Mass spectrometric study of three-body ion-molecule clustering reactions of C sub(p) H sub(q) super(+) ions with N sub(2) or CH sub(4) in N sub(2) -CH sub(4) mixtures

Considerable effort has been expended in recent years in attempting to describe the chemistry of the atmosphere of Titan. In its lower atmosphere, where ionization results from cosmic-ray bombardment, the relatively low tempertures and high neutral densities are favourable conditions for three-body ion-molecule association reactions. Thermochemical data for several ion-molecule clustering of hydrocarbon ions with N sub(2) or CH sub(4) were obtained from clustering equilibria studies in gas mixtures irradiated by alpha -particles. High-pressure mass spectrometery was used to determine the enthalpy and entropy changes of clustering ( Delta H degrees and Delta S degrees , respectively) for the reactions X super(+) (N sub(2) ) sub(n) sub(-) sub(1) +2N sub(2) reversible reaction X super(+) (N sub(2) ) sub(n) +N sub(2) with X=CH sub(5) sub(,) n=1-2; X=C sub(2) H sub(5) , n=1-4; and X=C sub(3) H sub(7) , n=1. For X=CH sub(5) , the values ( Delta H degrees ; Delta S degrees ) are found to be (-6.8 kcal mol super(-) super(1) ; -19.7 cal mol super(-) super(1) K super(-) super(1) ) for n=1, and (-5.3 kcal mol super(-) super(1) ; -15.9 cal mol super(-) super(1) K super(-) super(1) ) for n=2. For X=C sub(2) H sub(5) , ( Delta H degrees ; Delta S degrees )=(-6.9 kcal mol super(-) super(1) ; and -18.2 cal mol super(-) super(1) K super(-) super(1) ), for n=1, and (-4.6 kcal mol super(-) super(1) ; -20.8 cal mol super(-) super(1) K super(-) super(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, Delta G degrees , were -1.4 kcal mol super(-) super(1) for n=3, and -1.1 kcal mol super(-) super(1) for n=4. For X=C sub(3) H sub(7) we found Delta G degrees =-0.7 kcal mol super(-) super(1) at 213 K. The association reactions X super(+) +2CH sub(4) reversible reaction X super(+) (CH sub(4) )+CH sub(4) with X=CH sub(5) , C sub(2) H sub(5) , C sub(2) H sub(7) and C sub(3) H sub(7) were also studied, resulting in free energy values at 206 K of -3.1, -1.9, -0.5 and -1.3 kcal mol super(-) super(1) , respectively. The results for CH sub(5) , C sub(2) H sub(5) and C sub(3) H sub(7) , are compared with previously reported measurements.