Dissociative Thermal Electron Attachment

In 1964 a brief description of the ECD kinetic model was presented in Nature. This occurred in response to criticism of the use of ECD data to measure the affinity of biological molecules for free electrons. A new procedure for studying electron attachment in swarms and beams had been applied to chlorobenzene. Since the ECD response was originally referenced to that of chlorobenzene, critics emphasized the distinction between dissociative capture and nondissociative capture. They noted that dissociative capture can take place with thermal electrons. This was not disputed. It was realized that certain molecules could undergo dissociative electron capture and that the kinetic model would have to be expanded to include these types of compounds. 

The analyst is probably not too perturbed by the assertions of academic scientists that proves that the ECD detector does not work. I well recall the fierce attack on the 

This initial criticism led to an increase in the data for organic molecules in swarm 

In this region the phenomenological activation energy will be equal to I) Ea(X). Since the electron affinity of the halogen atoms is well established, this experimental property can be used to obtain the bond dissociation energy. 

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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. 

Nitromethane makes a nice transition from small molecules to large organic molecules. The Ea of N02 was determined using the ECD (above 425 K) from the activation energy for dissociative thermal electron attachment to CH3N02. The Ea of CH3N02 was determined from data in the lower-temperature region. The data were presented in Chapter 5 as an example of the determination of a Qan much smaller than 1. The ECD Ea of 0.50 0.02 eV for nitromethane is supported by AMB and TCT values. The PES value is 0.26 0.08 eV and could exist for an excited state. A dipole bound state is observed at 0.012 eV [1-13]. 

The temperature dependence of the alkyl halides was one of the first subjects to be studied using the ECD. These are the simplest to analyze because often there is only one temperature region when dissociative thermal electron attachment is exothermic. This means that the EDEA, the energy of dissociative electron attachment, is positive EDEA = a(X) - D(R —X). The alkyl bromides, iodides, and chlorides are among the few organic compounds that have positive EDEA. Like the homono-nuclear diatomic halogen ions, the ground-state anionic curves for these molecules are M(3), with positive values for all three Herschbach metrics—EDEA, Ea, and VEa. 

Some of the above alkyl halides contain fluorine. In the case of CF3C1 the activation energy indicated that two negative-ion curves contribute to the dissociative thermal electron attachment. In 1997 S. R. Sousa and S. E. Bialkowski carried out a classic study of the ECD temperature dependence of alternative fluorocarbon freon replacements [16]. The study used a commercial ECD and the fundamental concepts discussed in this book to obtain rate constants and energies for compounds. One of these, CF2C12, was used as an internal standard in all the measurements. The... 

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.

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. 

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. 

Thermal electron attachment to nitrous oxide has been studied for more than 75 years. In spite of this extensive experimental and theoretical work the adiabatic electron affinity of N20 remains uncertain. The reported electron affinities are 0.0 0.1 eV, 0.22 0.2 eV an upper limit of 0.76 0.1 eV is determined by PES [92, 98-103]. By fitting the ECD data to an expanded kinetic model, the data can be attributed to two states. The Ea obtained from the ECD data are —0.17 0.05 eV for the linear anion and 0.40 0.15 eV for the bent anion. The larger uncertainty in the ECD value results because the transition temperature to dissociative electron attachment cannot be determined. If the calculated curve is extended to the highest temperature, the Ea is 0.5 eV and the E value is 0.5 eV. If it is terminated as shown in the second curve at approximately 350 K, the Ea is 0.3 eV and the E1 is 0.4 eV. The best value of the AEa is thus 0.40(15) eV. 

Figure 11.2 Correlation of activation energies for thermal electron attachment with bond dissociation energies for DEC(l) and DEC(2) compounds.

From the data in Table 11.1 the consistency of A is apparent. If we assume a constant A about equal to the DeBroglie A1 and with relative molar responses of similar compounds in the ECD at the same temperature, the activation energy for thermal electron attachment can be obtained from E x /irc - = RT(ln(Rlef/Rx)). This is only applicable to alkyl chlorides, bromides, or iodides. The activation energy of dissociative reactions for alkylfluorides is too large to be observed in the ECD. 

The two molecules with a single positive slope (CHC1F2 and CF3CH2F) and the one with two slopes (CF3CHF2) indicate nondissociative thermal electron attachment. The negative slope for the latter is the activation energy for electron attachment to the ground state. There are no reasonable dissociative pathways. 

In 1972 Wentworth, Chen, and Steelhammer set out to write a monograph entitled Negative Ions Reaction and Formation in the Gas Phase, so scientists could plan future research using the ECD. At the time few fundamental properties of thermal electron reactions had been measured. Now many molecular electron affinities and rate constants for thermal electron attachment have been measured. Currently electron affinities and bond dissociation energies can be verified using theoretical SCF calculations on desktop computers. It is especially timely to review the techniques for studying reactions of thermal electrons with molecules and to evaluate the results. 

Rate constant for thermal electron attachment (kx or k12) from Mechanism I at low temperatures and high electron affinity Mechanism II for dissociative rate constant. 

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