A study of the reaction O₃^-+CO₂⇌ CO₃^-+O₂ and its implication on the thermochemistry of CO₃ and O₃ and their negative ions

The reaction O₃^-+CO₂⇌CO ₃^-+O₂ has been examined in the forward and reverse directions in a variable-temperature flowing afterglow from 200 to 600 K and in a flow-drift tube at mean relative kinetic energies from 0.04 to 1 eV. The forward direction is clearly established as the exothermic direction. Furthermore, collisional dissociation of CO₃^- and O ₃^- ions in the flow-drift tube at high E/N to form O ^- shows that CO₃^- is the more stable ion. All of this implies that D(CO₂+O^-)> D(O₂+O ^-). Kinetic-equilibrium studies at the higher temperatures show that the reverse rate constant is less than 6×10^-15 cm³ s^-1 below 600 K. When this is combined with the estimated entropy change of the reaction one obtains the quantitative lower limit D(CO ₂+O^-) -D(O₂+O^-)≥0.58 eV. The reaction OH^-+O₃→O₃+OH is found to be fast, thereby establishing lower limits for the electron affinity of O₃ and the O^- bond dissociation energy of O₃^-. When taken with the above limit for the relative CO₃^- and O₃^- bond dissociation energy one obtains lower limits for the electron affinity of CO₃ and the O^- bond dissociation energy of CO₃^-. The latter lower limit does not overlap an upper limit for D(CO₂+O^-) obtained from recent photodissociation studies. The relation of this discrepancy to the electron affinity of O₃ is discussed.