Thermodynamic values for the reaction CO2H++CH4?CH5+ +CO2 were measured by pulsed high pressure mass spectrometry as ΔH°=−2.8 kcal/mole, ΔS°=−2.4 e.u.; for CS2H+ +H2O?H3O++CS2 as ΔH°=−2.3 kcal/mole, ΔS°=−7.7 e.u.; and for CS2H++H2S?H3S++CS2 as ΔH°=−7.6 kcal/mole, ΔS°=−9.6 e.u. The proton affinities of CO2 and CS2 are found to be 124.0 and 166.6 kcal/mole, respectively. The large entropy changes in the reactions involving CS2 are explained by a nonlinear CS2H+ ion. Stabilities of cluster ions, i.e., −ΔH° for the ion–molecule association reactions leading to the cluster ions were measured (in kcal/mole) as follows: For radical ions: CO2+⋅CO2, 16.2; CO2+⋅2CO2, 6.0; CS2+⋅CS2, 21.9; S2+⋅CS2, 21.9. For protonated, even‐electron ions: CO2H+⋅CO2, 20.1; CS2H+CS2, 11.1; H3O+⋅CO2, 14.4. The stability of CO2H+⋅CO2 is unusually high for a small protonated cluster ion and indicates significant hydrogen bonding in this ion. Entropies of association were found in the range −17 to −26 e.u. Limits for the stabilities of O2+nCO2 (n=1–3) were also established. The application of these results in Martian ionospheric chemistry is discussed.

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