Dissociative capture attachment

Dissociative electron attachment is a radiation chemical reaction suitable for cutting a polymer just in half. If two equivalent polymer skeletons R are connected with a functional group XY that has a large cross section of dissociative electron attachment, the polymer captures an ejected electron at the center of the polymer skeleton and is broken into two fragments with similar molecular weight, as R-XY-R + e —>R-X + Y-R. The key to construct such a polymer is to find a functional group that is possible to connect two polymer chains, to capture an electron efficiently, and to dissociate into two fragments after the capture. 

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... 

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. 

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. 

The results show that DMF inhibits the copolymerization completely. However, CHC13 and CHjCL accelerate the copolymerization. It is known that halogenated hydrocarbons (RX) capture elctrons via dissociative electron attachment and stabilize monomer cation M +. 

When polymethylmethacrylate is irradiated in the presence of ethyl mercaptan, it is reasonable to consider that radiation ejected electrons in the matrix are scavenged by the additive. Indeed, in aqueous solution the efficiency of capture of solvated electrons by ethyl mercaptan [158] is about 500 times higher than by methyl pivalate, a low molecular weight model of polymethylmethacrylate [158]. Dissociative electron attachment has been observed by mass spectrometry [159]... 

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. 

While a positive EA is a necessary criterion for production of the radical anion [M] , the observation of a [M] anions in NlCl spectra also depends on the lifetime of these radical anions in the NlCl source. In other words, molecules with positive electron affinities may not be observed because they lose an electron (autodetachment) due to collisions with the reagent gas. Autodetachment of the radical anion is important for small molecule and organic molecules with small positive (<50 kJ/mol) EAs (75,110,111). Also, once a sample molecule has captured an electron, it may follow a dissociative electron attachment reaction pathway to produce [M—X] anions (Reaction 7.35) or may produce ion pair [X] anions (Reaction 7.36) (69,103). 

Quasiequilibrium statistical theory was applied to the negative ion mass spectra of diphenylisoxazoles. Electron capture by the isoxazole leads to molecular ions having excited vibrations of the ring and of bonds attached to it. The dissociation rate constants were also calculated (77MI41615, 75MI416U). 

Figure 2.2a illustrates the most general type of electron capture by a diatomic molecule. The asymptote of the dissociative AB curve lies below the asymptote of the AB curve by an amount equal to A(B), the electron affinity of B. Since the speed of the incident electron will be large, the nuclei may be considered at rest during the time in which attachment takes place. The Franck-Condon principle then states that dissociative attachment... 

Electron affinity (EA) is a measure of the ability of a molecule to attach an electron. This value is equal to an adiabatic IE of the corresponding negative ion. EAs for some EHal2 have been derived from appearance energies of the corresponding anions formed by low-energy electron dissociative resonance capture from in the conrse... 

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