Ion‐molecule equilibria in mixtures of N₂ and Ar

Equilibrium constants for the reactions $2N2+N2+=N4++N2 (I)$⁠; $N4++Ar=ArN2++N2 (II)$⁠; $ArN2++Ar=Ar2++N2 (III)$ have been determined mass spectrometrically. The reactions were studied as a function of E/P and temperature in a drift tube. It was found that ΔS_I° = −16.2 ± 2.9 eu and ΔH_I° = −24.4 ± 2.1 kcal/mole at 723°K. By extrapolation of enthalpy data to 0°K, the bond energies, D₀(AB⁺), were found to be 25.9 ± 2.1, 26.2 ± 2.2, and 27.8 ± 2.2 kcal/mole for $N2–N2+$⁠, $Ar–N2+$⁠, and Ar–Ar⁺, respectively. When presumably more accurate literature data for Reaction (I) were used, the D₀(AB⁺) for the same series were found to be 24.3, 24.5, and 26.3 kcal/mole. With literature data for Reaction (I) ΔS_II° = 3.57 ± 0.06 eu, ΔH_II° = −0.549 ± 0.026 kcal/mole, ΔS_III° = 1.87 ± 0.08 eu and ΔH_III° = 2.16 ± 0.03 kcal/mole at 298°K. Various models were used to determine the model dependence of the quantities derived from the equilibrium constant data. The ΔH_n°, ΔS_n° and D₀(AB⁺) values are only weakly dependent on the assumed bond length and weak mode vibrational frequency distribution of the ion‐molecule complex. However, $D 0(Ar–N2+)$⁠, ΔS_n°, and ΔH_n° (n = II or III) would be significantly different if $Ar–N2+$ were T shape rather than linear as assumed.