Electron attachment and detachment

Williamson, D.H. Knighton, W.B. Grimsrud, E.P. Effect of Buffer Gas Alterations on the Thermal Electron Attachment and Detachment Reactions of Azu-lene by Pulsed High Pressure Mass Spectrometry. Int. J. Mass Spectrom. 2000, 795/796,481-489. 

Electron propagator calculations 25-28 of electron-binding energies (that is, electron-attachment and -detachment energies) may be based on one-electron equations which read... 

Nondissociative electron attachment and detachment are general terms that refer to the process whereby an electron e(—) reacts with a molecule AB to form the molecular negative ion AB(-) and the reverse process whereby the electron is removed. It is also designated as the molecular anion formation ... 

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],... 

The reactions in the ECD and in NIMS were given in Chapter 2 as equations 2.1 to 2.6. They involve electron attachment and detachment, unimolecular dissociative... 

Thermal electron attachment to SF6 has been studied in the ECD and NIMS at atmospheric pressure and in a chemical ionization mass spectrometer at lower pressures. From these data rate constants for thermal electron attachment and detachment to SF6, and the Qan values and electron affinities of SF6 and SF5 have been determined. The Morse potential energy curves for multiple negative-ion states were calculated using electron impact data and electron affinities. 

A. V. Phelps, Laboratory studies of electron attachment and detachment processes of aeronomic interest, Can. J. Chem. 47, 1783-1793 (1969). 

Head-Gordon, M., Grana, A. M., Maurice, D., and White, C. A. (1995) Anal5 is of electronic transitions as the difference of electron attachment and detachment densities, J. Phys. Chem., 99, 14261-14270. 

To obtain the attachment reaction efficiency in the quasi-free state, we denote the specific rates of attachment and detachment in the quasi-free state by kf and kf respectively and modify the scavenging equation (10.10a) by adding a term kfn on the right-hand side, where is the existence probability of the electron in the attached state. From the stationary solution, one gets kf/kf = (kfk ikfkf), or in terms of equilibrium constants, K(qf) = Kr.Kr, where k, and k2 are the rates of overall attachment and detachment reactions, respectively. Furthermore, if one considers the attachment reaction as a scavenging process, then one gets (see Eq. 10.11) = k f fe/(ktf + kft) = fe,f/(l + Ku) and consequently k2 = kfKJ(l + KJ. 

The specific rate constants of interest to the ECD and NIMS are dissociative and nondissociative electron attachment, electron detachment, unimolecular anion dissociation, and electron and ion recombination. The reactions that have been studied most frequently are electron attachment and electron and ion recombination. To measure recombination coefficients, the electron concentration is measured as a function of time. The values are dependent on the nature of the positive and negative ions and most important on the total pressure in the system. Thus far few experiments have been carried out under the conditions of the NIMS and ECD. However, the values obtained under other conditions suggest that there is a limit to the bimolecular rate constant, just as there is a limit to the value of the rate constant for electron attachment. The bimolecular rate constants for recombination are generally large, on the order of 10 7 to 10-6 cc/molecule-s or 1014 to 1015 1/mole-s at about 1 atm pressure. Since the pseudo-first-order rate constants are approximately 100 to 1,000 s 1, the positive-ion concentrations in the ECD and NIMS are about 109 ions/cc. 

When the same heme is bound to the protein of cytochrome two donor atoms from the protein are bound to the iron atom, a nitrogen from a histidine imidazole and a sulfur from a methionine (Fig. 3). The porphyrin is held very tightly in the protein structure by the addition of two cysteine sulfhydryl groups across the double bonds of the two vinyl side chains of the porphyrin. Since the function of cytochrome c is electron transfer, and not the attachment and detachment of additional ligands, the cytochrome c structure is tailored to produce very stable binding of the porphyrin to the protein. 

In the discussion of the electron attachment we have to realize that it is in reality an equilibrium of attachment and detachment. 

Solutions of biphenyl in tetramethylsilane represent an example where the electron mobility is modulated by the formation of temporary negative ions due to electron attachment to the solute molecules. The electron mobility in tetramethylsilane at room temperature is about 100 cm V s The electrons are considered to be quasifree and to move in a conduction band. If biphenyl, Ph2, is dissolved electron attachment and subsequent thermally activated detachment occurs ... 

Potential energy surfaces (PES) of an anion formation (M ) after electron (e ) attachment to the neutral molecule (M). When the energy of an anion (M ) lies below the neutral molecule, positive electron affinity Is observed [left) otherwise the molecule is said to have negative electron affinity right). Energy changes shown for an electron attachment or detachment event are (1) the vertical electron affinity (VEA), (2) the adiabatic electron affinity (AEA), and (3) the vertical detachment energy (VDE). The VDE and VEA impose the upper and lower bounds to AEA... 

Simple textbook results for IPs and EAs serve as a starting point for thinking about quantum chemistry calculations. If we assume that the nonionized molecule is described by a Hartree-Fock wave function, and furthermore if we neglect any relaxation of the MOs on electron attachment or detachment - that is, if we use the neutral molecule s MOs to construct a Slater determinant for the ionized species, removing an electron from the HOMO or adding an electron to the LUMO - then the corresponding approximate IP and EA expressions are known as KT 35... 

Holroyd (1977) finds that generally the attachment reactions are very fast (fej - 1012-1013 M 1s 1), are relatively insensitive to temperature, and increase with electron mobility. The detachment reactions are sensitive to temperature and the nature of the liquid. Fitted to the Arrhenius equation, these reactions show very large preexponential factors, which allow the endothermic detachment reactions to occur despite high activation energy. Interpreted in terms of the transition state theory and taking the collision frequency as 1013 s 1- these preexponential factors give activation entropies 100 to 200 J/(mole.K), depending on the solute and the solvent. 

Mozumder (1996) has discussed the thermodynamics of electron trapping and solvation, as well as that of reversible attachment-detachment reactions, within the context of the quasi-ballistic model of electron transport. In this model, as in the usual trapping model, the electron reacts with the solute mostly in the quasi-free state, in which it has an overwhelmingly high rate of reaction, even though it resides mostly in the trapped state (Allen and Holroyd, 1974 Allen et ah, 1975 Mozumder, 1995b). Overall equilibrium for the reversible reaction with a solute A is then represented as... 

Electron attachment to solutes in nonpolar liquids has been studied by such techniques as pulse radiolysis, pulse conductivity, microwave absorption, and flash (laser) photolysis. A considerable amount of data is now available on how rates depend on temperature, pressure, and other factors. Although further work is needed, some recent experimental and theoretical studies have provided new insight into the mechanism of these reactions. To begin, we consider those reactions that show reversible attachment-detachment equilibria and therefore provide both free energy and volume change information. 

---