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

• Published in: Planetary and space science Vol. 47; no. 1; pp. 151 - 162
• Main Authors: Vacher, J.R., Duc, E.Le, Fitaire, M.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-12-1998
• ISSN: 0032-0633; 1873-5088
• DOI: 10.1016/S0032-0633(98)00089-0

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.