The Effect of Cluster Size on the Intra-Cluster Ionic Polymerization Process

Abstract

Polyaromatic hydrocarbons (PAHs) are widespread in the interstellar medium (ISM). The abundance and relevance of PAHs call for a clear understanding of their formation mechanisms, which, to date, have not been completely deciphered. Of particular interest is the formation of benzene, the basic building block of PAHs. It has been shown that the ionization of neutral clusters can lead to an intra-cluster ionic polymerization process that results in molecular growth. Ab-initio molecular dynamics (AIMD) studies in clusters consisting of 3–6 units of acetylene modeling ionization events under ISM conditions have shown maximum aggregation of three acetylene molecules forming bonded C6H6+ species; the larger the number of acetylene molecules, the higher the production of C6H6+. These results lead to the question of whether clusters larger than those studied thus far promote aggregation beyond three acetylene units and whether larger clusters can result in higher C6H6+ production. In this study, we report results from AIMD simulations modeling the ionization of 10 and 20 acetylene clusters. The simulations show aggregation of up to four acetylene units producing bonded C8H8+. Interestingly, C8H8+ bicyclic species were identified, setting a precedent for their astrochemical identification. Comparable reactivity rates were shown with 10 and 20 acetylene clusters.

Singlet Oxygen Insertion into Hydrocarbons: The Role of First- and Second-Generation Pathways in Astronomically Relevant Ices

“…Here, we focus on reactions of hydrocarbons with oxygen in its singlet form, O( 1 D). A possible explanation for the difference in the products in the gas phase and the ice environment is that the molecules in the ice alter the potential energy surfaces (PES) as observed previously in the case of molecular clusters [25,26,50,51]. However, stability analysis of the different products on the PES reveals that while changes in stability were observed, they do not significantly alter the PES, as demonstrated in Appendix A.…”

Intermolecular Forces: From Atoms and Molecules to Nanostructures

“…The importance of intermolecular forces in the ionic polymerization process on large clusters of acetylene molecules to form polyaromatic hydrocarbons (PAHs) has been studied by Molina and Stein [ 9 ]. They employed ab initio molecular dynamics (AIMD) simulations within interstellar medium (ISM) conditions and showed that four acetylene units can aggregate to form C 8 -bonded species, some of which are bicyclic structures.…”