Electron attachment reactions

Much of what is knotm about the structure response of the ECD is based on empirical observations. Clearly, the ability to correlate the response of the detector to fundamental molecular parameters would be useful. Chen and Wentworth have shorn that the information required for this purpose is the electron affinity of the molecule, the rate constant for the electron attachment reaction and its activation energy, and the rate constant for the, ionic recombination reaction [117,141,142]. in general, the direct calculation of detector response factors have rarely Jseen carried j out, since the electron affinities and rate constants for most compounds of interest are unknown. 

The electron affinity (EA) for a molecule is a quantity which is analogous to the ionization energy for cations. Thus, the electron affinity is defined as the negative of the enthalphy change for the electron attachment reaction ... 

It is natural to conclude that the high rate constants for electron attachment reactions in nonpolar liquids are associated with the high mobility of electrons. Early studies [96,104,105] of attachment to biphenyl and SFg emphasized the dependence of on mobility. This relationship is apparent if the expression for the rate constant for a diffusion-controlled reaction ... 

The occurrence of the dissociative electron attachment reactions was also established by direct observation of the transient product. Thus, the absorption spectra of the benzyl radical (21, 28) and the triphenyl methyl radical (28) were both observed. The other products formed in... 

As in the case of the positive ion chemistry, the problem is to describe the chemical steps which convert the primary negative ions to the observed clusters. The primary negative ions can only be O- and 02 formed in the electron attachment reactions ... 

Electron attachment reactions may end with IP formation or with the formation of a fragment ion(s) through cleavage to a stable anion and a neutral radical (which is not observed). These reactions, when halogenated (Cl,Br,I) compounds are involved, give the corresponding halide ion. Many examples of... 

In liquids the free energy change in electron attachment reactions is given... 

Both the GC/ECD and GC/NICI instruments commonly analyze compounds that form negative ions such as pesticides and polychlorinated biphenyls, making devices to carry out the study of thermal electron attachment reactions commercially available. These include the radioactive and nonradioactive ECD,... 

The electron attachment reactions for inorganic molecules were reviewed in 1974. Those for organic molecules were summarized in 1984 [12, 13]. In many cases activation energies were not measured. If a nominal value for A and A is assumed, the activation energies can be estimated. Recently, the flowing afterglow procedure has been extended to include electron and gas heating so that the dependence of the rate constants on thermal electron attachment can be examined for bulk temperature and electron temperature [14]. 

Curves for the negative-ion states of H2 and L are chosen to illustrate the procedures for the homonuclear diatomic molecules. Curves for benzene and naphthalene are examples of excited states for larger molecular negative ions. These illustrate the relationship between gas phase acidities and thermal electron attachment reactions. Such correlation procedures can be applied to systematic predictions for many different problems. 

The AMB Ea for 02 and NO are for excited states. In 1971 Freeman postulated that structure observed in the unfolded cross-section for the reaction of a Cs beam with O2 could give a measure of the activation energy for electron attachment to form the ground state of 02(—). In this case the extrapolated appearance potential is AP = IP + E. As shown in Figure 10.6, the two AP are 3.44 eV and 4.75 eV. With the 3.89 eV ionization potential of Cs, the first gives Ea = 3.89 3.44 = 0.45 eV for the excited state and the second E = 4.75 — 3.89 = 0.86 eV. This illustrates another interpretation of AMB data to give fundamental experimental information for thermal electron attachment reactions. This E has been subsequently measured and confirmed by isotopic exchange experiments and is incorporated in the ECD data analysis [33],... 

The production of H2 in the radiolysis of water has been extensively re-examined in recent years [8], Previous studies had assumed that the main mechanism for H2 production was due to radical reactions of the hydrated electron and H atoms. Selected scavenger studies have shown that the precursor to the hydrated electron is also the precursor to H2. The majority of H2 production in the track of heavy ions is due to dissociative combination reactions between the precursor to the hydrated electron and the molecular water cation. Dissociative electron attachment reactions may also play some role in y-ray and fast electron radiolysis. The radiation chemical yield, G-value, of H2 is 0.45 molecule/100 eV at about 1 microsecond in the radiolysis of water with y-rays. This value may be different in the radiolysis of adsorbed water because of its dissociation at the surface, steric effects, or transport of energy through the interface. 

Photoionization of the hydrocarbon followed by dissociative electron attachment (Reaction 1) should be considered since the ionization potential of a molecule is less in the liquid phase than it is in the gas phase. For hydrocarbons the ionization potential is 1 to 1.5 e.v. less in the liquid phase (24). The photon energy at 1470 A. is about 1.4 e.v. below the gas-phase ionization potentials of cyclohexane and 2,2,4-trimethylpentane (14). Some ionization may therefore occur, but the efficiency of this process is expected to be low. Photoionization is eliminated as a source of N2 for the following reasons. (1) If photoionization occurred and the electron reacted with nitrous oxide, then O" would be formed. It has been shown in the radiolysis of cyclohexane-nitrous oxide solutions that subsequent reactions of O result in the formation of cyclohexene and dicyclohexyl (I, 16, 17) and very little cyclohexanol (16, Table III). In the photolysis nitrous oxide reduces the yield of cyclohexene and does not affect the yield of dicyclohexyl. This indicates that O is not formed in the photolysis, and consequently N2 does not result from electron capture. (2) A further argument against photoionization is that cyclohexane and 2,2,4-trimethylpentane have comparable gas-phase ionization potentials but exhibit quite different behavior with respect to N2 formation. 

Fig. 13. Electron energy dependence of collision cross section for representative electron impact processes with molecular chlorine. Note threshold energy for endothermic processes (vibrational excitation, ionization, etc.). Qa, Qd, Qe, Qi, Qm. and Qv, correspond to electron attachment (reaction R18 in Table 4), dissociation (R19), electronic excitation (R12), momentum transfer (R20), ionization (R16) and vibrational excitation (not shown in Table 4), respectively. After [44].

General correlation between sodium flame reaction rates and activation energies for the dissociative electron attachment reaction is evident but further data are necessary to extend this to more specific cases. 

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