Titans atmosphere: possible clustering reactions ofHCNH+, HCNH+(N2) and HCNH+(CH4) ions withacetylene

The thermochemistry of several ion-molecule clustering reactions of HCNH+, HCNH+(N2) and HCNH+(CH4) ions with C2H2 was determined from chemical equilibrium measurements. Inorder to better understand the chemistry of Titan, a gas mixture containing 97% N2, 3% CH4 anda small amount of 0.005% C2H2 was irradiated by α-particles in the temperature range 140–320 K and analysed by high-pressure mass spectrometry. The enthalpy and entropy changes for thereactions HCNH+(C2H2)n+C2H2+N2←→HCNH+(C2H2)n+1+N2 (n=0–2), HCNH+(C2H2)n+N2+N2←→HCNH+(C2H2)n(N2)+N2 (n=1, 2), HCNH+(C2H2)n+CH4+N2←→HCNH+(C2H2)n(CH4)+N2 (n=1, 2), HCNH+(C2H2)n(N2)+C2H2+N2←→HCNH+(C2H2)n+1(N2)+N2 (n=0, 1) have been determined. For the HCNH+(C2H2)n series,the absolute enthalpy change decreases monotonically from 12.1 to 2.8 kcal mol−1 for n=1 to 3,showing no noticeable effect concerning the filling of shells. The nature of the bonding remainsmainly electrostatic. For the HCNH+(C2H2)n(X) series with X=N2 or CH4, the free energychanges ΔG° were calculated for T=90 K and 165 K. For both temperatures, ΔG°≈0 forreplacement of N2 by CH4 for n=0 to 2 wheareas ΔG° for the substitution of N2 or CH4 by C2H2are negative for n=0, 1. The ligand-exchange reaction leading to the formation of HCNH+(C2H2)and HCNH+(C2H2)2 could exist in the lower Titans atmosphere. Paths involving HCNH+(C2H2)(N2) or HCNH+(C2H2)(CH4) and yielding to HCNH+(C2H2)2 ions could also be possible and thuscould lead to heavier organic-nitrogen compounds and eventually large aerosols.