A model of ionization-induced reactions in CH4/N2 clusters in Titan's atmosphere: theoretical insights into mono- and divalent states

Abstract

We proposed a model for ionization–induced reactions between N2 and CH4, the main components of Titan’s atmosphere, and examined their mechanism using quantum mechanical and molecular dynamics methods. Bimolecular CH4—N2 clusters form through collision, and their conformation depends on the encounter cross section due to weak intermolecular interaction. These clusters acquire a driving force through vertical ionization because the vertically ionized structure is not situated at the minimum of the potential energy surface in the ionized state. This leads to multiple reactions, overcoming energy barriers in the process. In the divalent state, a robust attractive interaction occurs between CH4 and N2 through charge transfer. Subsequently, the H4C–N2 covalent bond forms prior to reactions, resulting in the production of N2H+, CH3+, CH3N2+, and CH2N2+; otherwise only N2H+, CH3+, and CH2+ are generated. In contrast, when ionized to monovalent state, although dissociation of N2H+ and isomerization to CH3NHN+ and CH3NNH+ occurs, a significant portion dissociates into CH4+ and N2 without undergoing further reactions. Additionally, the generation of N2H+ and CH3+ is limited in the monovalent state, primarily due to a lower driving force and the absence of Coulombic explosion. Our computational results highlight the pivotal role of divalent reactions within Titan’s atmosphere, which are more efficient than monovalent reactions.