Negative ion formation in collisions between potassium and fluoro- and chloromethanes: Electron affinities and bond dissociation energies

Formation, stability and structure of radical anions of chloroform, tetrachloromethane and fluorotrichloromethane in the gas phase

“…If the C1-ion transfer reactions from the molecular radical anions to the parent molecules are exothermic, this leads to even lower values for the electron affinities of chloroform and tetrachloromethane indicating that the given numbers represent upper limits for the true values. In conclusion, these considerations indicate that the adiabatic electron affinities of chloroform and tetrachloromethane are significantly lower than previously reported experimental [22,28] and also calculated values [29,30]. The previously reported experimental value [22] for the EA of tetrachloromethane was based upon a determination of the threshold energy for formation of CC14-ions in collisions between potassium atoms with a controlled kinetic energy and CCI 4 molecules.…”

“…In conclusion, these considerations indicate that the adiabatic electron affinities of chloroform and tetrachloromethane are significantly lower than previously reported experimental [22,28] and also calculated values [29,30]. The previously reported experimental value [22] for the EA of tetrachloromethane was based upon a determination of the threshold energy for formation of CC14-ions in collisions between potassium atoms with a controlled kinetic energy and CCI 4 molecules. The yield of the CC14-ions was extremely low and in addition, the relative cross-section displayed a non-gradual increase at low values of the relative kinetic energy.…”

“…For example, minor amounts of the radical anions of CF2C12 and CFC13 are reported to arise by electron transfer from the O~-ion [12,17] and in a recent study, we observed that CBr4-ions are formed by electron transfer from the radical anion of azobenzene to tetrabromomethane [14]. Low but measurable yields of the tetrachloromethane radical anion have been formed also in the collisions of CC14 molecules with neutral potassium atoms with a controlled kinetic energy [22]. Likewise, attachment of electrons to free halomethane molecules in the gas phase is referred to normally as an inherent dissociative process leading mostly to the predominant or exclusive formation of a halide ion [23,24].…”

“…The rare observations of stable molecular radical anions of simple halomethanes appear to be in conflict with the large and positive adiabatic electron affinities which have been suggested for species such as chloroform, tetrachloromethane and fluorotrichloromethane on the basis of results obtained by experiments [22,28] and high level calculations [29,30]. These apparently inconsistent findings raise questions as to the true adiabatic electron affinities of these molecules as well as to the intrinsic stability of the molecular radical anions with respect to dissociation into a halide ion and a halogen substituted methyl radical.…”

“…Under our experimental conditions only (15) The electron affinity of chloroform has been reported to be ~ 170kJmo1-1, whereas the value for tetrachloromethane has been given as ~ 190kJmo1-1 [22,28]. Based on an electron affinity of chloroform of 170kJmo1-1 in combination with known heats of formation of the other species involved in the reaction in Eq.…”