Dissociation energy Dissociative electron capture

HC1 is formed by a chain mechanism with a very high yield (M /N value —103), presumably because of the high reactivity of the Cl atom. The chain is initiated by the production of H and Cl atoms. Dissociative electron capture by Cl2 requires about 1.6 eV energy. Therefore, presumably both kinds of ions are initially produced by excitation and ionization. The chain is propagated simply as follows ... 

The second result relates to an electronation procedure in which field emission into liquids provides a source of low energy electrons which are capable of effecting selective chemical changes (Noda et al., 1979). Thus dissociative electron capture by alkyl halides, and electron capture followed by protonation, afford alternative routes to specific spin adducts, as exemplified in Scheme 8. 

Resonance electron capture directly yields the negative molecular ion, M"", whereas even-electron fragment ions are formed by dissociative electron capture and ion-pair formation. Molecular ions are generated by capture of electrons with 0-2 eV kinetic energy, whereas fragment ions are generated by capture of electrons from 0 to 15 eV. Ion-pair formation tends to occur when electron energies exceed 10 eV. [77]... 

Primary electron energy dissociative electron capture results from low-energy secondary electrons released from a metal surface. 

The formation of C5H5Co was observed at low ionizing energy, and is probably formed by a dissociative electron capture process. 

This definite failure to find G is importantlybecause Gaydoni has suggested that this appearance potential might correspond to the production of 0+ and C. Finally, O, without kinetic energy, is produced at 9-5 eV. Earlier workers obtained 0 at voltages between 9 3 and 9-5. The production of 0 at such low energies can only be due to a dissociative electron capture process... 

Our thermal functions Indicate that PF" is thermodynamically stable with respect to dissociation (P + F") below 2500 K. These predictions agree qualitatively with the results of MacNeil and Thynne ( ) who observed PF" in the negative ion mass spectrum of PF. Their reported appearance potential, AP(PF"/PFg) = 11.4 0.1 eV, gives EA(PF) = -0.47 eV assuming that the dissociative electron capture process is PF (g) + e" = PF"(g) + 2F(g). We believe this value is too low probably because of excess kinetic and/or excitation energies amounting to roughly 1.5 eV. 

The sulfates as a class were fragmented by a single pathway, neutral loss of 80 mass units corresponding to loss of SO,. Consistent with its tendency toward dissociative electron capture (or solvolysis) in filament-on ISP Q ) and hydrolysis, 4-nitrophenol sulfate was most susceptible to S03 loss in CAD fragmentation (Table V). With a collision energy of only 14 eV, the parent ion was either absent or only a minor product ion. 

Figure 4.8 ECD data plotted as In ftp versus 1,000/7. These alkyl halides dissociate via activation of the molecule. They are designated DEC(l) for dissociative electron capture via activation of the molecule. The slope multiplied by R is equal to the activation energy in the high-temperature region. The low-temperature data were originally not explained, but could be an indication of a low molecular electron affinity. The curves were fit using both dissociation and molecular ion formation. Data from [16-19].

Figure 4.10 BCD data plotted as In Kp versus 1.000/7. Chlorobenzene and chloro-naphthalene dissociate via an intermediate molecular ion. They are designated DEC(2) for dissociative electron capture by a two-step process. The slope in the high-temperature region multiplied by R is equal to the EDEA. Given the electron affinity of the dissociating species, in this case Cl(—), the C—Cl bond dissociation energy can be measured. Data from [17-19].

Compounds under dissociative electron capture in the ECD. DEC(l) refers to molecules that can dissociate unimolecularly via a single potential energy curve. DEC(2) refers to molecules that can dissociate via a negative-ion intermediate. 

Dissociative electron capture also occurs at energies greater than thermal but usually with significantly lower rate constants (cross sections) and various fragmentations. For example, SFg at low energies forms a metastable molecular anion whereas at higher energies direct impact dissociation to SFj" and F occurs but with a much lower cross section. 

This process, termed resonance electron capture (REC), occurs with high efficiency when electrons of near-thermal energies are available. These electrons are conveniently generated in the El source in the presence of a moderating gas such as H2, CH4, -C4Hio, NH3, N2, or Ar. With electrons of higher energies, the dissociative electron capture [reaction (2.15)] and ion-pair formation [reaction (2.16)] processes are also operative. 

Typical ion/molecule reactions between anions and neutral molecules [7, 9,152-155] can be classified as displacement (Scheme 2.1, Eq. (2.5)), proton transfer (Scheme 2.1, Eq. (2.6)), charge exchange (Scheme 2.1, Eq. (2.7)) and association (Scheme 2.1, Eq. (2.8)). Among these, the displacement reaction has been studied extensively in the gas phase [156,157], and the prototypical example is an anionic Sn2 reaction studied by Brauman [156]. In addition, interactions between a neutral molecule and an electron involving electron capture [158] and dissociative electron capture [159], are also important types of ion/molecule reactions in the gas phase. A molecule M vhth a positive electron affinity can form a stable anion M by capturing a thermal electron. In the case of dissociative electron capture, capture of an electron by a compound MX leads to a repulsive state of MX, which dissociates to form M and X vhth excess internal and/or translational energy. 

For CH3CI, it is transformed to the ed e with the ultrafast cleavage d stage (time of single oscillation, synchronous or concert mechanisms ). The kinetic rather than thermodynamic factors (such as the energy of the lowest unoccupied molecular orbital of the substrate, etc.) became now deciding. We drew attention to the analogy of [ed] reactions with gas-phase dissociative electron capture process and... 

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