Ion-molecule clustering reactions of HCNH + ion with N 2 and/or CH 4 in N 2CH 4 mixtures

Abstract

Thermochemical data for several ion-molecule clustering reactions of HCNH + ion with N 2 and (or) CH 4 have been studied by high-pressure mass spectrometry in order to better understand the chemistry of the atmosphere of Titan. Clustering equilibria were obtained in gas mixtures irradiated by α-particles in the temperature range 145–300 K. The enthalpy and entropy changes for the reactions HCNH +(N 2) n + CH 4+N 2↔HCNH +(N 2) n (CH 4)+N 2 ( n = 0–2), HCNH +(N 2) n−1 (CH 4) + N 2 + N 2↔HCNH +(N 2) n + (CH 4)+N 2 ( n = 1, 2), HCNH +(CH 4)+CH 4+N 2 ↔HCNH +(CH 4)+N 2 have been measured. The low enthalpy changes suggest that the nature of the bonding remains mainly electrostatic for these reactions. Enthalpy changes as a function of n for X +. (N 2) n and X +(CH 4) n ions (X + = CH 5 +, C 2H 5 + and C 3H 7 +) were compared with those for HCNH +(N 2) n and HCNH +(CH 4) n ions. The free energy changes for the step-wise replacement of N 2 by CH 4 in HCNH +(N 2) n (CH 4) n clusters ( n and m = 0, 1) have been determined. The ligand-exchange reaction HCNH +(N 2)+CH 4↔HCNH +(CH 4)+N 2, the first step of a possible path for the formation of organic-nitrogen compounds, could exist for T ∼ 160 K in Titan's atmosphere (∼ 150 km). Switching reactions from HCNH +(N 2)(CH 4) involving N 2, CH 4 and C 2H 2 and leading to HCNH +(C 2H 2) 2 could also be a path yielding heavier organic-nitrogen compounds in the lower Titan's atmosphere.