Electron attachment thermal

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. 

Figure 2 Dependence on C2H4 density of the effective two-body rate constant of thermal electron attachment in O2-C2H4 mixtures at room temperature. (From Ref. 52.) The dashed curve represents the expected contribution from the BB mechanism.

The EA of the isolated molecules are probably negative for the former, but they are large and jijC itive for the latter the for the former are most likely <10 sec, but they are >10 sec for the latter the thermal electron attachment rates, for the former are orders of magnitude smaller compared to those for the latter. 

Table 9. Positions of electron attachment cross section maxima, values of o ( e) at these maxima, energy integrated cross sections, and thermal electron attachment rates for haloethanes...

Shimamori H, Hatano Y. (1976) Mechanism of the thermal electron attachment in Oj-Nj mixtures. Chem Phys 12 439 45. 

The production of negative ions from CH3X (X = Br, I, CN, or N02) by both thermal electron attachment and electron capture from excited atoms has also been studied.121... 

Figure 2.1 Morse potential energy curves for the neutral and negative-ion states of F2. The vertical electron affinity VEa, adiabatic electron affinity AEa, activation energy for thermal electron attachment E, Err — AEa — VEa, EDEA — Ea(F) — D(FF), and dissociation energy of the anion Ez are shown.

Figure 2.3 Morse potential energy curves for the neutral and negative-ion states of CC14. The new quantity illustrated in this figure is photodetachment energy. It is larger than AEa and is the peak in the photodetachment spectmm. Thermal electron attachment is exothermic, that is, EDEA = a positive quantity. Two other states dissociating to Cl + CC13(—) and the polarization curve are not shown.

One of the early confirmations of the ECD model was based on the agreement of the ECD measurements for the rate constants and activation energies for thermal electron attachment to SF6 and C7F14 with the values obtained using the microwave method. In the interim other methods have been developed. Indeed, the determination of the rate constants as a function of both electron energy and temperature has been achieved [10-15]. 

This has been designated the p temperature region. The activation energy for thermal electron attachment and the pre-exponential term for the rate constant for thermal electron attachment are obtained. At that time neither of these two quantities had been measured so comparisons could not be made, ko was estimated from the electron concentration measured as a function of reaction time. The maximum value of k was determined by the DeBroglie wavelength of the electron and stabilization to the ultimate ions by collisions at high pressures. 

By 1967 the kinetic model for nondissociative thermal electron attachment and revised values for the electron affinities of 16 aromatic hydrocarbons and 7 aromatic carbonyl compounds were reported [24-26]. The ECD Ea values were correlated to theoretical calculations, electronegativities, spectroscopic data, and reduction potentials. The majority of these remain the most precise electron affinities for such compounds. Some values are assigned to excited states based on the multistate model of the ECD postulated in the 1990s [27, 28]. The electron affinities of atoms, molecules, and radicals were reviewed in 1966 [24]. The relative Ea of nitrobenzene, CS2, and SO2 were measured by the thermal charge transfer techniques and the Ea of O2 by photodetachment [30-32]. 

In 1967 B. H. Mahan and C. E. Young used a new microwave method to determine the rate constant for thermal electron attachment to molecules. These quantities were determined for SF6 and C7F14 using the ECD and agreed with the values reported using the microwave method at room temperature within the experimental error [37, 38]. In addition, the temperature dependence was determined so that activation energies were obtained. This was especially important in the case of strained molecules such as cyclooctatetrene [34],... 

Freeman also demonstrated the effect of a change in geometry on the formation of the anion of N20 and showed that the activation for thermal electron attachment... 

Jorge Ayala determined the rate constants for thermal electron attachment to aliphatic halides and the halogen molecules to confirm values measured by other techniques. The electron affinities of the halogen molecules had been determined by endothermic charge transfer experiments [57-59]. In the case of the halogen molecules, the ECD results lead to the rate constant for thermal electron attachment rather than the electron affinity of the molecule. Two-dimensional Morse potentials for the anions were constructed based on these data. Freeman and Ayala searched for a nonradioactive source for the ECD. In 1975 the data on the electron affinities of atoms were summarized and correlations examined between these values and the position of the atoms in the Periodic Table [60]. A large number of the atomic electron affinities were measured by photoelectron spectroscopy [61]. A similar compilation of the electronegativities of elements was carried out. In this case some of the values were obtained from the work functions of salts [62], These results will be updated in Chapter 8. 

Dissociative electron capture is observed with hyperthermal electrons in NIMS electron impact experiments. In order for dissociative electron capture to take place with thermal electrons, there must be a dissociative pathway that is accessible by the thermal activation of the neutral molecule or a low-lying negative-ion state. The quantity D(R — Le) — Ea(Le) must be less than about 1.0 eV. This limit has been established empirically. Two types of dissociative thermal electron attachment have been observed in NIMS and ECD. The first occurs by unimolecular dissociation in which there is only one temperature region for many compounds. In the original work a low-temperature low-slope region was observed but unexplained. We now believe this could represent the formation of a molecular ion with an electron affinity of about 0.1 eV. The exact nature of this ion is not known, but it could represent stabilization to an excited state. In Figure 4.8 ECD data are plotted for several... 

Only compounds that undergo sequential dissociative thermal electron attachment can exhibit a y region, where (k2 E> kN) and (k-1 k2), and... 

Examples of the temperature dependence for different classes of molecules are given as global plots of In KTm versus 1,000/T. The curves that are drawn used the equations for the complete model. Excited-state Ea have been measured with the ECD. The clearest indication of an excited state is structure in the data, as illustrated for carbon disulfide and C6F6. The temperature dependence of the ions formed in NIMS of the chloroethylenes indicate multiple states. NIMS also supports AEa, as in the case of SF6 and nitrobenzene. The quantity D Ea can be obtained from ECD data for DEC(2) dissociative thermal electron attachment. If one is measured, then the other can be determined. In the case of the chlorinated benzenes this quantity gives the C—Cl bond dissociation energy. The highest activation energy of 2.0 eV has been observed for the dissociation of the anion of o-fluoronitrobenzene. 

In this chapter the experimental ECD and NIMS procedures for studying the reactions of thermal electrons with molecules and negative ions are described. Gas phase electron affinities and rate constants for thermal electron attachment, electron detachment, anion dissociation, and bond dissociation energies are obtained from ECD and NIMS data. Techniques to test the validity of specific equipment and to identify problems are included. Examples of the data reduction procedure and a method to include other estimates of quantities and their uncertainties in a nonlinear least-squares analysis will be given. The nonlinear least-squares procedure for a simple two-parameter two-variable case is presented in the appendix. 

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 complementary techniques for determining rate constants for thermal electron attachment, detachment, and dissociation are the flowing afterglow, the microwave technique, the ion cyclotron resonance procedures, the swarm and beam procedures, the shock tube techniques, the detailed balancing procedures, the measurement of ion formation and decay, and the high-pressure mass spectrometer procedures. In all cases the measurement of an ion or electron concentration is made as a function of time so that kinetic information is obtained. In the determination of lifetimes for ions, a limiting value of the ion decay rate or k is obtained. 

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]. 

The rate constants for thermal electron attachment by alternative techniques are compared to those obtained with the ECD. A method of calculating activation energies for rate constants measured at a single temperature is suggested. 

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. 

---