Mass spectrometric study of three-body ion-molecule clustering reactions of C pH q+ ions with N 2 or CH 4 in N 2CH 4 mixtures

Abstract

Thermochemical data for several ion-molecule clustering of hydrocarbon ions with N 2 or CH 4 were obtained from clustering equilibria studies in gas mixtures irradiated by α-particles. High-pressure mass spectrometry was used to determine the enthalpy and entropy changes of clustering ( ΔH° and ΔS°, respectively) for the reactions X +(N 2) n−1 +2N 2⇋ X +(N 2) n +N 2 with X = CH 5, n = 1–2; X = C 2H 5, n = 1–4; and X = C 3H 7, n = 1. For X = CH 5, the values ( ΔH°; ΔS°) are found to be (−6.8 kcal mol −1; −19.7 cal mol −1 K −1) for n = 1, and (−5.3 kcal mol −1; −15.9 cal mol −1K −1) for n = 2. For X = C 2H 5, ( ΔH°; ΔS°) = (−6.9 kcal mol −1; −18.2 cal mol −1 K −1), for n = 1, and (−4.6 kcal mol −1; −20.8 cal mol −1 K −1) for n = 2. From the equilibrium measurements at 129 K, estimates of the thermochemical values could be obtained for n = 3–4. The results obtained for the free energy, ΔG°, were −1.4 kcal mol −1 for n = 3, and −1.1 kcal mol −1 for n = 4. For X = C 3H 7 we found ΔG° = −0.7 kcal mol −1 at 213 K. The association reactions X ++2CH 4⇋ X +(CH 4)+CH 4 with X = CH 5, C 2H 5, C 2H 7, and C 3H 7 were also studied, resulting in free energy values at 206 K of −3.1, −1.9, −0.5 and −1.3 kcal mol −1, respectively. The results for CH 5, C 2H 5 and C 3H 7 are compared with previously reported measurements.