Nondissociative Electron Capture

In the event that D(AB) — Ea(B) 1.5 eV], the value of k2 will be small and the data can exhibit both an a and a (I region. At most, four parameters will be determined. In some cases only a single slope and intercept are observed. We obtain the absolute electron affinity from the slope in the a region  

The g s are partition functions, k and h, the Boltzmann and Planck constants, and me, the electron mass. 

From this equation ln([g(A )/g(A)]) = ln(Ai/A i) — 12.43 — n AN/2AD). The value of 12.43 is obtained from fundamental constants. This involves the ratio of (An/2Ao) so that the concentration of the positive species and the temperature dependence of the intrinsic rate constants will cancel out and not affect the value of the slope. With an experimental intercept and n AN/2AD), a value for Qan, the partition function ratio Qan = [g A )/g A)]), can be calculated. The observed Qan values range from 1 to 10 The ECD and NIMS experiments are the primary source of Qan in the literature. A value ofAN/2AD can be obtained by measuring the ECD temperature dependence of a compound with an accurate Ea and a unit partition function ratio such as acetophenone. A unit value of Qan implies that the negative ion and neutral have the same partition functions except for spin multiplicity. Thus, equation 4.13 becomes 

Some of the Eql(l/1) compounds are predicted to have a second region at lower temperatures. If one or two data points exist in the second region, the slope will be lower and the intercept higher than the average value. In the case of benz[e]pyrene the linear least-squares fit gives an intercept of 15.10 1.60, which is higher than 

Originally, it was assumed that the (Q,m should always be unity and that the electron affinity should be obtained from the fixed intercept. Other experimental determinations of the electron affinities for CS2, CH3N02, tetracene, and benzopyrene demonstrate that Qan can be lower than 1. It is not expected that Qan is greater than 1 so the electron affinity is obtained by assuming it is 1, as in the case of benz[e]pyrene. If the intercept is lower than the average intercept, then the actual slope through the data should be used to obtain the electron affinity along with its associated errors [2-4]. 

---

Figure 2.2d shows the simplest type of nondissociative electron capture into discrete states of AB- that will occur between energies E1 and Ei, resulting in a vibrationally excited AB- molecular ion. If the capture process remains an isolated event, the electron will be ejected by autodetachment (Auger process) within a time comparable with a vibration time. 

A similar kinetic model has been developed for the measurement of ion complex formation kinetics and energies involving both dissociative and nondissociative electron capture, for example, the hydration of halide ions and of 02( ). The ratio of negative ions observed in NIMS can be used to determine energies of complex formation. In this case the sequential formation of the higher complexes must be added to the kinetic model. These studies are important because they demonstrate that the API mass spectrometer can be used to measure thermodynamic quantities. When we use the data for hydrates of 02(—) as an example, the kinetic expression is given by... 

The electron capture coefficient for nondissociative electron attachment is... 

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 mono and dichlorobenzenes do not show parent negative ions at 373 K or 523 K. This does not imply that the electron affinity is negative, but rather that other processes occur. Both dissociative and nondissociative capture are observed for the tri and tetrachlorobenzenes. For the penta and hexachlorobenzenes no dissociative attachment is observed. For the biphenyls with two chlorines ion molecule reactions predominate. With three chlorines, two on one ring and the third on the other ring, molecular ions are observed at 373 K. With four or more chlorines molecular ions are observed at both high and low temperatures, and dissociative electron... 

The electron affinities of the chlorinated biphenyls are lower than those of the chlorinated napthalenes. Nominal values are taken from Table 11.12. The temperature dependence of the three isomers of the monchlorobiphenyl will be similar to meta, ortho, and para dichlorobenzene data. Likewise, the temperature dependence of the compounds with two chlorines on the same ring will be similar to that of the trichlorobenzenes. The response of the fully chlorinated compound will be similar to that of hexachlorobenzene. The isomers with eight and nine chlorines only show nondissociative capture. Approximate curves for the chlorinated biphenyls are illustrated in Figure 11.13 and compared with experimental data obtained using the PDECD [45]. 

---