Clustering reactions of HCNH+, HCNH+(N2) and HCNH+(CH4) with ethane: application to Titan atmosphere

Thermochemical data for several ion-molecule clustering reactions of HCNH+, HCNH+(N2) and HCNH+(CH4) with C2H6 were determined by high-pressure mass spectrometry in order to better understand the role of ethane in the chemistry of Titan’s atmosphere. Clustering equilibria were obtained in a gas mixture (pure N2 with a small amount (10−5 of the total pressure) of C2H6) irradiated by alpha particles in the temperature range 150–320 K. The enthalpy and entropy changes for the reactions HCNH+(C2H6)n+C2H6+N2↔HCNH+(C2H6)n+1+N2 (n=0, 1), HCNH+(C2H6)(N2)n+N2+N2↔HCNH+(C2H6)(N2)n+1+N2 (n=0–2), HCNH+(CH4)+C2H6+N2↔HCNH+(CH4)(C2H6)+N2 and HCNH+(N2)+C2H6+N2↔HCNH+(N2)(C2H6)+N2 have been measured. For the HCNH+(C2H6)n series, the absolute enthalpy change for the trimer formation (4.5 kcal mol−1) is found to be much smaller than the relatively large enthalpy for the dimer formation (11.6 kcal mol−1). For the HCNH+(N2)n(C2H6) series, the enthalpy change appears to decrease monotonically from n=1 to 3, showing no discontinuity in the attachment energy of the ligand molecule. The measured low enthalpy changes suggest that the nature of the bonding remains mainly electrostatic for these cluster’s formations as it is for the formation of HCNH+(CH4)(C2H6). The free energy changes ΔG° for the substitution of N2 or CH4 by C2H6 in HCNH+(N2)n (n=1, 2), HCNH+(CH4) and HCNH+(N2)(C2H6) are negative for T=90 K and T=165 K. The ligand-exchange reaction leading to HCNH+(C2H6) and HCNH+(C2H6)2 is possible in Titan’s lower atmosphere (100–150 km). These processes could lead to the formation of heavier cyanides and eventually large aerosols.