Ion-molecule equilibria in mixtures of N2 and Ar

Abstract

Equilibrium constants for the reactions 2 N 2 + N 2 + = N 4 + + N 2 ( I ) ; N 4 + + Ar = ArN 2 + + N 2 ( II ) ; ArN 2 + + Ar = Ar 2 + + N 2 ( 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 ΔSI ° = −16.2 ± 2.9 eu and ΔHI ° = −24.4 ± 2.1 kcal/mole at 723°K. By extrapolation of enthalpy data to 0°K, the bond energies, D 0(AB+), were found to be 25.9 ± 2.1, 26.2 ± 2.2, and 27.8 ± 2.2 kcal/mole for N 2 –N 2 + , Ar–N 2 + , and Ar–Ar+, respectively. When presumably more accurate literature data for Reaction (I) were used, the D 0(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 ΔHn °, ΔSn ° and D 0(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–N 2 + ) , ΔSn °, and ΔHn ° (n = II or III) would be significantly different if Ar–N 2 + were T shape rather than linear as assumed.