Theoretical study of the formation of large, astronomically relevant PAH-organic molecule clusters

Abstract

Context. Polycyclic aromatic hydrocarbon (PAH) molecules play an essential role in the prebiotic compound evolution network in the interstellar medium (ISM). A recent experimental study revealed that large, astronomically relevant PAH-organic molecule clusters are gradually formed through the ion-molecule collision reaction pathway in the presence of a strong radiation field. Aims. We present a theoretical survey for the formation processes of PAH-organic molecule clusters (e.g., such as the graphene carbon cluster (C48) organic molecule (Pyroglutaminol, pgn, C5H9NO2) cluster cations, (pgn)nC48+, n = [1,6]), to illustrate the building block mechanism for the formation of large prebiotic compounds. Methods. To investigate the stability and the building block formation mechanisms of PAH-organic molecule clusters in the ion-molecule collision reaction process, we carried out theoretical calculations with DFT, including the hybrid density functional B3LYP, as implemented in the Gaussian 16 program. The basis set of the 6-311++G** and 6-31+G** was selected and used for different cluster systems. Results. We investigated the structure of newly formed species and the energy for these reaction pathways. The ion-molecule reaction between ((C5H9NO2)nC48+, n = [0,5]) with C5H9NO2 readily occur, resulting in a very large number of reaction pathways and very complex newly formed molecular clusters. An expanded tree (in building block pathways) shows the trunk and branches of these various formation pathways. These clusters (e.g., the graphene carbon cluster and its organic molecules) provide a possible formation and chemical-evolution route for the large complex prebiotic compounds in bottom-up and energy allowed processes in the ISM. Conclusions. The gas-phase reactions between large PAH species and organic molecules occur relatively easily, resulting in a very large number of reaction pathways and very complex newly formed molecular clusters. These PAH-organic molecule clusters will lead to large organic molecules, which may contain some of the critical molecular configurations that can characterize living material.