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

Abstract

The thermochemistry of several ion-molecule clustering reactions of HCNH +, HCNH +(N 2) and HCNH +(CH 4) ions with C 2H 2 was determined from chemical equilibrium measurements. Inorder to better understand the chemistry of Titan, a gas mixture containing 97% N 2, 3% CH 4 anda small amount of 0.005% C 2H 2 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 +(C 2H 2) n +C 2H 2+N 2←→HCNH +(C 2H 2) n +1 +N 2 ( n=0–2), HCNH +(C 2H 2) n +N 2+N 2←→HCNH +(C 2H 2) n (N 2)+N 2 ( n=1, 2), HCNH +(C 2H 2) n +CH 4+N 2←→HCNH +(C 2H 2) n (CH 4)+N 2 ( n=1, 2), HCNH +(C 2H 2) n (N 2)+C 2H 2+N 2←→HCNH +(C 2H 2) n +1 (N 2)+N 2 ( n=0, 1) have been determined. For the HCNH +(C 2H 2) 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 +(C 2H 2) n (X) series with X=N 2 or CH 4, the free energychanges Δ G° were calculated for T=90 K and 165 K. For both temperatures, Δ G°≈0 forreplacement of N 2 by CH 4 for n=0 to 2 wheareas Δ G° for the substitution of N 2 or CH 4 by C 2H 2are negative for n=0, 1. The ligand-exchange reaction leading to the formation of HCNH +(C 2H 2)and HCNH +(C 2H 2) 2 could exist in the lower Titans atmosphere. Paths involving HCNH +(C 2H 2)(N 2) or HCNH +(C 2H 2)(CH 4) and yielding to HCNH +(C 2H 2) 2 ions could also be possible and thuscould lead to heavier organic-nitrogen compounds and eventually large aerosols.