Dissociative capture

Irradiation of the molecular radical anion of DESO, which has a yellow color, with light of X = 350-400 nm partially restores the red color and the ESR spectrum of the radical-anion pair. Similarly to the case of DMSO-d6 a comparison of the energetics of the photodissociation of the radical anion and dissociative capture of an electron by a DESO molecule permits an estimation of the energy of the hot electrons which form the radical-anion pair of DESO. This energy is equal to 2eV, similarly to DMSO-d6. The spin density on the ethyl radical in the radical-anion pair of DESO can be estimated from the decrease in hfs in comparison with the free radical to be 0.81, smaller than DMSO-d6. 

Negative ions are also produced by electron bombardment through a variety of electron attachment or dissociative capture processes (Dillard, 1973). They may also be generated indirectly as the result of an ion-molecule reaction. The cross-sections for negative ion formation are at least an order of magnitude less than those for positive ions, a fact which accounts for the much weaker ion currents that are produced. Some of the common precursors for well-known negative ions are shown in (6)—(13). For several of these... 

F. Kjeldsen, K. F. Haselmann, B. A. Budnik, F. Jensen, andR. A. Zubarev, Dissociative capture of hot (3-13 eV) electrons by polypeptide polycations An efficient process accompanied by secondary fragmentation, Chem. Phys. Lett., 356 (2002) 201-206. 

In its reactions, Caq acts as a nucleophile. Thus, its reactivity is greatly enhanced by electron-withdrawing groups next to double bonds or on aromatic rings. Although the first step in the reaction is electron addition, this is in some cases followed by rapid dissociation of an anionic fragment so that the overall process can be viewed as one of dissociative capture. This is epitomized by organic halides ... 

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. 

For multiple states the expression is expanded. The value of k can refer to direct dissociation, previously designated k 2 or non-dissociative capture. 

In the case of negative-ion mass spectrometry it is possible to obtain an expression for both the parent negative ion and the products of dissociative capture using a kinetic model similar to that in the ECD. The reactions and rate constants are attachment k, detachment k-, dissociation k2, recombination of both the electrons kD, and the negative ions kN. In some cases excited states are present, but for simplicity we will not include these here. 

Figure 12.12 The calculated ECD temperature dependence for AGCUT if we assume dissociative capture.

In a previous study [1], NF has been found to form with low or no kinetic energy in a dissociative capture process at zero electron energy, possibly by the process NF3 + e- NF 2F. Its resonance maximum was at 2.8 eV. The cluster ion NF3-F was identified in electron swarm experiments with a mixture of 0.04% NF3 in N2 using a drift tube mass spectrometer system [13]. 

Electron impact (E<50 eV) on NF3 (p = 5x10" Torr) gaveNF" in addition to F"and Fgjfor E = 50 eV, the relative abundances were F" F2 NF =1000 5.0 0.7. The ionization efficiency curve of NF" exhibited two appearance potentials indicating two processes for NF" formation The dissociative capture process NF3 + e" NF + 2F forming NF" with little or no kinetic energy occurred at zero electron energy, the ion-pair process NF3 + e" NF" +F + F + e" presumably corresponded to the appearance potential at 23.0 eV [27]. 

Dissociative capture Resonance capture Fluorescence Ion pair production Autoionization Predissociation Parent ion Fragment ion... 

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