A study of the reaction O−3+CO2?CO−3+O2 and its implication on the thermochemistry of CO3 and O3 and their negative ions

Abstract

The reaction O−3+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 CO−3 and O−3 ions in the flow-drift tube at high E/N to form O− shows that CO−3 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 (CO2+O−) −D (O2+O−) ?0.58 eV. The reaction OH−+O3→O−3+OH is found to be fast, thereby establishing lower limits for the electron affinity of O3 and the O− bond dissociation energy of O−3. When taken with the above limit for the relative CO−3 and O−3 bond dissociation energy one obtains lower limits for the electron affinity of CO3 and the O− bond dissociation energy of CO−3. 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.