Ion-molecule clustering reactions of HCNH+ ion with N2 and/or CH4 in N2CH4 mixtures

Thermochemical data for several ion-molecule clustering reactions of HCNH+ ion with N2 and (or) CH4 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+(N2)n+ CH4+N2↔HCNH+(N2)n(CH4)+N2 (n = 0–2), HCNH+(N2)n−1(CH4) + N2 + N2↔HCNH+(N2)n + (CH4)+N2 (n = 1, 2), HCNH+(CH4)+CH4+N2 ↔HCNH+(CH4)+N2 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+. (N2)n and X+(CH4)n ions (X+ = CH5+, C2H5+ and C3H7+) were compared with those for HCNH+(N2)n and HCNH+(CH4)n ions. The free energy changes for the step-wise replacement of N2 by CH4 in HCNH+(N2)n(CH4)n clusters (n and m = 0, 1) have been determined. The ligand-exchange reaction HCNH+(N2)+CH4↔HCNH+(CH4)+N2, the first step of a possible path for the formation of organic-nitrogen compounds, could exist for T ∼ 160 K in Titanʹs atmosphere (∼ 150 km). Switching reactions from HCNH+(N2)(CH4) involving N2, CH4 and C2H2 and leading to HCNH+(C2H2)2 could also be a path yielding heavier organic-nitrogen compounds in the lower Titanʹs atmosphere.