion such as He+-is much less than is expected if account is taken only of the usual processes for energy transfer, including polarization of He by the positive ion. Similar effects are noted for the other inert gas ions and have been recorded also for N2+ in N2 (50). The effective collision cross section is increased by symmetry effects which include electron transfer as a component. Table I 

C0MPABI8ON OF Ckoss Section for Electron Transfer Ti WITH Gas Kinetic Cross Section Ta 

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Experiments with crossed molecular beams might entail a beam of ferrocene molecules against a beam of chloride ions in the close outer-sphere electron transfer in the gas phase. This should be a very similar process to what we have seen in solution. I d just like to leave the thought that these are two possible directions in which future work will go. 

The thermochemistry of sulfur radicals in the gas phase has been reviewed. Methylsulfonyl radicals and cations have been produced by femtosecond collisional electron transfer in the gas phase. When formed by vertical collisional electron transfer from cation CH3SO2+, radical CH3S02 dissociates to CH3 and SO2. Radical CH30S0 exists as a mixture of syn (19a) and anti (19b) isomers which are stable when formed by collisional electron transfer to the corresponding cation. Dissociation of both isomers of CH30S0 formed CH3 and SO2 via isomerization to methylsulfonyl radical. An ab initio study on the formation of the thiyl peroxyl radical has also been reported. Julolidylthiyl radicals (20) were formed by femtosecond photo-dissociation of the corresponding disulfide and have been observed... 

Electron transfer is usually carried out in bulk, condensed matter. In the gas phase, the lower concentrations of the donor and acceptor reduce the chance of an encounter between them in comparison with condensed phases. Furthermore, in the absence of a solvent, no stabilization of the separated ions by solvation is possible, enhancing the chance of charge recombination. The volume of published papers in this field is therefore much smaller for gaseous systems than for condensed matter. Nonetheless, gas-phase systems are in principle simpler to analyze and comparison with theory is more straightforward. The analysis of electron transfer in condensed systems usually starts from the (sometimes experimentally inaccessible) gaseous system. Therefore, efforts to study electron transfer in the gas phase continue, and have indeed shed much light on the mechanism of the process. 

Turecek, F Chen, X. Hao, C. Where does the electron go Electron distribution and reactivity of peptide cation radicals formed by electron transfer in the gas phase. J. Am. Chem. Soc. 2008, 130, 8818-8833. 

The example considered shows that, due to electron tunneling at large distances, the non-resonance (as well as resonance) charge transfer in the gas phase can occur at distances which substantially exceed the size of the colliding particles themselves. 

The above combinations of electricity with chemistry deal with the generation of charged species in either a gas of a liquid medium. This requires ionization, which occurs by electron transfer and transport of charged species in a closed electrical circuit. The charged species themselves, or the radicals generated by them (e.g., radicals generated by electron impact in the gas phase, in a plasma) can be used as activated species taking part in chemical reactions. 

In Section II, A we defined the processes of electron ionization in the gas phase and of electron transfer to the continuum in the solution phase ... 

There are many other types of solution data that support the half-wave reduction potential and charge transfer complex data. These include the measurement of cell potentials or equilibrium constants for electron transfer reactions. Another important condensed phase measurement involving a negative ion is the determination of electron spin resonance spectra. In these studies the existence of a stable molecular anion is established and the spin densities can be measured [79]. The condensed phase measurements support the electron affinities in the gas phase and extend the measurements to lower valence-state electron affinities. 

Although electron bombardment is rapid and the predominant form of heat transfer in the gas phase, transport processes within the pellet are quite different and much slower. Increased char yield and condensed aromatics found in this study are consistent with the following description of the processes occurring within the pellet. Upon collision with the pellet surface, the flux of electrons relases large amounts of heat which volatilizes and cracks the polymeric lignin. Depending on the gas composition as in Figure VI, the stoichiometry (or C/O/H ratios) of the biomass, and the mass transport situation, an amount of residual or char forms inward from the pellet surface, while the volatiles outflow increases the gas pressure near the pellet. 

Tables I and II summarize gas phase basicity and acidity results for a number of compounds. In Table I most compounds are oxygen or nitrogen bases with a lone pair of electrons. The basicities in the left side of Table I have been expressed as dC°(kcal mole ) for proton transfer in the gas phase from the base B to H2O.

Although the dynamical problem of the reacting molecular system is of the same complexity as that encountered in gas-phase reaction dynamics, the presence of a surface adds additional processes and phenomena. Such phenomena are, for instance, not only the importance of the structure, including corrugation, steps, and surface anomalities, but also the interaction with the possible excitation processes in the solid, such as phonon (surface vibrations) and electronic excitations. Also for charge transfer and other nonadiabatic electronic processes in the gas phase, the importance of the surface temperature adds additional features to the problem. Aside from this, the various processes of interest occur on different time scales, from fast reactive chemisorption processes on the sub-pico second time scale to the relatively slow diffusion and desorption processes. Thus different theoretical tools are needed in order to describe the variety of processes and the large time span one needs to cover. Also the many-body problem of the solid combined with the few-body gas-phase problem makes it necessary to introduce different methods for treating the dynamics, from classical trajectories and... 

A type of molecular resonance scattering can also occur from the formation of short-lived negative ions due to electron capture by molecules on surfrices. While this is frequently observed for molecules in the gas phase, it is not so important for chemisorbed molecules on metal surfaces because of extremely rapid quenching (electron transfer to the substrate) of the negative ion. Observations have been made for this scattering mechanism in several chemisorbed systems and in phys-isorbed layers, with the effects usually observed as smaU deviations of the cross section for inelastic scattering from that predicted from dipole scattering theory. 

We shall see in Chapter 2 that the formation of a bond in an ionic compound depends on the removal of one or more electrons from one atom and their transfer to another atom. The energy needed to remove electrons from atoms is therefore of central importance for understanding their chemical properties. The ionization energy, /, is the energy needed to remove an electron from an atom in the gas phase ... 

Radicals can be prepared from closed-shell systems by adding or removing one electron or by a dissociative fission. Generally speaking, the electron addition or abstraction can be performed with any system, the ionization potential and electron affinity being thermodynamic measures of the probability with which these processes should proceed. Thus, to accomplish this electron transfer, a sufficiently powerful electron donor or acceptor (low ionization potential and high electron affinity, respectively) is required. If the process does not proceed in the gas phase, a suitable solvent may succeed. 

Abstract Sonoluminescence from alkali-metal salt solutions reveals excited state alkali - metal atom emission which exhibits asymmetrically-broadened lines. The location of the emission site is of interest as well as how nonvolatile ions are reduced and electronically excited. This chapter reviews sonoluminescence studies on alkali-metal atom emission in various environments. We focus on the emission mechanism does the emission occur in the gas phase within bubbles or in heated fluid at the bubble/liquid interface Many studies support the gas phase origin. The transfer of nonvolatile ions into bubbles is suggested to occur by means of liquid droplets, which are injected into bubbles during nonspherical bubble oscillation, bubble coalescence and/or bubble fragmentation. The line width of the alkali-metal atom emission may provide the relative density of gas at bubble collapse under the assumption of the gas phase origin. 

The extension of analytical mass spectrometry from electron ionization (El) to chemical ionization (Cl) and then to the ion desorption (probably more correctly ion desolvation ) techniques terminating with ES, represents not only an increase of analytical capabilities, but also a broadening of the chemical horizon for the analytical mass spectrometrist. While Cl introduced the necessity for understanding ion—molecule reactions, such as proton transfer and acidities and basicities, the desolvation techniques bring the mass spectrometrist in touch with ions in solution, ion-ligand complexes, and intermediate states of ion solvation in the gas phase. Gas-phase ion chemistry can play a key role in this new interdisciplinary integration. 

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